CA1168304A - High rate battery charger - Google Patents
High rate battery chargerInfo
- Publication number
- CA1168304A CA1168304A CA000368280A CA368280A CA1168304A CA 1168304 A CA1168304 A CA 1168304A CA 000368280 A CA000368280 A CA 000368280A CA 368280 A CA368280 A CA 368280A CA 1168304 A CA1168304 A CA 1168304A
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- battery
- charging
- pulse
- charge
- current
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- Charge And Discharge Circuits For Batteries Or The Like (AREA)
Abstract
ABSTRACT
A battery charger is provided which supplies pulses of charging current to a battery and dis-charges the battery between each charging pulse.
The change in the battery terminal voltage over a predetermined period during each charging pulse is measured, and the pulse charging is terminated when a parameter of the measured voltage change exhibit a predetermined characteristic. Pulse charging may be terminated as a result of the measured voltage change exceeding a predetermined threshold or as a result of the detection of a point of inflexion in the variation of measured voltage change with time.
A battery charger is provided which supplies pulses of charging current to a battery and dis-charges the battery between each charging pulse.
The change in the battery terminal voltage over a predetermined period during each charging pulse is measured, and the pulse charging is terminated when a parameter of the measured voltage change exhibit a predetermined characteristic. Pulse charging may be terminated as a result of the measured voltage change exceeding a predetermined threshold or as a result of the detection of a point of inflexion in the variation of measured voltage change with time.
Description
2 ~ 3~
The present inven-tion relates to battery chargers, and in particular to battery chargers for charging batteries at a high rate. The invention is applicable -to nickel-cadmium and other types of battery.
Nickel-cadmium (NiCd) batteries have found wide acceptance in many field~s, including portable radio telephone systems, because of their ability to be charged and discharged over a very wide opera-tional temperature range, their inherently low inter-nal resistance, their long life, and the fact that they can be fully sealed. These features make the battery particularly suitable for military use.
It is well known that NiCd batteries are damaged if overcharged. The equation which describes the chemical behaviour of a NiCd battery is given as follows:
2 Ni (OH)2 + Cd (OH)2 charge _ Cd ~ 2 Ni O(OH) + 2 H20 ~discharge ~ ~ r Cathode Anode This equation does not give any indication of the intermediate ionic states which must occur, for example the at-tachment of one of the hydrogen atoms to an (OH) group to provide water.
Overcharging of a NiCd battery returns hydrogen to the cathode as if a partial discharge had taken place. This precarious balance between apparent dis-charge and charge at the cathode means that a surfeitof energy must be present 9 in this case heat. As hydrogen is used up in this fashion, a build up of oxygen occurs, and in the case of excessive overcharge the oxygen is released through safety ventsO It is . . . ~
The present inven-tion relates to battery chargers, and in particular to battery chargers for charging batteries at a high rate. The invention is applicable -to nickel-cadmium and other types of battery.
Nickel-cadmium (NiCd) batteries have found wide acceptance in many field~s, including portable radio telephone systems, because of their ability to be charged and discharged over a very wide opera-tional temperature range, their inherently low inter-nal resistance, their long life, and the fact that they can be fully sealed. These features make the battery particularly suitable for military use.
It is well known that NiCd batteries are damaged if overcharged. The equation which describes the chemical behaviour of a NiCd battery is given as follows:
2 Ni (OH)2 + Cd (OH)2 charge _ Cd ~ 2 Ni O(OH) + 2 H20 ~discharge ~ ~ r Cathode Anode This equation does not give any indication of the intermediate ionic states which must occur, for example the at-tachment of one of the hydrogen atoms to an (OH) group to provide water.
Overcharging of a NiCd battery returns hydrogen to the cathode as if a partial discharge had taken place. This precarious balance between apparent dis-charge and charge at the cathode means that a surfeitof energy must be present 9 in this case heat. As hydrogen is used up in this fashion, a build up of oxygen occurs, and in the case of excessive overcharge the oxygen is released through safety ventsO It is . . . ~
3. ~ 3 0 ~
imperative that any long life consideration of batter-ies must include methods ~or minimising excessive gass-ing, as once oxygen has been vented the resulting imbalance within the battery cannot be corrected.
The risks of damage due to overcharging are relatively slight if the charging current is limited, but if batteries are to be rapidly charged, e.g. at a rate of the order of C x 7 where C i5 the rated charg-ing capacity o~ the battery, as is desired ln many of the environments where NiCd bat-teries find appli-cations, means must be provided for detecting when the bat-tery is fully charged or almost ~ully charged and then terminating the charge. Overcharging at high rates can make NiCd batteries dangerously explosive.
Many proposals ha~e been made for charging batteries by ~orcing pulses of current through the battery and monitoring the battery charge state at the termination of each pulse. For example, British Patent No. 1 219 324 describes a pulse charg-ing system in which the battery is initially charged continuously and then pulse-charged. The battery is discharged after each pulse and the charge is stopped in dependence upon the average discharge current or the ratio of the energy removed by the discharge to the energy put in during the preceding charging step.
British Patent No. 1 293 983 describes a modified arrangement in which the terminal voltage of the battery is sampled between pulses a predetermined time after the commencement of the discharge. None of the proposed systems have proved altogsther satisfactory
imperative that any long life consideration of batter-ies must include methods ~or minimising excessive gass-ing, as once oxygen has been vented the resulting imbalance within the battery cannot be corrected.
The risks of damage due to overcharging are relatively slight if the charging current is limited, but if batteries are to be rapidly charged, e.g. at a rate of the order of C x 7 where C i5 the rated charg-ing capacity o~ the battery, as is desired ln many of the environments where NiCd bat-teries find appli-cations, means must be provided for detecting when the bat-tery is fully charged or almost ~ully charged and then terminating the charge. Overcharging at high rates can make NiCd batteries dangerously explosive.
Many proposals ha~e been made for charging batteries by ~orcing pulses of current through the battery and monitoring the battery charge state at the termination of each pulse. For example, British Patent No. 1 219 324 describes a pulse charg-ing system in which the battery is initially charged continuously and then pulse-charged. The battery is discharged after each pulse and the charge is stopped in dependence upon the average discharge current or the ratio of the energy removed by the discharge to the energy put in during the preceding charging step.
British Patent No. 1 293 983 describes a modified arrangement in which the terminal voltage of the battery is sampled between pulses a predetermined time after the commencement of the discharge. None of the proposed systems have proved altogsther satisfactory
4 1 IB~304 however because of the particular characteristics of NiCd batteries.
NiCd batteries have a very low internal resis-tance and therefore simply loading the battery as is done with lead/acid batteries does not give a clear charge status indication. The -terminal voltage of NiCd batteries does not change very much in response to variations in the charge status and is therefore no-t an adequate measure of the charge status. Further-more, the terminal voltage can be affected by temper-ature variations by an amount similar to charge vari-ations. This latter point is of particular consequence as battery temperature increases by a significant amount during rapid charging. In fact, one rapid charger has been proposed which relies on the differential temperature between the inside and ou-tside of the battery casing to terminate rapid charging A further problem with known rapid battery chargers is that it is difficult to achieve better than 80/~ charge acquisition.
It is an object of the present invention to provide an improved rapid battery charger which can overcome or reduce the problems re~erred to above.
According to the present invention there is provided a battery charger comprising means for supplying pulses of charging current to a battery and means for discharging the battery during the interpulse period, characterised by means for measuring the change in the battery terminal voltage over a predetermined period d~ring a charging pulse or the interpulse period,
NiCd batteries have a very low internal resis-tance and therefore simply loading the battery as is done with lead/acid batteries does not give a clear charge status indication. The -terminal voltage of NiCd batteries does not change very much in response to variations in the charge status and is therefore no-t an adequate measure of the charge status. Further-more, the terminal voltage can be affected by temper-ature variations by an amount similar to charge vari-ations. This latter point is of particular consequence as battery temperature increases by a significant amount during rapid charging. In fact, one rapid charger has been proposed which relies on the differential temperature between the inside and ou-tside of the battery casing to terminate rapid charging A further problem with known rapid battery chargers is that it is difficult to achieve better than 80/~ charge acquisition.
It is an object of the present invention to provide an improved rapid battery charger which can overcome or reduce the problems re~erred to above.
According to the present invention there is provided a battery charger comprising means for supplying pulses of charging current to a battery and means for discharging the battery during the interpulse period, characterised by means for measuring the change in the battery terminal voltage over a predetermined period d~ring a charging pulse or the interpulse period,
5 . 1 ~ 3 ~ ~
and means for -termlna-ting t,he pulse charging of the battery when a parameter of the measured voltage change exhibits a predetermined characteristic.
In a simple embodiment of the invention, -the pulse charging of the bat-tery is terminated when the measured voltage change exceeds a predetermined thres-hold.
Preferably the change in the battery terminal voltage is measured during a period starting at the beginning of a charging pulse.
Preferably the chargercornprises a first current control device connected in series wi-th the battery across a charging current supply, a second current control device connected across the -terminals of the battery for the purpose of discharge, a timing circuit for cyclically turning the second current control device on and off, and means for turning the first current control device on when the second current control device is off. The charger may fur-ther comprise a monostable circuit triggered by the timing circuit for sampling the battery terminal voltage when the first current control device is turned on and at a predeter-mined interval thereafter, a comparator for comparing the two sampled voltages 9 and a bistable circuit for holding both current control devices off when the comparator detects a predetermined difference between the two sampled vol-tages. Conveniently~ the current control devices are transistors.
Means may be provided for changing the rate of charge from a rapid charge to a normal charge rate ,;
-`` 6. ~ ~6~3~ql of approximately 0.1 x C after a battery has completed a ~ast charge cycle.
In one embodiment of the charger, pulses of current of for example 10 seconds duration are supplied to a battery to be charged, the battery being dis-charged between each charging pulse for a period of for example 2 seconds. The battery terminal voltage is measured at the beginning of each charging pulse and again a predetermined period such as 2 seconds thereafter. When the battery has been charged, the difference between the two terminal voltage measure-ments increases. Pulse charging of -the battery is automatically terminated as soon as the difference between the two terminal voltages exceeds a predeter-mined threshold.
The above described charger according to theinvention essentially relies on the fact that if a charge/discharge se~uence is used with rapid current transitions, the battery terminal voltage does not follow this square wave current envelope, but rather changes voltage at a much slower rate. The ampli tude of this change in voltage increases as the battery nears completion of charge. This change in voltage, is used as a sense voltage and compared with a reference voltage so that charge may be term-inated at a threshold defined by the reference voltage.
The simple charger described above is effective if used in carefully controlled circumstances, but it ~annot be relied upon to handle mixed batches o~
7.
batteries of various types, conditions and temperatures.
The ideal relationship between the measured voltage difference and the "state of charge" would be that the voltage difference remains small until the battery is fully charged, whereafter the voltage difference would increase by, say, one order in a relatively short period of time. This would allow the charge to be terminated easily and accurately. Unfortunately5 the voltage difference proves to be variable in amplitude by about 2:1 from one cell to another, does not change dramatically in amplitude when the battery reaches its fully charged sta-te and is subject to an overall amplitude shift of about 301 with a change in temper-ature of 0c to 50c. The result of this is that it is not possible to select a measured voltage difference threshold which will always be certain to detect a fully charged cell at 50c and also allow the commencement of charge at 0c. As batteries may be used in situations where they are exposed to extreme temperatures, e.g.
in Arctic regions, but are generally charged in a charging station which is sheltered, the battery tem-perature can vary considerably during a charging operation simply because of the change of environment.
Thus it is of considerable importance to provide a charger which can cope with batteries of widely varying internal temperatures.
~ Irther problems are experienced with the simple charger described above. For example, when a fully discharged battery is to be recharged, the battery terminal voltage initially increases very rapidly.
8. l~304 This can result in the difference between the two measured voltages exceeding the predetermined threshold so that pulse charging is terminated before any signi-ficant charging of the ba-ttery has been achieved.
Furthermore, the difference voltage which is to be detected is proportional to the nominal voltage of the ba-ttery to be charged, and therefore if a charger is to be adapted to charge a range of batteries by having a voltage selector switch, a single fixed thres~
hold will not be sufficierlt. Even the simple selec-tion of an operating voltage ccm cause problems in dif~icult situations, particularly with inexperienced personnel.
In addition the described charger is not able to indicate if the condition of a battery has deteriora-ted.
Thus a further object of the present inventionis to obviate or mitigate the limitations of the simple charger described above.
Accordingly9 the present invention also provides a battery charger of the above type, characterised in that means are provided for monitoring the internal condition of the battery and means are provided for modifying the current through the battery to compen-sate for the monitored internal battery condition such that variations in the measured voltage change with internal battery condition are minimised.
In a commercially available NiCd bat-tery an output is provided which indicates the temperature difference between the interior of the battery and its casing. With such a battery, the charger terminal plugs 9 ~ ~6~30~
could incor~orate a temperature sensing device so that the internal battery temperature could be derived from the sensing device output in combination with the battery casing/interior temperature output.
The current ~odifying means may comprise means for modifying the amplitude of the current in the first portion of each charging pulse such tha-t the amplitude increases with increasing internal battery temperature, and means for modifying the amplitude of the current in a second portion of each charging pulse to maintain the mean charging current substan-tially constan-t.
Preferably the charging current amplitude modifying means comprises a first solid state device connected in series with the battery, a second solid state device connected between two resistors forming a potential divider and controlled by a signal repre-sentative of the internal ba-ttery temperature) and switches connecting a control terminal of the first solid state switch to respective sides of the second solid state switch.
Where a battery has an output which indicates the internal battery temperature this important condition of the bat-tery can be directly monitored.
Unfortunately most available batteries do not have such an output and it is therefore necessary to in-directly monitor the internal condition of the battery.
Accordingly the in~ention provides a battery charger of the above type c~aracterised in t~at the means for monitoring the internal condition of a 10. ~83~
battery to be charged comprise means for conducting a test charge/discharge sequence before the main charging sequence, means for monitoring the measured voltage change at the beginning of the test sequence, means for monitoring the measured vol-tage change after discharge, and means for reducing the charge rate of the main charging sequence if the second monitored voltage change is not substantially less than -the first.
I~ a high voltage change is monitored at the beginning of the test sequence, this could ei-ther indicate a cold battery or a charged warmer battery.
After the discharge step of the tes-t se~uence, if the battery is warm and had been previously fully charged, it will no longer be fully charged so the measured voltage change will be reduced~ If the measured voltage change is still high, the battery must be cold and therefore it is essential to select a lower charge rate appropriate to a cold battery.
The above described embodiments of the invention a]low for compensation for internal battery temperature but are not always able to handle batteries which exhibit other non-standard conditions. Studies of the variation of the measured voltage change with time have shown that when a viable battery is being charged, in almost all cases a curve of measured voltage change versus time exhibits a point of inflexion shortly before the battery is fully charged. Accordingly a further feature of the invention is the provision of means for detecting a point of inflexion in the varia-.:, 11. 116~304tion of the measured vol-tage change with time, and means ~or terminating a charge sequence after the point of inflexion is de-tected.
Some non-viable ba-tteries exhibit no point of inflexion but the curve of measured voltage change versus time goes through a maximum due to increasing temperature if charging is attempted. Accordingly a further feature of the invention is the provision of means for detecting a maximum in the variation of measured voltage change versus time and means for -terminating a charge ~equence if a maximum is detected in the absence of a point of inflexion.
A further feature of the invention is the provision of timing means to automatically terminate a charging sequence after a predetermined period related to the maximum period for which a ba-ttery can be effectively charged. This fur-ther feature is necessa~y as very cold non~viable batteries can accept a charge almost indefinitely without any significant variation in the measured voltage change.
It has been discovered that, to a first approx-imation, the "charge out/charge in" efficiency of NiCd batteries is reasonably constant over the charge cycle and with cells in various states of decrepitude.
Prior art devices for determining the capacity of a battery have initially charged the battery, then fully discharged it to measure the actual capacity, and then recharged it. This is obviously a lengthy and in-efficien-t process.
According to a further feature of the invention, 12~ 3~
there is provided a battery charger of the above type, characterised in that means are provided -to discharge the battery to a predetermined voltage per cell, and means are ~ovided for determining the charge supplied - 5 to the battery prior to termination of the charging sequence, the charge supplied being a measure of the capacity of the battery.
Effectively the present invention makes i-t possible to accurately determine when the battery is charged, and therefore the charge supplied prior to termina-tion is a reasonably accurate measure of the actual battery capacity.
The charge supplied can be determined by measur-ing the duration o~ a predetermined charging sequence.
Embodiments of the present invention will now be described, by way ~ example9 with reference to the accompanying drawings, in which :
Fig. 1 is a waveform diagram of the terminal voltage of a battery being charged by a first embodi-ment of the invention;
Figs. 2 to 4 together show the circuit diagram of the first embodiment of the invention;
Fig. 5 illustrates the effects of battery temperature upon battery charging characteristics;
Figs. 6 and 7 show charging current waveforms of a second embodiment of the invention in which battery temperature is compensated for, Fig. 6 illustrating the case of a cold battery and Fig. 7 the case of a warm battery;
Figs. 8 and 9 illustrate charging characteristics 3 03.
resulting from the waveforms of Figs. 6 and 7 respect-ively;
Fig. 10 is a schematic circuit diagram of the curren-t supply circuitry of the second embodiment of the invention;
Fig. 11 is a flow diagram illustrating the operating sequence of the second ernbodiment of the invention;
Figs. 12 and 13 show further curves of measured voltage change versus timefor hot and cold batteries respec-tively;
Fig. 14 shows further curves of measured voltage change versus time for hot and cold batteries to illustrate the effects of varying the mean charging current;
Fig. 15 schematically illustrates a third embodiment of the invention;
Figs. 16 and 17 show further details of component parts of the -third embodiment; and Fig. 18 is a flow diagram illustrating the operation sequence of the third embodiment.
Referring to Fig. 1, it is assumed that a discharged battery is connected to a rapid charger according to the invention at time to~ and that pulse charging is initiated at once. A-t the first inter-ruption of the charging current the terminal voltage drops to a minimum value of V1. At the commencement of the next charging pulse the voltage rises very rapidly from V1 to V2 at t1~ then rises slowly to the level V3 at time t2. It then varies only marginally 14. ~6l'33~4 during the remainder of -the charging pulse. The period t1 - t2 represents the time taken for the voltage to reach a sensible steady s-tate. At time t3 the passage of charging current through the battery is automatically terminated and the ba-ttery is then discharged for a relatively short period until time t~. Fur-ther pulses of current are then driven through the battery and the cycle is repeated as show~, the charging cycle even-tually being terminated. In practice, when a flat ba-ttery is being charged the cycle will be repeated many times,but for the purposes of explanation only six cycles are shown in Fig. 1.
It has been found that -the change in the battery terminal voltage which occurs during the ini-tiation f a charging pulse, i.e. voltage change (V3 - V2) over periods such as t1 ~ t2~ provides an accurate measure of the state of charge of the battery. When the battery is reaching the fully charged condition7 this voltage change increases, i.e. to (V3a - V
between times t6 and t7. It should be noted tha~
V3a is greater than V3, and that (~3a ~ V2a) is substantially greater than (V3 - V2). By monitoring the voltage change and terminating charging when the monitored change exceeds a predetermined threshold 9 NiCd batteries can be charged rapidly and reliably.
An embodiment of the invention will now be described with reference to Figs. 2 to 4. ~eferring to Fig. 2, two batteries may be connected to respec-tive terminals 1, 2 and 3, 4. High current capacity switching transistors S, 6 when in their conductive , ~
15. ~L16~30d~
states connect the -terminals 1, 2 and 3, 4 respectively across terminals 7, 13 and 8, 14 which in turn are connected to a D.C. power supply. The power supply may be the mains suitably transformed and rectified or another battery, i.e. a car battery.
The circuit of Fig. 2 is connected to the circuit of Fig. 3 via terminals 9 to 14 respectively.
Referring to Fig~ 3, a timer based on a 555 timing circuit 15 cyclically turns on transistors 16t 17 for a period of approximat;ely 2 seconds and then holds transistors 169 17 off for approximately 10 seconds. When transistors 16, 17 are off, trans-istors 18 and 19 turn on, thereby turning on trans-istors 5 and 6 (Fig. 2). A high charging current passes via transi~tors 5 and 6 to th0 batteries connected to the terminals 1, 2 and 3, 4. ~nen trans-istors 16, 17 are on, the batteries are discharged therethrough. Thus a charge/discharge cycle of the type described with reference to Fig. 1 is established, each charging pulse having a duration of 10 seconds, and each discharging period a dura-tion of 2 seconds.
When the charger has a pair of batteries connected to it, a transistor 20 turns on to supply the timing and pulse control circuits. This lights LED's 21, 22 to indicate that both the batteries are on charge, as -transistors 23, 24 are in their conducti~e states~
Re~erring to Fig. 4, which is connected to Fig. 3 via terminals 25 to 31, two integrated mono 16. ~B~30~
stable circuits 32 (i.e. a 4538 circui-t) and a switch 33 (i.e. a 4016 circuit) sample the voltage of the two batteries immediately the ba-ttery goes on charge (i.e. at the beginning of each 10 second charge pulse times t1 and t4 in Fig. 1) and three seconds later (times t2 and t5 in Fig. 1). This sampling is initiated by -the timer 15 (Fig. 3) ~hich triggers the monostable 32 via terminal 31 (Figs. 3 and 4). This causes the capacitors 34, 35 to charge via switch 33 which is closed for approximately 100mS
in dependence upon the volta~es across the batteries which appear at terminals 25, 26, i.e. at times t17 t4, and -t6. These sampled voltages are compared with the voltage across the batteries appearing at terminals 25, 26 by comparator circuits 36, 37. Three seconds after the capacitors34 and 35 are charged the output voltages of the comparator circuits 36 and 37 are sampled by the switch 33 under the control of the monostable 32. These output voltages appear at terminals 29 and 30. If at the time of sampling the output of the comparators the terminal battery voltage sampled at one input of the comparators exceeds the stored battery terminal voltage sampled at e.g. times t1 and t4 by a predetermined percentage, -the associated comparator turns on the transistor 38 or 39 (Fig. 3) to which its output is connected.
If for example the battery being moni-tored by comparator 37 reaches full charge, comparator 37 turns on transistor 38, illuminating LED 40 ~ indicate "battery charged" and extinguishing LED 21. In addition transistor 38 disables the charge and dis-charge circui-ts via diodes 43 and 44. The transistors 38 and 23 form a bistable circuit. The 'charged' battery does however continue to receive a trickle charge via the base of transistor 20 and resistor 42.
Equivalent circuitry reacts in the same manner if comparator 36 turns on transistor 39.
The above description refers to the charging of two batteries. I~ will of course be apprecia-ted tha-t the charger can be u.sed to charge the two halves of for example a 2 x 5 cell 1.2 Ampere Hour bat-tery. With the described charger a fully dis-charged battery can be fully charged in approximately 15 minutes compared with for example 1 hour with previously available chargers. If a fully charged battery is placed on charge~ it will be rejected in one cycle of the charging circuit, i.e. 10 seconds.
In the event that the two halves of the battery take substantially different times to charge1 this gener-ally indicates a failing battery or a charge fault.When the "battery charged" LED's light the battery can be either removed immediately or left inde~inately on the charger.
A normal (C x 0.1) charge outlet may be provided if desired in addition to the rapid charge circuits described.
In sealed NiCd batteries~ the electrodes are very closely spaced. This directly affects its fast charging capability. Although the chemical equation which describes the process taking place in NiCd 18. 1~68304 batteries appears to show a balance of oxygen and hydrogen in their initial and final states~ the physical construction of a NiCd battery li~its the rate at which conversion can take place. This arises from the differences in the ionic velocities which prevail. In general, limits on charging current are set to cater for this effect (in the case of practical batteries). Inthe practical situation of closely spaced electrodes, this matter is aggrev--ated by the diameter of the generated gas bubblesbecoming co~parable to electrode spacing. When this is the case, uneven chemical forming of -the e]ectrode plates can occur.
It is believed tha-t gas bubbles, arising as a result of the 'plate-forming-reaction-time' being less than that o~ the intermediate r~ctions, may be the major cause why most reports of prior high current charging systems do not show a full charge acquisition.
This phenomenon does not seem apparent in the case of the described charger, presumably because the regular discharging periods, irrespective o~ the state o~ the battery, repeatedly degas the battery, thus ensuring an even forming of the surface. This constitutes a further substantial improvement over previously available rapid charge.
The 10 second charge to 2 second discharge relationship selected for the described charger has been found to produce very good results. A dis-charge period of less than about 2 seconds is found to be less desirable, presumably because the degassing 19. ~ ~ ~81304 of the bat-tery is a relatively slow process. Equally it takes abou-t 3 seconds ~or the charging voltage to build up, probably because of the ionic velocities involved. The elapsed time between -the two ter~inal voltage measurements should be as short as possible to minimise temperature change effects and the like, and it is for this reason that in the described charger the two measurements are made a-t the start of the charging pulse and 3 seconds later. Although the change of battery voltage during a charge period has been utilised to determine the state of charge of a ba-t-tery a similar indication can be obtained by sampling the battery voltage during the discharge periods.
The circuit of Figs. 2 -to 4 operates as described when connected to a particular battery type and condition at a particular temperature. Unfortunately such well ordered circumstances are often not found in practice, particularly when dealing with military equipment.
Referring now to Fig. 5, the relationship between the measured change in voltage (~V) and time for a fixed rate of charging is illustrated ~or internal battery temperatures of 0c, 10c, 20c and 50 c. (It will be appreciated that the internal temperature of a battery to be charged will depend upon the ambient temperature to which the battery was expo~ed before being brough-t in for charging and can therefore vary widely, e.g. between -30c and ~55c.) If the threshold level were set at a level indicated 20. ~ 3 0 ~
by differential voltage Th, it can be seen that at 10c the measured voltage change would pass through the threshold level satisfactorily, but at 0c the threshold would always be exceeded (charging could not start) whereas at 20c and 50c the threshold level would never be reached (overcharging would occur).
The variation in these cw~ves has been exaggerated in order to illustrate the points more clearly.
Referring now to Figs. 6 and '7, modified charging currentpulse waveforms are illustrated which are compensated for internal battery temper~tures of -25c (Fig. 6) and +20c (Fig. 7). Each waveform comprises a discharge portion of dura-tion t8, and a charge portion made up of two portions of durations tg and t10 respectively. In the case of Fig. 6, a charging current of 3.1A is passed for three seconds, and a charging current of 6.1A is passed for seven seconds. In the case of Fig. 7, a charging current of 8A is passed for three seconds, and a charging current of 4A is passed for seven seconds. The total energy supplied in the two cases is thus the same, i.e. the mean charging current is the same.
The change in battery terminal voltage is measured over the period tg~ By decreasing the charg-ing current during the period tg with decreasinginternal battery temperatures, the variation of the change in battery terminal voltage with time can be minimised. Figs. 8 and 9 show the variations in the change :in terminal voltage with time corresponding to 3 Figs. 6 and 7 respectively. Although the curves of 21. 1 1 6 8 3 0 4 Figs. 8 and 9 are not identical, they are sufficiently similar to enable the reliable control of a charging cycle by reference to a fixed voltage change threshold.
Fig. 10 is a schematic circuit diagram of a circuit for supplying current waveforms of the type shown in Figs. 6 and 7. A battery 101 which is to be charged is connected to -terminals 102, 103. During a discharge period (t8) a switch 104 is actuated to turn on the constan-t current d:ischarge transistor 105. Dur-ing a charging period (tg~ t10) current is .supplied tothe battery by -transistor 106 the conduc-tivity of which is determined by switches 107 and 108 and a transistor 109. The conductivity of transistor 109 is determined by the o~tput of a temperature sensing circuit 110 which is co~lected by means not shown to sense the internal temperature of the battery 101.
The transistor 109 is connected between two resistors which effectively form a potential divider.
The switch 107 is closed during the period tg and applies the collector voltage of transistor 109, reduced by the breakdown voltage of zener diode 111, to the base of transistor 106. The switch 108 is open during period tg~ The switch 108 is closed during the period t10 and applies the emitter voltage of transistor 109 to the base of transistor 106. The switch 107 is open during period t10. Thus as the sensed internal battery temperature falls the collector voltage of transistor 109 sw~ngs more negative, increas-ing the t10 current, and the emitter voltage of trans-istor 109 swings more positive, decreasing -the tg current.
22, ~ 4 The change in the terminal voltage of the battery 101 over the perio~ tg is sensed by circuit 112. The circuit 112 may be of any suitable type and may comprise for example a monostable circuit triggered by a timing circuit controlling the cyclical charge/discharge sequence to sample the batt;ery terminal voltage at two instants within period tg~ a comparator for comparing the two sampled voltages, cmd a bistable clrcuit for providing a maintained output when the comparator detects a predetermined dii~ference between the two sampled voltages.
A test switch 113 is provided which when actuated connects a simulated battery 114 to the charger circuit to enable the operation of the sensing circuit 112 to be checked. A further test switch 115 is provided which when actuated connects a simulated "battery at 20c" input on te~minal 116 to transistor 109 to enable the operation of the charging current supply circuit to be tested.
It will be appreciated tha-t the switches 104, 107, 108, 113 and 115 which are schematically illustrated will in general be solid state devices.
Referring now to Fig. 11, an operating cycle flow chart of a charger according to the invention will be described. The operating cycle is initiated manually by an operator who connects a bat-tery to the charger and actuates a start switch. Thereafter operation is automatic.
After manual initiation by the operator, a test is made 1;o see that the voltage change sensing circuit : -112 (Fig. 10) is operating, as this is a crucial part of the system. This is done by actuating switch 113.
If a failure is found, this is indicated and the sequence stops. If no failure is found, a check is made to ensure that the main current carrying charger plug and the voltage change and temperature sense plugs are connected to the battery. Throughout the charge sequence these "plugs connec-ted" monitors are operative. Should there be a disconnection, the faul-t is visually indicated7 the charge sequence is stopped and all timing circuits are reset.
Following the plug check, if no failure is indicated, the battery is charged for long enough (e.g. 30 seconds) to allow a 'flat' battery to reach its nominal voltage. This voltage is then sensed to enable the charger to compensate for the nominal voltage of the battery being charged.
The battery is then discharged for four minutes.
This is because the battery could already be fully charged, and under the charge sequence to be followed it could get a four minute charge even if initially fully charged. Thus overcharging is prevented. In parallel with the four minute discharge, the battery voltage is sensed and if it drops to 1.OVDC per cell, which is indicative of a flat battery the four minute discharge is inhibited. This prevents unnecessarily discharging a flat battery which would not only waste 2 L~
-time but could cause a cell or cells to becorne excessively reverse charged.
The fast charge sequence is then star-ted. Temper-ature probes sense the internal battery temperature.
This informa-tion is used to set the discharge current (z) during period t8 and the charge currents (X and Y) during periods tg and t10 (see Figs- 6 and 7). As described above, by decreasing the ''tgll current X with decreasing temperature9 it is possible to get QV/a time curves which are similar even for widely differ-ing tempera-tures. It is also possible to slow down the charge rate at the very extremes of the temperature range.
The voltage change sensing circuit becomes operative after a delay of approximately one minute.
This prevents the circuit responding -to -the large voltage change transients which appear during the start of charge of a flat battery. When the predetermined voltage change threshold is reached, a time delay circuit is activated. This sets up a time-to-go for the completion of charge. That delay time may be a function of temperature, and if so -the temperature sense circuits will be arranged to compute and set the length of time delay needed to allow the battery to become fully charged. Thus it is seen -that the threshold detection is not used directly -to switch off the fas~ charge but rather to determine a "time-to-go"
for completion of charge. This enables the threshold level to be set at a safe level below the average peak of the vcltage change versus time characteristic.
~ 1 16~30 25 .
This greatly reduces the probability of the threshold not being reached. At the completion of fast charge, the charge continues at -the low rate of approxima-tely 0.1C (C being the reated charging capacity~ until the charger is switched off. This allows an extra "topping up" to take place if the operator is not ready to remove the battery from the charger.
The fast charge sequence will also be terminated if the temperature sensors either detect an excess in-ternal battery temperature (e.g. 60c) or an excessively high internal~to-case battery temperature (e.g. 30c).
Figs. 12 and 13 show further measured voltage change versus time curves, Fig. 12 showing curves for a series of different batteries of varying ages and general condition all at high temperatures (greater than 50c) and Fig. 13 showing curves for a similar series of cold batteries (-25c)~ The charging cycle in Fig. 12 is 3 seconds at 8A, 7 seconds at 4A, and 2 seconds at minus 0.7A. The charging cycle in Fig.
13 is 3 seconds at 3.1A, 7 seconds at 5.9A, and 2 seconds at -0.7A.
It can be seen tha-t given similar thermal conditions and the same charging pulse sequence, an assortment of different batteries can exhibit different characteristics~ In particular, the maximum measured voltage change varies widely which causes problems 3 ~ ~
26.
when using detec-tion of a fixed threshold level to terminate charging. It will however be noted that in each curve of Figs. 12 and 13 (and Fig. 5) there is a point of inflexion where the rate of change o~ the slope of the curve changes from positive to negative.
This ~akes it possible to terminate a charging sequence by detec-ting the poin-t of inflexion rather than an absolu-te value of the measured voltage change.
Fig. 1l~ shows four curves 117, 118, 119 and 120 each representing variations in the measured voltage change with time for the same battery. Curves 117 and 118 represen-t the case of the battery being warm (~21c), curves 119 and 120 the case of the battery being cold (-25c). The charge rate is 10 seconds at 2A, 2 seconds at -2A for curves 117 and 120, 3 seconds at 8A, 7 seconds at 4A, 2 seconds at -0.7A
for curves 118 and 119. It can be seen that the charge rate radically alters the variation o~
voltage change versus time, a high charge rate being suitable for warm batteries, a low charge rate being suitable for cold batteries. A low charge ra-te must be used for cold batteries as at a high charge ra-te the voltage change actually decreases with time. It is believed that this decrease is due to the charging current warming up the battery.
With a cold battery and high charge rate, the voltage change is high regardless of whether or not the battery is charged. With a warm battery the voltage change is high only if the b~ttery is charged.
Accordingly if a battery is discharged and then the 27.
voltage change is measured9 only a cold battery will exhibit a high voltage change. This makes it possible to identify a cold battery as such even when no direct temperature sensing circuit is incorporated in the battery.
Figs. 15 to 18 illustrate details of a system which detects points of inflexion in the variation of voltage change versus time and then terminates pulse charging~ and which detects cold batteries by performing an initial test charge/discharge sequence on all batteries and selects lower charging rates when a cold battery is detected.
Fig. 15 is a block diagram of the complete system. A battery to be charged is connected between terminals 121, 122 which are themselves connected to an analogue-to-digital converter 123, a programmable current source 124 and a programmable current load 125. The converter, source and load are controlled by a microprocessor 126 which is turned on by a switch 127.
As shown in Fig. 16, the converter 123 may be a 12 bit binary A/D converter, for example an Intersil ICL 7109. The source 124 may be for example a power supply model SP 684 as available from Power Electronics (London) Limited. The microprocessor may be type MPE-2 as available from Cyfas Limited. The particular components selected are however merely a matter of design choice.
Fig. 17 shows the current load circuit 125 in some detail. The ENABLE input 139 can be derived from a monostable (not shown~ which is continually being 3 0 ~
28.
re-triggered by an AWAKE pulse from the microprocessor.
If the microprocessor or its programme should fail, the AWAKE pulses would cease, the monostable would not be re-triggered, and the charge and discharge circuits 5 would be inhibited to prevent possible damage to the battery.
The ENABLE input 139 controls gate 129 which ENABLES the programmable power source (124 in Fig. 15) when required by the microprocessor. It also con-trols 10 gates 130 which admit the discharge current code inputs from the microprocessor to a digital -to analogue converter 131 (type ZN 425E). The discharge current code provides for 0.5A steps in the discharge current.
The analogue ou-tpu-t of -the converter controls a pair of transistors 132 (-type IRF 130) which are connected in series with resistors 133, 134. Lines 135, 136 are connected to the terminals of the battery being charged.
A link 137 is incorporated in the battery terminal plugs when it is desired to charge a twelve volt 20 battery, the link shorting out resistor 134, The link is not provided in the terminal plugs used for connect-ing to 24 volt batteries. This means that 24 volt and 12 volt batteries can be handled by the same unit.
A potentiometer 138 can be used for fine adjustment of 25 the discharge current as desired.
Referring now to Fig. 18, the operating cycle of the embodiments of Figs. 15 to 17 will be described.
Six separate battery connector stations are provided, and the first step is to connect batteries to one or 30 more of -these stations. Connectors are used which ,~
29 ~ 4 identify the type of battery and the unit initially checks that the batteries connected to it are of recognisable type. If so, a charge programme appropriate to that type is selected from the micro-processor memory. If not, a lead fault is indicatedand the sequence is termina-ted. Assuming that the battery type is recognised, the unit looks at the first battery station. If there is no battery there, it then looks at the other stations until one is found.
Once a battery is found, the unit conducts a test charge/discharge cycle. If the measured change in the terminal voltage of the battery is initially higher than a predetermined threshold and remains so during the test, this is assumed to indicate a cold battery and a low charge rate is selected from the microprocessor memory.
The appropriate charge rate having been selected, the charge sequence is run. The input to the power supply unit is monitored and the sequence is termin-ated if this input is less than a preset limit. Thisprevents batteries being over charged because the interrogating current pulses are not high enough to enable the charge cond~ion of the battery to be correctly assessed in the event of a power supply fault The voltage change is monitored during each charge/discharge cycle and averaged over five cycles.
the slope of the averaged voltage change with respect to time and the rate of change o~ the slope are monitored and charging is continued until the slope ~ 3 0 ~
is positive and its rate of change is negati~e, i.e.
a point of inflexion near to the fully charged condi-tion of the battery is detected, Charging of the battery is then terminated and the unit looks for another battery at the battery stations. The unit then repeats the above cycle on any located battery or termina-tes its operation if no other batteries are located.
Averaging o~ a number of voltage change samples effec-tively delays the detection of the point of inflexion as well as providing a more reliable smoothed measurement, The delay means that charging is termin-ated shortly after the point of inflexion is reached so that the charge acquisition is increased and is in 15. fact very near to the maximum possible without risk of damage to the battery.
The unit operates two further routine checks to prevent overcharging. Firstly the duration of each charge sequence is measured and the sequence is automatically terminated if it exceeds a threshold related to the type of battery and the selected rate of charge. Secondly if a maximum occurs in the measured voltage change (i~e. slope zero but going negative) and no point of inflexion has been detected, the sequence is automatically terminated. The maximum indicates that a faulty battery is warming up.
The measured duration of each charge sequence is, assuming the sequence is not terminated due to an excessive duration, a measure of the charge acquired by the battery as a result of the charge sequence.
Accordingly if the battery is fully discharged prior to charging, and gi~en that charging is terminated accurately when the battery is fully charged, the duration of each charge sequence when considered in combination with the charge rate is a measure of the ability of the battery to accept charge. This is a very important parameter as it enables failing batteries to be discarded when for example charge acquisition drops below 70% of a nominal value for the battery in question The operating sequences illustrated in Figs.
11 and 18 are implemented using conventional micro-processor equipment and programming techniques within the normal competence of persons experienced in -the application of microprocessors.
The embodiments of the battery charger of the invention described above are concerned with charging NiCd batteries. Similar techniques are capable however of satisfactorily charging batteries other ~o than NiCd batteries, in particular lead/acid batteries.
In addition, silver/zinc, nickel/zinc and nickel/iron batteries for example exhibit characteristics such that they too can be satisfactorily charged.
It will be appreciated that as the charger described can handle bat-teries at a wide range of internal temperatures, in some circumstances it will be possible to dispense with the thermal insulation normally incorporated in NiCd batteries of the sort used in mobile radio equipment for example.
In the case of the embodiment of Figs. 15 to 18, 32, ~ o ~
no special tempera-ture sensing arrangements are required in the ~attery since only the normal two battery connections provide all the information required.
and means for -termlna-ting t,he pulse charging of the battery when a parameter of the measured voltage change exhibits a predetermined characteristic.
In a simple embodiment of the invention, -the pulse charging of the bat-tery is terminated when the measured voltage change exceeds a predetermined thres-hold.
Preferably the change in the battery terminal voltage is measured during a period starting at the beginning of a charging pulse.
Preferably the chargercornprises a first current control device connected in series wi-th the battery across a charging current supply, a second current control device connected across the -terminals of the battery for the purpose of discharge, a timing circuit for cyclically turning the second current control device on and off, and means for turning the first current control device on when the second current control device is off. The charger may fur-ther comprise a monostable circuit triggered by the timing circuit for sampling the battery terminal voltage when the first current control device is turned on and at a predeter-mined interval thereafter, a comparator for comparing the two sampled voltages 9 and a bistable circuit for holding both current control devices off when the comparator detects a predetermined difference between the two sampled vol-tages. Conveniently~ the current control devices are transistors.
Means may be provided for changing the rate of charge from a rapid charge to a normal charge rate ,;
-`` 6. ~ ~6~3~ql of approximately 0.1 x C after a battery has completed a ~ast charge cycle.
In one embodiment of the charger, pulses of current of for example 10 seconds duration are supplied to a battery to be charged, the battery being dis-charged between each charging pulse for a period of for example 2 seconds. The battery terminal voltage is measured at the beginning of each charging pulse and again a predetermined period such as 2 seconds thereafter. When the battery has been charged, the difference between the two terminal voltage measure-ments increases. Pulse charging of -the battery is automatically terminated as soon as the difference between the two terminal voltages exceeds a predeter-mined threshold.
The above described charger according to theinvention essentially relies on the fact that if a charge/discharge se~uence is used with rapid current transitions, the battery terminal voltage does not follow this square wave current envelope, but rather changes voltage at a much slower rate. The ampli tude of this change in voltage increases as the battery nears completion of charge. This change in voltage, is used as a sense voltage and compared with a reference voltage so that charge may be term-inated at a threshold defined by the reference voltage.
The simple charger described above is effective if used in carefully controlled circumstances, but it ~annot be relied upon to handle mixed batches o~
7.
batteries of various types, conditions and temperatures.
The ideal relationship between the measured voltage difference and the "state of charge" would be that the voltage difference remains small until the battery is fully charged, whereafter the voltage difference would increase by, say, one order in a relatively short period of time. This would allow the charge to be terminated easily and accurately. Unfortunately5 the voltage difference proves to be variable in amplitude by about 2:1 from one cell to another, does not change dramatically in amplitude when the battery reaches its fully charged sta-te and is subject to an overall amplitude shift of about 301 with a change in temper-ature of 0c to 50c. The result of this is that it is not possible to select a measured voltage difference threshold which will always be certain to detect a fully charged cell at 50c and also allow the commencement of charge at 0c. As batteries may be used in situations where they are exposed to extreme temperatures, e.g.
in Arctic regions, but are generally charged in a charging station which is sheltered, the battery tem-perature can vary considerably during a charging operation simply because of the change of environment.
Thus it is of considerable importance to provide a charger which can cope with batteries of widely varying internal temperatures.
~ Irther problems are experienced with the simple charger described above. For example, when a fully discharged battery is to be recharged, the battery terminal voltage initially increases very rapidly.
8. l~304 This can result in the difference between the two measured voltages exceeding the predetermined threshold so that pulse charging is terminated before any signi-ficant charging of the ba-ttery has been achieved.
Furthermore, the difference voltage which is to be detected is proportional to the nominal voltage of the ba-ttery to be charged, and therefore if a charger is to be adapted to charge a range of batteries by having a voltage selector switch, a single fixed thres~
hold will not be sufficierlt. Even the simple selec-tion of an operating voltage ccm cause problems in dif~icult situations, particularly with inexperienced personnel.
In addition the described charger is not able to indicate if the condition of a battery has deteriora-ted.
Thus a further object of the present inventionis to obviate or mitigate the limitations of the simple charger described above.
Accordingly9 the present invention also provides a battery charger of the above type, characterised in that means are provided for monitoring the internal condition of the battery and means are provided for modifying the current through the battery to compen-sate for the monitored internal battery condition such that variations in the measured voltage change with internal battery condition are minimised.
In a commercially available NiCd bat-tery an output is provided which indicates the temperature difference between the interior of the battery and its casing. With such a battery, the charger terminal plugs 9 ~ ~6~30~
could incor~orate a temperature sensing device so that the internal battery temperature could be derived from the sensing device output in combination with the battery casing/interior temperature output.
The current ~odifying means may comprise means for modifying the amplitude of the current in the first portion of each charging pulse such tha-t the amplitude increases with increasing internal battery temperature, and means for modifying the amplitude of the current in a second portion of each charging pulse to maintain the mean charging current substan-tially constan-t.
Preferably the charging current amplitude modifying means comprises a first solid state device connected in series with the battery, a second solid state device connected between two resistors forming a potential divider and controlled by a signal repre-sentative of the internal ba-ttery temperature) and switches connecting a control terminal of the first solid state switch to respective sides of the second solid state switch.
Where a battery has an output which indicates the internal battery temperature this important condition of the bat-tery can be directly monitored.
Unfortunately most available batteries do not have such an output and it is therefore necessary to in-directly monitor the internal condition of the battery.
Accordingly the in~ention provides a battery charger of the above type c~aracterised in t~at the means for monitoring the internal condition of a 10. ~83~
battery to be charged comprise means for conducting a test charge/discharge sequence before the main charging sequence, means for monitoring the measured voltage change at the beginning of the test sequence, means for monitoring the measured vol-tage change after discharge, and means for reducing the charge rate of the main charging sequence if the second monitored voltage change is not substantially less than -the first.
I~ a high voltage change is monitored at the beginning of the test sequence, this could ei-ther indicate a cold battery or a charged warmer battery.
After the discharge step of the tes-t se~uence, if the battery is warm and had been previously fully charged, it will no longer be fully charged so the measured voltage change will be reduced~ If the measured voltage change is still high, the battery must be cold and therefore it is essential to select a lower charge rate appropriate to a cold battery.
The above described embodiments of the invention a]low for compensation for internal battery temperature but are not always able to handle batteries which exhibit other non-standard conditions. Studies of the variation of the measured voltage change with time have shown that when a viable battery is being charged, in almost all cases a curve of measured voltage change versus time exhibits a point of inflexion shortly before the battery is fully charged. Accordingly a further feature of the invention is the provision of means for detecting a point of inflexion in the varia-.:, 11. 116~304tion of the measured vol-tage change with time, and means ~or terminating a charge sequence after the point of inflexion is de-tected.
Some non-viable ba-tteries exhibit no point of inflexion but the curve of measured voltage change versus time goes through a maximum due to increasing temperature if charging is attempted. Accordingly a further feature of the invention is the provision of means for detecting a maximum in the variation of measured voltage change versus time and means for -terminating a charge ~equence if a maximum is detected in the absence of a point of inflexion.
A further feature of the invention is the provision of timing means to automatically terminate a charging sequence after a predetermined period related to the maximum period for which a ba-ttery can be effectively charged. This fur-ther feature is necessa~y as very cold non~viable batteries can accept a charge almost indefinitely without any significant variation in the measured voltage change.
It has been discovered that, to a first approx-imation, the "charge out/charge in" efficiency of NiCd batteries is reasonably constant over the charge cycle and with cells in various states of decrepitude.
Prior art devices for determining the capacity of a battery have initially charged the battery, then fully discharged it to measure the actual capacity, and then recharged it. This is obviously a lengthy and in-efficien-t process.
According to a further feature of the invention, 12~ 3~
there is provided a battery charger of the above type, characterised in that means are provided -to discharge the battery to a predetermined voltage per cell, and means are ~ovided for determining the charge supplied - 5 to the battery prior to termination of the charging sequence, the charge supplied being a measure of the capacity of the battery.
Effectively the present invention makes i-t possible to accurately determine when the battery is charged, and therefore the charge supplied prior to termina-tion is a reasonably accurate measure of the actual battery capacity.
The charge supplied can be determined by measur-ing the duration o~ a predetermined charging sequence.
Embodiments of the present invention will now be described, by way ~ example9 with reference to the accompanying drawings, in which :
Fig. 1 is a waveform diagram of the terminal voltage of a battery being charged by a first embodi-ment of the invention;
Figs. 2 to 4 together show the circuit diagram of the first embodiment of the invention;
Fig. 5 illustrates the effects of battery temperature upon battery charging characteristics;
Figs. 6 and 7 show charging current waveforms of a second embodiment of the invention in which battery temperature is compensated for, Fig. 6 illustrating the case of a cold battery and Fig. 7 the case of a warm battery;
Figs. 8 and 9 illustrate charging characteristics 3 03.
resulting from the waveforms of Figs. 6 and 7 respect-ively;
Fig. 10 is a schematic circuit diagram of the curren-t supply circuitry of the second embodiment of the invention;
Fig. 11 is a flow diagram illustrating the operating sequence of the second ernbodiment of the invention;
Figs. 12 and 13 show further curves of measured voltage change versus timefor hot and cold batteries respec-tively;
Fig. 14 shows further curves of measured voltage change versus time for hot and cold batteries to illustrate the effects of varying the mean charging current;
Fig. 15 schematically illustrates a third embodiment of the invention;
Figs. 16 and 17 show further details of component parts of the -third embodiment; and Fig. 18 is a flow diagram illustrating the operation sequence of the third embodiment.
Referring to Fig. 1, it is assumed that a discharged battery is connected to a rapid charger according to the invention at time to~ and that pulse charging is initiated at once. A-t the first inter-ruption of the charging current the terminal voltage drops to a minimum value of V1. At the commencement of the next charging pulse the voltage rises very rapidly from V1 to V2 at t1~ then rises slowly to the level V3 at time t2. It then varies only marginally 14. ~6l'33~4 during the remainder of -the charging pulse. The period t1 - t2 represents the time taken for the voltage to reach a sensible steady s-tate. At time t3 the passage of charging current through the battery is automatically terminated and the ba-ttery is then discharged for a relatively short period until time t~. Fur-ther pulses of current are then driven through the battery and the cycle is repeated as show~, the charging cycle even-tually being terminated. In practice, when a flat ba-ttery is being charged the cycle will be repeated many times,but for the purposes of explanation only six cycles are shown in Fig. 1.
It has been found that -the change in the battery terminal voltage which occurs during the ini-tiation f a charging pulse, i.e. voltage change (V3 - V2) over periods such as t1 ~ t2~ provides an accurate measure of the state of charge of the battery. When the battery is reaching the fully charged condition7 this voltage change increases, i.e. to (V3a - V
between times t6 and t7. It should be noted tha~
V3a is greater than V3, and that (~3a ~ V2a) is substantially greater than (V3 - V2). By monitoring the voltage change and terminating charging when the monitored change exceeds a predetermined threshold 9 NiCd batteries can be charged rapidly and reliably.
An embodiment of the invention will now be described with reference to Figs. 2 to 4. ~eferring to Fig. 2, two batteries may be connected to respec-tive terminals 1, 2 and 3, 4. High current capacity switching transistors S, 6 when in their conductive , ~
15. ~L16~30d~
states connect the -terminals 1, 2 and 3, 4 respectively across terminals 7, 13 and 8, 14 which in turn are connected to a D.C. power supply. The power supply may be the mains suitably transformed and rectified or another battery, i.e. a car battery.
The circuit of Fig. 2 is connected to the circuit of Fig. 3 via terminals 9 to 14 respectively.
Referring to Fig~ 3, a timer based on a 555 timing circuit 15 cyclically turns on transistors 16t 17 for a period of approximat;ely 2 seconds and then holds transistors 169 17 off for approximately 10 seconds. When transistors 16, 17 are off, trans-istors 18 and 19 turn on, thereby turning on trans-istors 5 and 6 (Fig. 2). A high charging current passes via transi~tors 5 and 6 to th0 batteries connected to the terminals 1, 2 and 3, 4. ~nen trans-istors 16, 17 are on, the batteries are discharged therethrough. Thus a charge/discharge cycle of the type described with reference to Fig. 1 is established, each charging pulse having a duration of 10 seconds, and each discharging period a dura-tion of 2 seconds.
When the charger has a pair of batteries connected to it, a transistor 20 turns on to supply the timing and pulse control circuits. This lights LED's 21, 22 to indicate that both the batteries are on charge, as -transistors 23, 24 are in their conducti~e states~
Re~erring to Fig. 4, which is connected to Fig. 3 via terminals 25 to 31, two integrated mono 16. ~B~30~
stable circuits 32 (i.e. a 4538 circui-t) and a switch 33 (i.e. a 4016 circuit) sample the voltage of the two batteries immediately the ba-ttery goes on charge (i.e. at the beginning of each 10 second charge pulse times t1 and t4 in Fig. 1) and three seconds later (times t2 and t5 in Fig. 1). This sampling is initiated by -the timer 15 (Fig. 3) ~hich triggers the monostable 32 via terminal 31 (Figs. 3 and 4). This causes the capacitors 34, 35 to charge via switch 33 which is closed for approximately 100mS
in dependence upon the volta~es across the batteries which appear at terminals 25, 26, i.e. at times t17 t4, and -t6. These sampled voltages are compared with the voltage across the batteries appearing at terminals 25, 26 by comparator circuits 36, 37. Three seconds after the capacitors34 and 35 are charged the output voltages of the comparator circuits 36 and 37 are sampled by the switch 33 under the control of the monostable 32. These output voltages appear at terminals 29 and 30. If at the time of sampling the output of the comparators the terminal battery voltage sampled at one input of the comparators exceeds the stored battery terminal voltage sampled at e.g. times t1 and t4 by a predetermined percentage, -the associated comparator turns on the transistor 38 or 39 (Fig. 3) to which its output is connected.
If for example the battery being moni-tored by comparator 37 reaches full charge, comparator 37 turns on transistor 38, illuminating LED 40 ~ indicate "battery charged" and extinguishing LED 21. In addition transistor 38 disables the charge and dis-charge circui-ts via diodes 43 and 44. The transistors 38 and 23 form a bistable circuit. The 'charged' battery does however continue to receive a trickle charge via the base of transistor 20 and resistor 42.
Equivalent circuitry reacts in the same manner if comparator 36 turns on transistor 39.
The above description refers to the charging of two batteries. I~ will of course be apprecia-ted tha-t the charger can be u.sed to charge the two halves of for example a 2 x 5 cell 1.2 Ampere Hour bat-tery. With the described charger a fully dis-charged battery can be fully charged in approximately 15 minutes compared with for example 1 hour with previously available chargers. If a fully charged battery is placed on charge~ it will be rejected in one cycle of the charging circuit, i.e. 10 seconds.
In the event that the two halves of the battery take substantially different times to charge1 this gener-ally indicates a failing battery or a charge fault.When the "battery charged" LED's light the battery can be either removed immediately or left inde~inately on the charger.
A normal (C x 0.1) charge outlet may be provided if desired in addition to the rapid charge circuits described.
In sealed NiCd batteries~ the electrodes are very closely spaced. This directly affects its fast charging capability. Although the chemical equation which describes the process taking place in NiCd 18. 1~68304 batteries appears to show a balance of oxygen and hydrogen in their initial and final states~ the physical construction of a NiCd battery li~its the rate at which conversion can take place. This arises from the differences in the ionic velocities which prevail. In general, limits on charging current are set to cater for this effect (in the case of practical batteries). Inthe practical situation of closely spaced electrodes, this matter is aggrev--ated by the diameter of the generated gas bubblesbecoming co~parable to electrode spacing. When this is the case, uneven chemical forming of -the e]ectrode plates can occur.
It is believed tha-t gas bubbles, arising as a result of the 'plate-forming-reaction-time' being less than that o~ the intermediate r~ctions, may be the major cause why most reports of prior high current charging systems do not show a full charge acquisition.
This phenomenon does not seem apparent in the case of the described charger, presumably because the regular discharging periods, irrespective o~ the state o~ the battery, repeatedly degas the battery, thus ensuring an even forming of the surface. This constitutes a further substantial improvement over previously available rapid charge.
The 10 second charge to 2 second discharge relationship selected for the described charger has been found to produce very good results. A dis-charge period of less than about 2 seconds is found to be less desirable, presumably because the degassing 19. ~ ~ ~81304 of the bat-tery is a relatively slow process. Equally it takes abou-t 3 seconds ~or the charging voltage to build up, probably because of the ionic velocities involved. The elapsed time between -the two ter~inal voltage measurements should be as short as possible to minimise temperature change effects and the like, and it is for this reason that in the described charger the two measurements are made a-t the start of the charging pulse and 3 seconds later. Although the change of battery voltage during a charge period has been utilised to determine the state of charge of a ba-t-tery a similar indication can be obtained by sampling the battery voltage during the discharge periods.
The circuit of Figs. 2 -to 4 operates as described when connected to a particular battery type and condition at a particular temperature. Unfortunately such well ordered circumstances are often not found in practice, particularly when dealing with military equipment.
Referring now to Fig. 5, the relationship between the measured change in voltage (~V) and time for a fixed rate of charging is illustrated ~or internal battery temperatures of 0c, 10c, 20c and 50 c. (It will be appreciated that the internal temperature of a battery to be charged will depend upon the ambient temperature to which the battery was expo~ed before being brough-t in for charging and can therefore vary widely, e.g. between -30c and ~55c.) If the threshold level were set at a level indicated 20. ~ 3 0 ~
by differential voltage Th, it can be seen that at 10c the measured voltage change would pass through the threshold level satisfactorily, but at 0c the threshold would always be exceeded (charging could not start) whereas at 20c and 50c the threshold level would never be reached (overcharging would occur).
The variation in these cw~ves has been exaggerated in order to illustrate the points more clearly.
Referring now to Figs. 6 and '7, modified charging currentpulse waveforms are illustrated which are compensated for internal battery temper~tures of -25c (Fig. 6) and +20c (Fig. 7). Each waveform comprises a discharge portion of dura-tion t8, and a charge portion made up of two portions of durations tg and t10 respectively. In the case of Fig. 6, a charging current of 3.1A is passed for three seconds, and a charging current of 6.1A is passed for seven seconds. In the case of Fig. 7, a charging current of 8A is passed for three seconds, and a charging current of 4A is passed for seven seconds. The total energy supplied in the two cases is thus the same, i.e. the mean charging current is the same.
The change in battery terminal voltage is measured over the period tg~ By decreasing the charg-ing current during the period tg with decreasinginternal battery temperatures, the variation of the change in battery terminal voltage with time can be minimised. Figs. 8 and 9 show the variations in the change :in terminal voltage with time corresponding to 3 Figs. 6 and 7 respectively. Although the curves of 21. 1 1 6 8 3 0 4 Figs. 8 and 9 are not identical, they are sufficiently similar to enable the reliable control of a charging cycle by reference to a fixed voltage change threshold.
Fig. 10 is a schematic circuit diagram of a circuit for supplying current waveforms of the type shown in Figs. 6 and 7. A battery 101 which is to be charged is connected to -terminals 102, 103. During a discharge period (t8) a switch 104 is actuated to turn on the constan-t current d:ischarge transistor 105. Dur-ing a charging period (tg~ t10) current is .supplied tothe battery by -transistor 106 the conduc-tivity of which is determined by switches 107 and 108 and a transistor 109. The conductivity of transistor 109 is determined by the o~tput of a temperature sensing circuit 110 which is co~lected by means not shown to sense the internal temperature of the battery 101.
The transistor 109 is connected between two resistors which effectively form a potential divider.
The switch 107 is closed during the period tg and applies the collector voltage of transistor 109, reduced by the breakdown voltage of zener diode 111, to the base of transistor 106. The switch 108 is open during period tg~ The switch 108 is closed during the period t10 and applies the emitter voltage of transistor 109 to the base of transistor 106. The switch 107 is open during period t10. Thus as the sensed internal battery temperature falls the collector voltage of transistor 109 sw~ngs more negative, increas-ing the t10 current, and the emitter voltage of trans-istor 109 swings more positive, decreasing -the tg current.
22, ~ 4 The change in the terminal voltage of the battery 101 over the perio~ tg is sensed by circuit 112. The circuit 112 may be of any suitable type and may comprise for example a monostable circuit triggered by a timing circuit controlling the cyclical charge/discharge sequence to sample the batt;ery terminal voltage at two instants within period tg~ a comparator for comparing the two sampled voltages, cmd a bistable clrcuit for providing a maintained output when the comparator detects a predetermined dii~ference between the two sampled voltages.
A test switch 113 is provided which when actuated connects a simulated battery 114 to the charger circuit to enable the operation of the sensing circuit 112 to be checked. A further test switch 115 is provided which when actuated connects a simulated "battery at 20c" input on te~minal 116 to transistor 109 to enable the operation of the charging current supply circuit to be tested.
It will be appreciated tha-t the switches 104, 107, 108, 113 and 115 which are schematically illustrated will in general be solid state devices.
Referring now to Fig. 11, an operating cycle flow chart of a charger according to the invention will be described. The operating cycle is initiated manually by an operator who connects a bat-tery to the charger and actuates a start switch. Thereafter operation is automatic.
After manual initiation by the operator, a test is made 1;o see that the voltage change sensing circuit : -112 (Fig. 10) is operating, as this is a crucial part of the system. This is done by actuating switch 113.
If a failure is found, this is indicated and the sequence stops. If no failure is found, a check is made to ensure that the main current carrying charger plug and the voltage change and temperature sense plugs are connected to the battery. Throughout the charge sequence these "plugs connec-ted" monitors are operative. Should there be a disconnection, the faul-t is visually indicated7 the charge sequence is stopped and all timing circuits are reset.
Following the plug check, if no failure is indicated, the battery is charged for long enough (e.g. 30 seconds) to allow a 'flat' battery to reach its nominal voltage. This voltage is then sensed to enable the charger to compensate for the nominal voltage of the battery being charged.
The battery is then discharged for four minutes.
This is because the battery could already be fully charged, and under the charge sequence to be followed it could get a four minute charge even if initially fully charged. Thus overcharging is prevented. In parallel with the four minute discharge, the battery voltage is sensed and if it drops to 1.OVDC per cell, which is indicative of a flat battery the four minute discharge is inhibited. This prevents unnecessarily discharging a flat battery which would not only waste 2 L~
-time but could cause a cell or cells to becorne excessively reverse charged.
The fast charge sequence is then star-ted. Temper-ature probes sense the internal battery temperature.
This informa-tion is used to set the discharge current (z) during period t8 and the charge currents (X and Y) during periods tg and t10 (see Figs- 6 and 7). As described above, by decreasing the ''tgll current X with decreasing temperature9 it is possible to get QV/a time curves which are similar even for widely differ-ing tempera-tures. It is also possible to slow down the charge rate at the very extremes of the temperature range.
The voltage change sensing circuit becomes operative after a delay of approximately one minute.
This prevents the circuit responding -to -the large voltage change transients which appear during the start of charge of a flat battery. When the predetermined voltage change threshold is reached, a time delay circuit is activated. This sets up a time-to-go for the completion of charge. That delay time may be a function of temperature, and if so -the temperature sense circuits will be arranged to compute and set the length of time delay needed to allow the battery to become fully charged. Thus it is seen -that the threshold detection is not used directly -to switch off the fas~ charge but rather to determine a "time-to-go"
for completion of charge. This enables the threshold level to be set at a safe level below the average peak of the vcltage change versus time characteristic.
~ 1 16~30 25 .
This greatly reduces the probability of the threshold not being reached. At the completion of fast charge, the charge continues at -the low rate of approxima-tely 0.1C (C being the reated charging capacity~ until the charger is switched off. This allows an extra "topping up" to take place if the operator is not ready to remove the battery from the charger.
The fast charge sequence will also be terminated if the temperature sensors either detect an excess in-ternal battery temperature (e.g. 60c) or an excessively high internal~to-case battery temperature (e.g. 30c).
Figs. 12 and 13 show further measured voltage change versus time curves, Fig. 12 showing curves for a series of different batteries of varying ages and general condition all at high temperatures (greater than 50c) and Fig. 13 showing curves for a similar series of cold batteries (-25c)~ The charging cycle in Fig. 12 is 3 seconds at 8A, 7 seconds at 4A, and 2 seconds at minus 0.7A. The charging cycle in Fig.
13 is 3 seconds at 3.1A, 7 seconds at 5.9A, and 2 seconds at -0.7A.
It can be seen tha-t given similar thermal conditions and the same charging pulse sequence, an assortment of different batteries can exhibit different characteristics~ In particular, the maximum measured voltage change varies widely which causes problems 3 ~ ~
26.
when using detec-tion of a fixed threshold level to terminate charging. It will however be noted that in each curve of Figs. 12 and 13 (and Fig. 5) there is a point of inflexion where the rate of change o~ the slope of the curve changes from positive to negative.
This ~akes it possible to terminate a charging sequence by detec-ting the poin-t of inflexion rather than an absolu-te value of the measured voltage change.
Fig. 1l~ shows four curves 117, 118, 119 and 120 each representing variations in the measured voltage change with time for the same battery. Curves 117 and 118 represen-t the case of the battery being warm (~21c), curves 119 and 120 the case of the battery being cold (-25c). The charge rate is 10 seconds at 2A, 2 seconds at -2A for curves 117 and 120, 3 seconds at 8A, 7 seconds at 4A, 2 seconds at -0.7A
for curves 118 and 119. It can be seen that the charge rate radically alters the variation o~
voltage change versus time, a high charge rate being suitable for warm batteries, a low charge rate being suitable for cold batteries. A low charge ra-te must be used for cold batteries as at a high charge ra-te the voltage change actually decreases with time. It is believed that this decrease is due to the charging current warming up the battery.
With a cold battery and high charge rate, the voltage change is high regardless of whether or not the battery is charged. With a warm battery the voltage change is high only if the b~ttery is charged.
Accordingly if a battery is discharged and then the 27.
voltage change is measured9 only a cold battery will exhibit a high voltage change. This makes it possible to identify a cold battery as such even when no direct temperature sensing circuit is incorporated in the battery.
Figs. 15 to 18 illustrate details of a system which detects points of inflexion in the variation of voltage change versus time and then terminates pulse charging~ and which detects cold batteries by performing an initial test charge/discharge sequence on all batteries and selects lower charging rates when a cold battery is detected.
Fig. 15 is a block diagram of the complete system. A battery to be charged is connected between terminals 121, 122 which are themselves connected to an analogue-to-digital converter 123, a programmable current source 124 and a programmable current load 125. The converter, source and load are controlled by a microprocessor 126 which is turned on by a switch 127.
As shown in Fig. 16, the converter 123 may be a 12 bit binary A/D converter, for example an Intersil ICL 7109. The source 124 may be for example a power supply model SP 684 as available from Power Electronics (London) Limited. The microprocessor may be type MPE-2 as available from Cyfas Limited. The particular components selected are however merely a matter of design choice.
Fig. 17 shows the current load circuit 125 in some detail. The ENABLE input 139 can be derived from a monostable (not shown~ which is continually being 3 0 ~
28.
re-triggered by an AWAKE pulse from the microprocessor.
If the microprocessor or its programme should fail, the AWAKE pulses would cease, the monostable would not be re-triggered, and the charge and discharge circuits 5 would be inhibited to prevent possible damage to the battery.
The ENABLE input 139 controls gate 129 which ENABLES the programmable power source (124 in Fig. 15) when required by the microprocessor. It also con-trols 10 gates 130 which admit the discharge current code inputs from the microprocessor to a digital -to analogue converter 131 (type ZN 425E). The discharge current code provides for 0.5A steps in the discharge current.
The analogue ou-tpu-t of -the converter controls a pair of transistors 132 (-type IRF 130) which are connected in series with resistors 133, 134. Lines 135, 136 are connected to the terminals of the battery being charged.
A link 137 is incorporated in the battery terminal plugs when it is desired to charge a twelve volt 20 battery, the link shorting out resistor 134, The link is not provided in the terminal plugs used for connect-ing to 24 volt batteries. This means that 24 volt and 12 volt batteries can be handled by the same unit.
A potentiometer 138 can be used for fine adjustment of 25 the discharge current as desired.
Referring now to Fig. 18, the operating cycle of the embodiments of Figs. 15 to 17 will be described.
Six separate battery connector stations are provided, and the first step is to connect batteries to one or 30 more of -these stations. Connectors are used which ,~
29 ~ 4 identify the type of battery and the unit initially checks that the batteries connected to it are of recognisable type. If so, a charge programme appropriate to that type is selected from the micro-processor memory. If not, a lead fault is indicatedand the sequence is termina-ted. Assuming that the battery type is recognised, the unit looks at the first battery station. If there is no battery there, it then looks at the other stations until one is found.
Once a battery is found, the unit conducts a test charge/discharge cycle. If the measured change in the terminal voltage of the battery is initially higher than a predetermined threshold and remains so during the test, this is assumed to indicate a cold battery and a low charge rate is selected from the microprocessor memory.
The appropriate charge rate having been selected, the charge sequence is run. The input to the power supply unit is monitored and the sequence is termin-ated if this input is less than a preset limit. Thisprevents batteries being over charged because the interrogating current pulses are not high enough to enable the charge cond~ion of the battery to be correctly assessed in the event of a power supply fault The voltage change is monitored during each charge/discharge cycle and averaged over five cycles.
the slope of the averaged voltage change with respect to time and the rate of change o~ the slope are monitored and charging is continued until the slope ~ 3 0 ~
is positive and its rate of change is negati~e, i.e.
a point of inflexion near to the fully charged condi-tion of the battery is detected, Charging of the battery is then terminated and the unit looks for another battery at the battery stations. The unit then repeats the above cycle on any located battery or termina-tes its operation if no other batteries are located.
Averaging o~ a number of voltage change samples effec-tively delays the detection of the point of inflexion as well as providing a more reliable smoothed measurement, The delay means that charging is termin-ated shortly after the point of inflexion is reached so that the charge acquisition is increased and is in 15. fact very near to the maximum possible without risk of damage to the battery.
The unit operates two further routine checks to prevent overcharging. Firstly the duration of each charge sequence is measured and the sequence is automatically terminated if it exceeds a threshold related to the type of battery and the selected rate of charge. Secondly if a maximum occurs in the measured voltage change (i~e. slope zero but going negative) and no point of inflexion has been detected, the sequence is automatically terminated. The maximum indicates that a faulty battery is warming up.
The measured duration of each charge sequence is, assuming the sequence is not terminated due to an excessive duration, a measure of the charge acquired by the battery as a result of the charge sequence.
Accordingly if the battery is fully discharged prior to charging, and gi~en that charging is terminated accurately when the battery is fully charged, the duration of each charge sequence when considered in combination with the charge rate is a measure of the ability of the battery to accept charge. This is a very important parameter as it enables failing batteries to be discarded when for example charge acquisition drops below 70% of a nominal value for the battery in question The operating sequences illustrated in Figs.
11 and 18 are implemented using conventional micro-processor equipment and programming techniques within the normal competence of persons experienced in -the application of microprocessors.
The embodiments of the battery charger of the invention described above are concerned with charging NiCd batteries. Similar techniques are capable however of satisfactorily charging batteries other ~o than NiCd batteries, in particular lead/acid batteries.
In addition, silver/zinc, nickel/zinc and nickel/iron batteries for example exhibit characteristics such that they too can be satisfactorily charged.
It will be appreciated that as the charger described can handle bat-teries at a wide range of internal temperatures, in some circumstances it will be possible to dispense with the thermal insulation normally incorporated in NiCd batteries of the sort used in mobile radio equipment for example.
In the case of the embodiment of Figs. 15 to 18, 32, ~ o ~
no special tempera-ture sensing arrangements are required in the ~attery since only the normal two battery connections provide all the information required.
Claims (17)
1. A battery charger comprising means for supplying a cyclic sequence of charging pulses of charging current of a predetermined charging pulse period of the order of several seconds to a battery interspersed with discharge pulses of a predetermined discharge pulse period of the order of greater than a second for discharging the battery during the charging pulse interpulse period, means for measuring the change in the battery terminal voltage over a further predeter-minded period during one of said pulse periods, and means for processing said change of battery voltage to derive a condition for terminating the pulse charging of the battery in response to the measured voltage change.
2. A battery charger according to claim 1, wherein the measuring means is adapted to measure the change in the battery terminal voltage during a period starting at the beginning of a charging pulse.
3. A battery charger according to claim 1 wherein the terminating means is adapted to terminate pulse charging when the measured voltage change exceeds a predetermined threshold.
4. A battery charger according to claim 3, characterised by a first current control device connected in series with the battery across a charging current supply, a second current control device connected across the terminals of the battery, a timing circuit for cyclically turning the second current control device on and off, and means for turning the first current control device on when the second current control device is off.
5. A battery charger according to claim 4, characterised by a monostable circuit triggered by the timing circuit for sampling the battery terminal voltage when the first current control device is turned on and at a predetermined interval thereafter, a comparator for comparing the two sampled voltages, and a bistable circuit for holding both current control devices off when the comparator detects a predetermined difference between the two sampled voltages.
6. A battery charger according to claim 4 or 5, wherein the timing circuit is effective to supply charging current pulses of 10 second duration and to discharge the battery for periods of 2 seconds duration.
7. A battery charger according to claim 4, in which the current control devices are transistors.
8. A battery charger according to claim 1 or 2, wherein means are provided for monitoring the internal temperature of the battery by means of discharging the battery with a predetermined charge and testing the voltage change magnitudes at two different times to determine temperature, and means are provided for modifying the current through the battery to compensate for the monitored internal battery temperature such that variations in the measured voltage change with internal battery temperature are minimised.
9. A battery charger according to claim 1 or 2, wherein the terminating means is operative to terminate pulse charging a predetermined period after the measured voltage change exceeds a predetermined threshold.
10. A battery charger according to claim 1, comprising means for modifying the current through the battery in response to changes in battery temperature, wherein the current modifying means comprise means for modifying the amplitude of the current in the first portion of each charging pulse such that the amplitude increases with increasing internal battery temperature, and means for modifying the amplitude of the current in a second portion of each charging pulse to maintain the mean charging current substantially constant.
11. A battery charger according to claim 10, characterised in that the charging current ampli-tude modifying means comprises a first solid state switch connected in series with the battery, a second solid state switch connected between two resistors forming a potential divider and controlled by a signal representative of the internal battery temperature, and switches connecting a control terminal of the first solid state switch to respective sides of the second solid state switch.
12. A battery charger according to claim 11, wherein the means for monitoring the internal temperature of a battery to be charged comprise means for conducting a test charge/discharge sequence before the main charging sequence, means for determining the measured voltage change at the beginning of the test sequence, means for determining the measured voltage change after test discharge, and means for reducing the charge rate of the main charging sequence if the voltage change determined after the test discharge is not substantially less than the voltage change at the beginning of the test sequence.
13. A battery charger according to claim 1, wherein the processing means is adapted to detect a point of inflexion in the variation of the measured voltage with time, and to terminate pulse charging when a point of inflexion is detected, and wherein a series of consecutively sampled measurements of the voltage change are averaged to smooth the measurement of the voltage change and to delay termination of pulse charging.
14. A battery charger according to claim 13, wherein means are provided to detect when the slope of the variation of the measured voltage change is posltive, the processing means being adapted to terminate pulse charging only when the slope is positive at the time that a point of inflexion is detected.
15. A battery charger according to claim 1, comprising means for measuring the duration of a pulse charging sequence, and means for terminating pulse charging if the measured duration exceeds a predetermined limit.
16. A battery charger according to claim 15, comprising means for fully discharging a battery prior to a pulse charging sequence, axed means for recording the duration of the pulse charging sequence to provide a measure of the total charge acquired by the battery in the subsequent pulse charging sequence.
17. A battery charger according to claim 1, comprising means for changing the rate of charge from a rapid charge rate to a normal charge rate of approximately 0.1 x C after temination of a first charge pulse sequence.
Applications Claiming Priority (6)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB8000959A GB2044333B (en) | 1979-01-12 | 1980-01-11 | Temporary tunnel support for holing operations in tunnels of underground mines |
GB80-959 | 1980-01-11 | ||
GB80-10580 | 1980-03-28 | ||
GB80-10578 | 1980-03-28 | ||
GB8010578 | 1980-03-28 | ||
GB8010580 | 1980-03-28 |
Publications (1)
Publication Number | Publication Date |
---|---|
CA1168304A true CA1168304A (en) | 1984-05-29 |
Family
ID=27260842
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
CA000368280A Expired CA1168304A (en) | 1980-01-11 | 1981-01-12 | High rate battery charger |
Country Status (2)
Country | Link |
---|---|
AU (1) | AU549684B2 (en) |
CA (1) | CA1168304A (en) |
Families Citing this family (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
AU622300B2 (en) * | 1989-03-31 | 1992-04-02 | Commonwealth Of Australia, The | Apparatus for charging storage cells |
AU636006B2 (en) * | 1990-04-30 | 1993-04-08 | George Langford | Battery management systems |
-
1981
- 1981-01-09 AU AU66101/81A patent/AU549684B2/en not_active Ceased
- 1981-01-12 CA CA000368280A patent/CA1168304A/en not_active Expired
Also Published As
Publication number | Publication date |
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AU549684B2 (en) | 1986-02-06 |
AU6610181A (en) | 1981-07-16 |
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