AU622300B2 - Apparatus for charging storage cells - Google Patents

Description

iii

AUSTRALIA

Patents Act 2 2 0 COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number: Lodged: Complete Specification Lodged: Accepted: Published: *1i *r Priority Related Art: APPLICANT'S REF.: PAL 8821 Name(s) of Applicant(s): COMMONWEALTH OF AUSTRALIA CAP OF PJ 3475 i i i i i 1i g "i a~i xm~cr i Address(es) of Applicant(s): Department of Defence, Anzac Park West, Canberra, Australian Capital Territory 2600 Actual Inventor(s): Address for Serviceis: PHILLIPS, ORMONDE AND FITZPATRICK Patent and Trade Mark Attorneys 367 Collins Street Melbourne, Australia, 3000 Complete Specification for the invention entitled: APPARATUS FOR CHARGING STORAGE CELLS The following statement is a full description of this invention, including the best method of performing it known to applicant(s): P19/3/84 r: APPARATUS FOR CHARGING STORAGE CELLS The present invention relates to apparatus for charging electric storage cells or batteries. In particular the present invention relates to apparatus i suitable for fast charging of Nickel Cadmium Batteries.

i A need currently exists for a relatively low-cost V general purpose fast charger for the many and varied V Nickel Cadmium batteries currently in service.

i i Generally, fast charging schemes require a higher V 10 charging current than usual, together with a suitable j method for detecting state of charge of the battery.

State of charge detection is a critical task as I under-ci. rging can limit operation life of battery powered i equipment in the field, whilst over-charging can damage S"the batteries or at best shorten their life.

tit: One widely used prior art scheme for detecting end-of-charge (COC) has been temperature sensing. This scheme uses temperature probes on each cell or battery to sense temperature rise which occurs when charge is no S 20 longer being accepted efficiently and/or is being converted to waste heat. This method has disadvantages in that it results in unnecessary heating of the cell or Sbattery. In addition, it is difficult, in practice, to achieve mechanical placement of temperature probes so as to provide reliable and repeatable sensing.

Other prior art schemes perform testing of batteries for end-of-charge whilst the batteries are being charged.

Applicants investigations have shown that these schemes do not offer reliable indication of true battery capacity.

30 Specifically the investigations have shown that iirrespective of the state of charge the terminal voltage of a battery under charge may vary considerably with age, condition and construction of the battery.

Another prior art scheme utilizes a circuit which interrupts the charging cycle with short discharge periods and compares battery voltage under charge to that under discharge. The difference between these two voltages is compared to a hard-wired reference voltage. When the 39 difference exceeds the reference voltage, the battery is 4520L -2-

A

1 Li ii deemed to be no longer accepting charge, and charging is terminated. The latter scheme has been found to perform unreliably in practice. One problem is that the scheme relies heavily on the shape of the charge and discharge curves. In practice, these vary considerably with cell construction. For instance, the scheme performs satisfactorily when standard size cell packs are used, but fails when button cell packs are used. A comparison of charge and discharge curves reveals that button cell packs cannot be charged reliably using this technique.

An object of the present invention is to alleviate disadvantages of prior art charging schemes.

The present applicant has conducted extensive evalu.tions of charge-discharge curves of various cells in 1 an effort to arrive at a more universal, more reliable and, ultimately, simpler EOC detection technique. The present invention is based in part on a recognition that the gradient of the discharge curve, which may be derived by periodic discharge cycles during charging, may be 20 utilized to reveal a more reliable EOC estimate.

The charging apparatus of the present invention includes control means adapted to control charging. The control means may comprise a microcontroller or microprocessor such as a Motorola device type 68HC811A2.

The control means may be adapted to control the charging process such that it interrupts charging of cells with relatively short discharging periods. In one form the charg ng process may comprise a charging cycle for, say, seconds at 1 Amp. This may be followed by a discharging cycle for, say, 10 seconds at The control means may read cell voltage during each discharge cycle allowing a past history of the state of charge to be built up. Figure 10 of the enclosed drawings shows a graphical representation of the history of the state of charge which may be built up in the memory of the control means. The premise on which End-of-Charge may be determined is that a cell's terminal voltage under discharge will continue to rise only as long as charge 39 continues to be accepted during a charging cycle. The 4520L -3control means may include an end-of-charge detection algorithm. The algorithm may be adapted to terminate charging when the point A on the general discharge voltage curve shown in Figure 10 is detected, indicating that charge is no longer being accepted.

Whilst the above premise is true in general, heating effects may cause a decrease in charge acceptance capability. This may occur because when the state of charge reaches approximately 75%, oxygen begins to be generated at the positive electrode. As this oxygen accumulates, the pressure gradually rises, causing the cell to go into an 'overcharge' condition, in which most subsequent charge current goes into producing more oxygen, which in turn produces heat when it reacts with the cadmium of the negative electrode.

As a result, point A on the general discharge curve shown in Figure 10 may not necessarily indicate maximum capacity of the cell although it is the maximum capacity for the applied charge rate (and subsequent heating). It is estimated that for a charge current of 1 A on a 600 mAh cell, a capacity of at least 90% of nominal maximum capacity may be achieved using this method. In cells of greater capacity, the degree of overcharge may be less for the same current, resulting in less generation of heat, and consequently a greater usable capacity can be achieved.

The algorithm, then, may rely on detecting point A on the genieral discharge curve. This may be done by analyzing the gradient of the discharge curve at each discharge interval. A 'history' of, say, 6 successive points may be analyzed. Charging may be terminated if the analysis reveals a substantially negative gradient. This would indicate a point just over the 'hump'. Parameters such as: a. the number of points analyzed in the 'history' of the state of charge, and b. the degree of negativity of the gradient required to cease charging, may be varied to maximize the charge accepted, while 39 minimizing heating effects. In addition, it may be 4520L possible to maximize charge accepted using two other strategies: a. as the 'hump' approaches, charge current may be decreased so as to minimize heating effects and thus increase charge acceptance, and/or b. as the 'hump' approaches, the system may be rested, allowing the cell to cool. Charging may then be restarted.

The apparatus of the present invention includes power supply means. The power supply means may include a main power supply and an auxiliary power supply. The main power supply is adapted to supply charging power to a cell being charged. The main power supply may be arranged to provide, say 200W charging power in a range of output off# voltages to suit various battery packs, e.g. DC voltages *between 20 and 50 volts. The main power supply may be controlled by the control means allowing remote selection of output voltage. The purpose of being able to adjust the voltage of the power supply is to keep dissipation in charging circuits to a minimum by keeping the charge voltage close to the minimum required, while extending the upper voltage limit of cells or batteries capable of being charged. The main power supply may be of a switch-mode type and in one form may accept 240V AC 50Hz mains power.

The auxiliary power supply may be adapted to supply power Ato the controller/processing means and other low power devices in the apparatus.

The power supply means may alternatively comprise a DC/DC converter. The converter may be adapted for operation from a portable D.C. power supply such as a vehicle in the field. In the latter case the power supply means may operate with an input of say 10-30VDC. The output voltage may be variable as above although the power output of the power supply means may be limited to, say 100W. Thus the range of cells or batteries which may be charged in the field may be more limited.

The apparatus of the present invention may include one or more charging channels. This may enable a 39 plurality of cells or batteries to be charged 4520L simultaneously. Each charging channel may be controlled by the control means and may be connected thereto via suitable interface means. Each charging channel may be connected to a battery via a suitable interface such as a charge/discharge circuit. The or each charge/discharge circuit may be adapted for performing charge, discharge and/or disconnect functions. Each charging channel may optionally include a switch selectable deep discharge function. Deep discharging is occasionally desirable as it allows batteries to be periodically refreshed, correcting such reversible failures as: a. memory effect, i.e. loss of capacity due to repeated shallowness of discharge; b. loss of capacity due to prolonged operation at high temperatures; and c. loss of capacity and cell voltage due to long term overcharging at elevated temperatures.

The control means may control selection of charge, discharge, disconnect and deep discharge functions. The control means may also select magnitude of charge current. Where a plurality of charging channels is employed the control means may be adapted to switch charging current to each charge/discharge circuit in a staggered fashion such that at least one channel does not IL draw charging current at any given time. This is referred to herein as "charge current interleaving" and reduces current demand on the power supply. The channel not being charged may be placed in discharge mode. Cells or batteries to be charged may be connected to terminals of respective charge/discharge circuits.

The interface means connecting the or each t 41 charge/discharge circuit to the control means may include a reference generator, an analogue to digital (A to D) converter and a conditioning circuit. The A to D converter may be incorporated with the control means. The reference generator may be adapted to generate a reference or base voltage for the A to D converter. The reference generator may include a digital to analogue (DAC) 39 converter. The reference or base voltage may be 4520L i,;;i 1 subtracted from the battery voltage by the conditioning circuit such that the voltage to be read or presented to the A to D converter is within a preset window. This may allow the A to D converter to maintain an acceptable level of resolution notwithstanding that cell or battery voltage may fluctuate significantly. The reference or base voltage generated by the reference generator may be adjusted on a continuous basis by the control means to ensure valid A to D conversions for any cell or battery voltage.

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1 a i q I ;i jl i i: j- i, i" i i When presence of a cell or battery is detected by the control means (by a non-zero voltage reading), the reference voltage may be immediately adjusted, such that the cell or battery voltage read by the control means 15 falls within the pre-set window. For example, the window may be approximately 1.25 volts and if the battery voltage is 6 volts, the reference voltage may be adjusted to 5.4 volts, so that the voltage being read is 0.6 volts, being approximately mid-range of the A to D converter. The reference voltage may be adjusted by the control means on a continuous basis for each battery being reed.

The apparatus may include display means. The display means may be connected to the control means and adapted to display data relating to cells or batteries 25 under charge. In one form the display means may provide a voltage display of cells or batteries under charge. The display means may include a voltmeter display panel for each charging channel or it may include a single display panel. In the case of a single display panel a scroll switch may be connected to the control means to allow scrolling through the charging channels. The displayed value for each channel may be updated at the end of each discharge period for that channel.

According to one aspect of the present invention there is provided apparatus for charging an electric storage cell having a pair of terminals, said apparatus comprising: power supply means for supplying charging power to said terminals; 4c means for interrupting said supply of charging power 26L -7- s n at least periodically; means for connecting a discharging load to said terminals when said supply of charging power is interrupted; means for sampling voltage under load when said discharging load is connected to said terminals; and means for terminating said supply of charging power when the rate of increase of successively sampled voltages falls below a predetermined threshold.

Aorngtorthr ape of hte prst invention t ere is provided a method of charging an electric stor ge cell having at terminals of said cell a voltage-under-l ad which rises so long as said cell is accepting charge comprising the steps of: 15 supplying harging power to said terminals of said cell; sampling s id voltage-under-load at least :periodically during charging by interrupting said supply of charging power a d connecting a discharging load to 20 said terminals; and terminating said supply of charging power when the rate of increase of su cessively sampled voltages falls below a predetermined thr shold.

According to one em odiment the charging apparatus of the present invention ay incorporate four charging channels for charging up to four cells or batteries simultaneously. A four chann 1 battery charging apparatus may function as follows. Bat eries to be charged may be connected to the output termina s of each Charge/Discharge circuit. The battery voltages t be read are fed to A to A D conversion inputs of the \control means, after subtraction of a reference volta e by the conditioning circuit.

The entire function of the c arging apparatus is controlled by the control means which embodies all program memory, I/O lines, A/D lines and timi g functions. When presence of batteries is detected an a valid voltage reading is established the control mea s estimates the i. ,minimum voltage required from the Main \ower Supply to \ba ~J charge the batt-ry and sets it,. g n0 ,r 05Q- -8- According to a further aspect of the present invention there is provided a method of charging an electric storage cell having a pair of terminals, comprising the steps of: supplying charging power to said terminals; interrupting said charging power at least periodically; connecting a discharging load to said terminals when said charging power is interrupted; 0 sampling voltage under load when said discharging load is connected to said terminals; and terminating said charging power when the rate of increase of successively sampled voltage falls below a predetermined threshold.

According to one embodiment the charging apparatus of the present invention may incorporate four charging channels for charging u,3 to four cells or batteries simultaneously. A four channel battery charging apparatus may function as follows. Batteries to be charged may be connected to the output terminals of each Charge/Discharge circuit. The battery voltages to be read are fed to A to D conversion inputs of the control means, after subtraction of a reference voltage by the conditioning circuit.

The entire function of the charging apparatus is controlled by the control means which embodies all program memory, i/O lines, A/D lines and timing functions. When presence of batteries is detected and a valid voltage reading is established the control means estimates the minimum voltage required from the Main Power Supply to charge the battery and sets it, e.g. 20, 30, 40 or 39 MJP -8ai volts.

As the Main Power Supply is common to the four channels in this embodiment, only batteries of similar terminal voltage can be charged simultan-ously. However, in principle each channel could have an independent power supply allowing greater flexibility, although this may not be economically feasible in all circumstances.

Actual power required from the Main Power Supply may be minimized by interleaving the charging process between the four charging channels, as illustrated in Figure 9.

Charge and discharge periods may be cycled for 30 seconds and 10 seconds respectively. Discharge periods will occur in only one channel at any given time. Hence, at any given time, only three of the four channels are in charge mode, thereby requiring only three times maximum battery charge current from the power supply, not four.

As the terminal voltage on each battery increases during charging, reference voltage and power supply setting may be varied dynamically. When end-of-charge has been determined by the control means for any channel, that battery may be disconnected, while the remaining batteries i may continue to charge.

I The display means, e.g. a three digit segment i display, displays battery voltage to the user indicating i that the battery is fully functional.

A preferred embodiment of the present invention will now be described with reference to the accompanying drawings wherein: Figllre 1 shows a schematic diagram of a general j 30 purpose charging apparatus according to one embodiment of the present invention; Figure 2 shows a detailed diagram of the microcontroller section; Figure 3 shows a detailed diagram of the A to D conditioning circuit and reference generator; Figure 4 shows a detailed diagram of one of the charge discharge modules; Figure 5 shows a detailed diagram of the display 39 section; 4520L -9r Ii Figure 6 shows the main and auxiliary power supplies; Figure 7 shows a detailed diagram of the main power supply; Figure 8 shows a typical front panel for the apparatus; Figure 9 is a graphical representation of charge current interleaving; Figure 10 is a graphical representation of the history of state of charge of a battery undergoing charging; and Figures 11 to 31 show flow diagrams representing tsoftware used in the microcontroller section.

Referring to Figure 1, the charging apparatus includes microcontroller 10 connected via interface circuit 11 to respective charge/discharge circuits 12, 13, 14 and 15. The apparatus includes power supply 16 and display 17 connected respectively to microcontroller 10 as shown.

Figure 2 shows connection details to microcontroller 10. Microcontroller 10 comprises a Motorola device type 68HC811A2 and embodies all software required to control the apparatus. Flow diagrams representing software K programs which may be embodied in microcontroller 10 are illustrated in Figures 11-31 of the accompanying drawings. Microcontroller 10 is connected to various I parts of the apparatus via control lines as shown.

i The interface circuit 11 is shown in detail in 7Figure 3. The interface circuit 11 includes a ji conditioning circuit shown generally at 30 and a reference generator shown generally at 31. Reference generator 31 comprises digital-to-analogue (DAC) converter 32 and buffer 33 connected to the output of DAC 32 as shown.

Conditioning circuit 30 comprises four summing amplifiers 35-38, one per charge/discharge circuit. A reference voltage is available at the output 34 of reference generator 31. The reference voltage is available at the output terminal of buffer 33 and is connected to the inverting input terminals of respective summing amps 39 35-38. The non-inverting inputs of summing amps 35-38 are 4520L connected to the BATT+ charging terminals of respective charge/discharge circuits 12-15. The arrangement is such that each summing amp 35-38 subtracts the reference voltage generated at the output of reference generator 31 from the battery voltage inputted to the non-inverting terminals of respective summing amps 35-38. This voltage difference is adjusted by microcontroller 10 such that it falls within a preset window. Microcontroller 10 sets the window by dynamically varying the reference voltage generated by reference generator 31. The voltage difference is supplied to the A to D converter input of microcontroller 'i i "f A representative charge/discharge circuit is shown 4 4 S, in Figure 4. The charge/discharge circuit comprises a 3 to 8 line decoding circuit 40. Decoding circuit 40 may be a circuit device type 74HC259 and is responsible for decoding instructions from microcontroller 10, e.g.

charge, discharge, deep discharge, end-of-charge or 4 standby.

When the charge function is required decoder enables transistor TI. Tl switches transistor T2 which in lof turn connects supply of power +V from the power supply to I the positive terminal of the battery BATT+ under charge S...via voltage regulator VR1, which may be a circuit type LM317, and diode Dl. VR1 and T2 effectively function as a fixed current source for providing a fixed charging current from the power supply. The current of VR1 is set via resistor Rl.

The discharge function is performed by enabling transistor T3. T3 switches transistors T4 and T5 which in 1 turn connects the negative terminal of the power supply to SSi the positive terminal of the battery BATT+ via voltage regulator VR2 and diode D2. VR2 and T5 also function as a fixed current source for providing a fixed discharge current from the battery. The current of VR2 is set via resistor R2.

The deep discharge function is performed by enabling transistor T6 or T7. T7 is enabled when battery voltage 39 is above 6 volts, connecting the positive battery terminal 4520L -11- A iBATT+ to ground via voltage regulator VR3 and diode D2.

The discharge current is controlled via voltage regulator VR3 and associated resistor R3. If battery voltage is below 6 volts deep discharge is performed by enabling transistor T6. T6 connects the positive battery terminal BATT+ to ground via resistor RDD.

SThe state of the charge/discharge circuit is indicated via tri-colour LED 41. LED 41 is driven via the output lines of decoder 40. LED 41 illuminates red, green or orange indicating charge, discharge or deep discharge functions respectively. No illumination indicates end-of-charge or standby.

Figure 5 shows details of the display 17. Display 17 comprises counter/segment driver 50 which may be a circuit type 74C925. Counter/driver 50 is connected to 4 LED segment displays 51-54. Display 51 is used to display the number of the charging channel, i.e. 1 to 4 in the present example. Displays 52-54 are used to display the voltage of the battery associated with the charging channel identified by display 51. The voltage to be dispUaved is obtained by counting pulses sent by microprocessor 10 to the clock input of counter/driver A momentary switch 55 is provided for scrolling charging channels through display 51.

Figure 6 shows details of power supply 16. Power v supply 16 includes a main power supply 60 and an auxiliary power supply shown generally at 61. Main power supply provides at V+ charging power for charging the or each battery and is described in greater detail below w,7ith reference to figure 7.

The auxiliary power supply 61 provides a ±5 volt supply to microcontroller 10 and other powered devices in An the apparatus. Power supply 61 comprises step down Transformer 62 having separate windings connected to respective bridge rectifiers 63, 64. The DC outputs of rectifiers 63,64 are connected to respective smoothing capacitors Cl, C2 and hash filter capacitors C3, C4. The outputs of smoothing capacitors Cl, C2 are connected to 39 voltage regulators VR4, VR5 which may be circuit device 4520L -12- I ~ICIU~- I. 1_1_ CIUPI~II~III~-ill**Ii-lll f i-..i:lii.i( I:)i-i;jli.-iiil~i~ :Ill~L types LM317, LM337 respectively. The voltages at the outputs of VR4, VR5 are set via respective voltage dividers formed via resistor pairs R4/R5 and R6/R7.

Additional smoothing capacitors C5, C6 are connected to the outputs of VR4, VR5 as shown.

Main power supply 60 provides an output voltage V which is stepwise adjustable via microcontroller 10 to four voltage settings, namely 20, 30, 40 or 50 volts, to cater for a wide variety of batteries to be charged.

Power supply 60 is adapted to be switched by controller between charge/discharge cycles lasting 30 seconds and seconds respectively. Charging cycles for the four charging channels are staggered at 10 second intervals i "(refer Figure 9) promoting charge current interleaving and reducing peak current demand on the power supply. The 0 discharge cycle occurs in only one channel at any given S. time. Hence, at any given time, only three of the four channels are in a charging cycle, thus drawing only three times battery charge current from the power supply, not four.

15 Referring to figure 7, power supply 60 comprises a 240V AC supply connected via Electomagnetic interference filter 65 to bridge rectifier 66 which may be a device type BR1510. The DC output of bridge rectifier 66 is filtered to provide a high unregulated voltage at point DD. The output of filter 65 is also connected via a reactive voltage divider comprising resistors R8, R9 and capacitors C7, C8 to bridge rectifier 67, which may be a device type W04. The output of bridge rectifier 67 is filtered and regulated via linear regulator 68 which may be a circuit type LM 317T, to provide a stable 12 volt DC output for powering power-supply c -troller 69 which may be an industry standard 3524 RegL.lating Pulse Width Modulator configured to operate as a half-bridge Switch-Mode-oPower-Supply (SMPS) controller.

Alternating outputs El and E2 of controller 69 drive respective power MOS transistors T8, T9. Power transistor T8 is driven directly via transistor drivers T10, Til 39 whilst power transistor T9 is driven via level shifting 4520L -13transformer TR1.

Power MOS transistor T8, T9 are connected to drive the primary winding of transformer TR2 at a high frequency and with variable duty cycle causing a voltage to be induced at its secondary windings. The ind.,-d secondary voltage is rectified and filtered to provide a DC output voltage V A current transformer TR3 samples the output current I. Current induced in the secondary winding of transformer TR3 is rectified via diode bridge 70 which may be a device type W04, and filtered and provides current feedback to controller 69 at CC. To ensure a regulated trr output voltage a sample is taken of the output voltage 1W V. The voltage sample is modified by a voltage V selecting circuit shown generally at 71.

Circuit 71 modifies the voltage sample according to the state of programming switches SWI, SW2 before feeding this back to controller 69 at EE. When both switches SW1, SW2 are turned on, feedback loading applied to the voltage sample is at a minimum causing controller 69 to stabilize the output voltage V at a level of 20 Volts DC. When c, switch SWl is off and SWl is on, the feedback loading is slightly higher causing the output voltage V to be stabilized at a level of 30 Volts DC. When SWI is on and SW2 is off feedback loading is slightly higher again, causing the output voltage V+ to be stabilized at a level of 40 Volts DC. Finally, when both switches SWI, SW2 are Off, feedback loading is at a maximum causing the output voltage V+ to be stabilized at a level of 50 Volts DC.

The current and voltage samples are fed back to controller 69 via optical couplers 72, 73 respectively.

'Figure 8 shows one form of front panel layout for Sthe charging apparatus of the present invention. The panel incorporates features described herein.

The General Purpose Fast NiCd Charger described has the following features: a. Fast charging algorithm; b. Automatic battery recognition; 39 c. Four batteries of similar type may be charged 4520L -14simultaneously; d. Display of battery voltage; e. Selectable deep discharge option; f. Simplicity of operation (insert batteries and start); and g. Low cost.

It will be appreciated that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the present invention.

W ir 1 i t 41 i l j| 39 4520L

Claims (9)

1. Apparatus for charging an electric storage cell having a pair of terminals, said apparatus comprising: power supply' means for supplying charging power to said terminals; means for interrupting said supply of charging power at least periodically; means for connecting a discharging load to said terminals when said supply of charging power is interrupted; means for sampling voltage under load when said discharging load is connected to said terminals; and means for terminating said supply of charging power when the rate of increase of successively sampled voltages falls below a predetermined threshold.

2. Apparatus according to Claim 1 wherein said supply of charging power is interrupted and said discharging load is connected to said terminals for a period of 10 seconds each 40 seconds.

3. Apparatus according to any one of the preceding Sclaims wherein said sampling means and said terminating S: means comprise a microcontroller including a memory.

4. Apparatus according to Claim 3 wherein said memory is adapted to store a past history of said sampled voltages.

Apparatus according to any one of the preceding claims having four charging channels adapted to charge up to four cells or batteries simultaneously, and incorporating charge current interleaving whereby at any 1 30 one time at least one channel has its supply of charging power interrupted.

6. Apparatus according to any one of the preceding Claims wherein said threshold is indicative of a negative rate of increase of successively sampled voltages.

7. Apparatus according to any one of the preceding claims wherein said power supply means is adapted to provide a range of output voltages selectable to suit the cell or battery being charged. f..A me-thon of c-harcing an electric sto-j rage cG-ci ha'v:ing t terminals of said ccli a volt-ae under load which rics- -16-

8. A method of charging an electric storage cell having a pair of terminals, comprising the steps of: supplying charging power to said terminals; interrupting said charging power at least periodically; connecting a discharging load to said terminals when said charging power is interrupted; sampling voltage under load when said discharging load is connected to said terminals; and terminating said charging power when the rate of increase of successively sampled voltage falls below a predetermined threshold.

9. Apparatus for charging an electric storage cell substantially as herein described with reference to the accompanying drawings. A method of charging an electric storage cell substantially as herein described with reference to the accompanying drawings. i i .,20 fi 1 ii ,i tI. DATED: 22 January, 1992. PHILLIPS ORMONDE FITZPATRICK Attorneys for: COMMONWEALTH OF AUSTRALIA &e 39 3488u MJP -17-