GB2337336A

GB2337336A - Battery condition monitoring

Info

Publication number: GB2337336A
Application number: GB9810000A
Authority: GB
Inventors: John Stuart Lewanewsky
Original assignee: THIRD GALAXY ELECTRONICS Ltd
Current assignee: THIRD GALAXY ELECTRONICS Ltd
Priority date: 1998-05-11
Filing date: 1998-05-11
Publication date: 1999-11-17
Also published as: GB9810000D0

Abstract

An apparatus for periodically testing the condition of a rechargeable backup battery 16 for an alarm system comprises a first switch 17 for disconnecting the battery charging current, a monitoring circuit 19 for measuring the battery voltage, a load 20 and second switch 21 for connecting the load 20 across the battery 16. In use, the battery voltage monitoring circuit 19 takes a first voltage measurement when the battery 16 is on charge, whereupon the first switch 17 is actuated to disconnect the battery charging current before the second switch 21 connects the load 20 across the battery 16. After a predetermined time period, the battery voltage monitoring circuit 19 takes a second voltage measurement before the load 20 is disconnected. The condition of the battery 16 is then determined by examining the difference between the first and second voltage measurements. If a cell of the battery 16 is open circuit, the battery voltage will fall by a corresponding amount when the battery charging current is disconnected and this fall in voltage will be detected by the apparatus, in addition to faults which cause the battery 16 to discharge quickly when the load 20 is applied.

Description

1 Battery Condition monitoring 2337336 This invention relates to a method

of monitoring the condition of a rechargeable battery, in order to determine whether it is faulty and to an apparatus for carrying out the method.

Rechargeable batteries are used in fire alarm systems to provide a source of backup power, in the event of a power failure. Typically, there is a requirement that the backup battery must be able to provide sufficient power to enable the alarm to be activated, even after it has been operating on its backup battery for 72 hours.

Alarm systems consume a considerable amount of power, when in their standby mode and thus it will be appreciated that the rechargeable battery needs to be tested regularly, in order to ensure that it can hold a sufficient amount of power.

Hitherto, it has been proposed to monitor the condition of rechargeable batteries in fire alarm systems, by removing the charging current, measuring the battery voltage, applying a load and measuring the battery voltage again, after a predetermined time period has elapsed. The condition of the battery can then be determined by the amount that the battery voltage has fallen by between the two readings.

Batteries of the lead acid type generally comprise a plurality of low voltage cells connected in series. A common fault with this type of battery is that one of the cells becomes short circuit. However, a disadvantage of the abovementioned method of battery monitoring is that batteries having a short circuit cell are generally able to drive a load for a considerable period, without the battery voltage falling substantially from its initial value. Thus, many faulty batteries have hitherto gone undetected, since the slope of their discharge curve is the same as a good battery.

We have now devised a method of monitoring the condition of a rechargeable battery, which alleviates the above-mentioned problems.

In accordance with this invention there is provided a method of monitoring the condition of a rechargeable battery, 2 the method comprising applying a charging current to the battery, measuring the battery voltage, removing the charging current, applying a load, measuring the battery voltage a predetermined time period after the load has been applied and determining the difference between the first and second battery voltage measurements.

In use, if the battery has lost its ability to hold charge, it will rapidly discharge, once the load is applied and as such a large difference will be found between the first and second voltage measurements.

Alternatively, if the battery has a short circuit cell, the battery voltage will fall by a voltage corresponding to the missing cell voltage, when the charge is removed. Thus, a large difference will again be found between the first and second voltage measurements, even if the slope of the voltage discharge curve is not substantially different from that of a good battery.

Finally, if the first voltage measurement is below a predetermined limit, it will be apparent that the battery is faulty.

Preferably the load is disconnected after the second battery voltage measurement has been taken.

Preferably the charging current is re-applied to the battery after the load has been disconnected.

Preferably the above-mentioned steps are repeated periodically.

Preferably the period in which the charging current is disconnected is short compared with the period in which the charging current is connected, so that the battery remains fully charged.

Preferably the period in which the load is connected is short compared with the period in which it is disconnected, so that the battery does not become exhausted.

Also in accordance with this invention, there is provided an apparatus for testing a rechargeable battery, the apparatus comprising first switch means for connecting and disconnecting a charging current for the battery, voltage measurement means, a load and second switch means for connecting and disconnecting the load across the battery, 3 wherein the voltage measurement means is arranged to take a first voltage measurement when the battery is on charge, the first switch means then being arranged to disconnect the charging current from the battery, whereupon the second switch means is arranged to connect the load, the voltage measurement means being arranged to take a second voltage measurement a predetermined time period after the load has been applied, means being provided for determining the difference between said first and second voltage measurements and for providing a fault signal in the event that said difference is above predetermined level.

is Preferably the second switch means is arranged to disconnect the load, once the second voltage measurement has been taken.

Preferably the first switch means is arranged to reconnect the charging current once said load has been disconnected.

Preferably a delay is provided between disconnecting the charging current and connecting the load, so as to ensure that the load is not applied when the charging current is connected to the battery.

Preferably, the apparatus is arranged to produce an fault signal if said first voltage measurement is below a predetermined level, thereby indicating that a fault on the battery is pulling the charging voltage down.

Preferably the apparatus is arranged to periodically test the battery.

An embodiment of this invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a block diagram of a f ire alarm system, incorporating a battery condition monitor in accordance with this invention; Figure 2 is a graph of battery voltage against time for a good and a faulty battery, when being monitored by the battery condition monitor of Figure 1.

Referring to Figure 1 of the drawings, there is shown an alarm system comprising a control panel 10 connected to an alarm circuit 11 and a detector circuit 12.

4 In use, the control panel 10 is arranged to energise sounder devices 13 that are connected across the alarm circuit 11, when smoke or heat is detected by one of the sensors 14 connected across the detector circuit 12.

The control panel 10 is normally powered from the mains supply. However, a rechargeable standby battery 15 is provided for maintaining the operation of the alarm system, in the event of a mains power failure. The rechargeable battery 15 in fact comprises two 12 volt lead acid batteries connected in series, thereby providing an output of 24 volts.

The battery 16 is connected via a switching circuit 17 to a charging circuit 16, which is arranged to charge the battery 15 from the mains supply. A voltage monitoring circuit 19 is connected across the battery 15, in order to measure the battery voltage V,,. A load 20 having a resistance of 50n or less is also selectively connectable across the battery 15 by means of a second switching circuit 21.

The control panel 10 also comprises a microprocessor 18. which controls the first and second switching circuits 17, 21. together with the voltage monitoring circuit 19, in such a manner that a test routine is regularly performed on the battery 15, in order to test its condition.

Referrina to Ficrure 2 of the drawinas the 01 microprocessor 18 is normally configured to enable the switching circuit 17, so that the battery 15 is recharged by the charging circuit 16. Af ter the battery 15 has been on charge for at least 30 seconds, the microprocessor 18 conf igures the voltage monitoring circuit 19 to read the "on charge" voltage V,,, across the battery 15 at time T1, whereupon the battery 15 is disconnected from the charging circuit 16 by disabling the switching circuit 17.

In the good battery the "off charge" voltage remains at substantially the same as the "on charge" voltage VB11 whereas in the faulty battery the voltage falls by several volts to an "off charge" voltage V,,4.

After 100ms, the microprocessor 18 applies the load 20 across the battery 16 at time T2, by enabling the switching circuit 19: this immediately causes the battery voltage VB to fall to a value VW in both the good and faulty batteries.

The load 20 is applied for 500 ms, during which time the battery voltage V,, falls, as the battery discharges through the load 20. After 500ms, the microprocessor 18 configures the voltage monitoring circuit 19 to read the battery discharge 5 voltage V,,.,, following discharge at time T3.

The load 20 is disconnected from the battery 15, once the voltage V.3 has been read at time T3, whereupon the battery 15 is put back on charge at time T4, after a suitable time delay has occurred.

The microcomputer 18 is arranged to determine the difference between V,,, and V,,., by subtracting the second voltage reading from the first. The microprocessor 18 is arranged to trigger a warning device 22 on the control panel 10, if the difference between the two voltages is above a predetermined level.

In Figure 2 the faulty battery has one cell which is short circuit. However, it will be seen from Figure 2, that the slope of the voltage discharge curve, when the load is applied is substantially the same for both the good and the faulty battery. Accordingly, a conventional battery monitor, which monitors the difference between the voltages at the start and end of the discharge curve, will not detect the faulty battery.

We have f ound that the on charge 1 voltage V,, is substantially the sane for good batteries and for faulty batteries. However, in faulty batteries we have observed that the battery voltage V,, falls immediately when the charging current is disconnected, by a value which is substantially equal to the voltage of one battery cell. Thus, a battery having a faulty cell is detected by the present invention, since the battery condition monitor determines the difference between voltage readings obtained before and after the charging current is disconnected.

Most other types of faults on batteries cause the battery to discharge rapidly when the load is applied and thus such faulty batteries are also detected, because the battery condition monitor determines the difference between voltage readings obtained before and after the load is applied.

The control panel may also comprise means for measuring 6 the charging current and/or voltage and for triggering the warning device 22, in the event that the measurement is outside predetermined limits.

The microprocessor 18 is preferably arranged to monitor the battery condition regularly. However, it is preferred that the battery is charged for at least 30 seconds between each test, so that the battery does not become discharged.

The battery is preferably only taken off charge for a fraction of a second on each test and thus the condition of the 10 battery will not be substantially affected by the test.

7

Claims

1) A method of monitoring the condition of a rechargeable battery, the method comprising applying a charging current to the battery, measuring the battery voltage, removing the charging current, applying a load, measuring the battery voltage a predetermined time period after the load has been applied and determining the difference between the first and second battery voltage measurements.

2) A method as claimed in claim 1, in which the load is disconnected after the second battery voltage measurement has been taken.

3) A method as claimed in claim 2, in which the charging current is re-applied to the battery after the load has been disconnected.

4) A method as claimed in claim 3, in which the period in which the charging current is disconnected is short compared with the period in which the charging current is connected.

5) A method as claimed in claim 2, in which the period in which the load is connected is short compared with the period in which it is disconnected.

6) A method as claimed in any preceding claim, in which the method is repeated periodically.

7) A method substantially as herein described with reference to the accompanying drawings.

8) An apparatus for testing a rechargeable battery, the apparatus comprising first switch means for connecting and disconnecting a charging current for the battery, voltage measurement means, a load and second switch means for connecting and disconnecting the load across the battery, wherein the voltage measurement means is arranged to take a first voltage measurement when the battery is on charge, the 8 first switch means then being arranged to disconnect the charging current f rom the battery, whereupon the second switch means is arranged to connect the load, the voltage measurement means being arranged to take a second voltage measurement a predetermined time period after the load has been applied, means being provided for determining the difference between said first and second voltage measurements and for providing a fault signal in the event that said difference is above predetermined level.

9) An apparatus as claimed in claim 8, in which the second switch means is arranged to disconnect the load, once the second voltage measurement has been taken.

10) An apparatus as claimed in claim 9, in which the first switch means is arranged to reconnect the charging current once said load has been disconnected.

11) An apparatus as claimed in any of claims 8 to 10, in which a delay is provided between disconnecting the charging current and connecting the load.

12) An apparatus as claimed in any of claims 8 to 11, which 20 is arranged to produce an fault signal if said first voltage measurement is below a predetermined level.

13) An apparatus as claimed in any of claims 8 to 12, which is arranged to periodically test the battery.

14) An apparatus substantially as herein described with reference to the accompanying drawings.