GB2046923A - Automatic checking of batteries or cells - Google Patents

Abstract

The drawing illustrates one form of apparatus for continuous monitoring of the internal resistance of a battery or cell. Periodically, a capacitor 16, after charging to a higher voltage than the battery or cell 10, is discharged by thyristor 23 through the battery or cell 10 and a resistor 22. The magnitude of the peak discharge current is compared with a reference by comparator 33 and a display 37 is operated if this difference indicates that the internal resistance is too high. The reference signal from comparator 33 is derived from a sample and hold circuit 31 and is dependent on the difference between the voltage on the capacitor and the open circuit voltage of the battery or cell. <IMAGE>

Description

SPECIFICATION Improvements in or relating to the automatic checking of batteries or cells This invention relates to the automatic checking of batteries or cells.

Standby batteries are commonly used for a wide variety of purposes. It is desirable to check the condition of the battery at regular intervals.

A check of the mechanical and electrical condition of the cells is not only desirable to estimate the electrochemical deterioration but also to examine possible structural weaknesses in the cell, for example in the lead plates and connectors of a lead-acid accumulator type of cell. We have found that the measurement of internal resistance is one way of producing information in a useful form and it is an object of the present invention to provide a method and apparatus for periodically monitoring the internal resistance of a cell or battery.

According to one aspect of the present invention a method of testing a cell or battery comprises the steps of periodically charging a capacitor to a voltage higher than the open circuit voltage of the cell or battery and subsequently discharging the capacitor through a circuit comprising the cell or battery and a resistor and indicating if the peak current is less than a predetermined amount dependent on the magnitude of the difference between the voltage to which the capacitor is charged and the voltage of the cell or battery.

According to another aspect of the present invention, apparatus for testing a cell or battery comprises a capacitor, terminals for connecting the cell or battery in series with a resistor and a switch, means for charging the capacitor to a potential higher than the open circuit voltage of a cell or battery to be tested, time-controlled means for periodically operating said switch to discharge the capacitor through said resistor and cell or battery, and means for determining if the peak discharge current is below a predetermined magnitude dependent on the difference between the voltage to which the capacitor is charged and the voltage of the cell.

Conveniently the means for determining if the peak current is below a predetermined magnitude comprise a comparator comparing a reference voltage from a sample and hold circuit controlled by said timer and responsive to the difference between the voltage to which the capacitor is charged and the voltage of a battery or cell connected to said terminal with the voltage developed across said resistor or a part thereof.

Means, including a further sample and hold circuit may be provided for indicating the difference between the two voltages applied to the comparator.

A two-state indicator may be provided which is operated by said timer shortly before the time when the capacitor is discharged and is reset if the comparator shows that the peak current is of sufficient magnitude. This indicator conveniently is a light-emitting diode.

When the current pulse is discharged through the battery or cell and the resistor, the voltage developed across the resistor will depend on the ratio of the magnitude of this resistance to the sum of this resistance and the internal resistance of the cell. It will also depend on the difference between the voltage to which the capacitor is charged and the open circuit voltage to the battery or cell. By utilising this difference voltage in the comparator, the result of the comparison may be made independent of the voltage difference and hence the comparison indicates whether the internal resistance of the battery is acceptable or not.

By using a two-state indicator as described above which is switched on to time when the capacitor is discharged and then switched off if the result is acceptable, indication is given that the tester is operating to check the internal resistance and also that the internal resistance is satisfactory. If the internal resistance is not satisfactory, the indicator will remain operative and it may be arranged, if operative for more than a predetermined period, to initiate operation of an alarm.

Preferably, in the comparator, the voltage across said resistor or a voltage related thereto is compared with a reference voltage of smaller magnitude, derived from the difference between the voltage to which the capacitor is charged and the voltage of the battery or cell. If the battery or cell has high internal resistance, the peak current on discharge of the capacitor is reduced and the alarm is operated in the manner described above. It will be seen that the alarm will be operated if no current pulse is applied to the battery or cell.

Preferably a voltage limiter, e.g. a Zener diode, is provided for limiting the voltage applied to the comparator from said resistor.

Thus, if the voltage to the battery or cell falls, the increasing reference voltage, that is the difference voltage, will cause the alarm to operate.

The following is a description of one embodiment of the invention, reference being made to the accompanying drawing which is a schematic diagram illustrating apparatus for monitoring the condition of a battery.

Referring to the drawing, a battery to be tested is shown at 10 and is connected across terminals 11, 1 2 of the monitoring apparatus.

Purely by way of example, it is convenient to refer to specific voltages and, in this particular embodiment, the battery is a 1 10 volt battery.

The monitoring equipment is energised from an alternating current mains supply shown at 1 3. In this particular embodiment this is a 50 Hz supply and the supply frequency is utilised in a counter-timer 14 to provide required timing signais as will be explained later. This counter-timer comprises NAND gates and also monostables to produce short duration pulses.

The alternating supply feeds a rectifier unit 1 5 providing a d.c. output which is applied across a capacitor 1 6 to charge the capacitor, in this particular example, to 1 40 volts. The voltage across the capacitor is stabilised by Zener diodes 17, 1 8. A high impedance voltage divider 19, connected across the capacitor 16, is tapped at 20 to provide a voltage signal on a line 21 proportional to the voltage applied to the capacitor 16. The capacitor 16 is periodically discharged through the battery 10 and a series resistor 22, typically of 1 ohm resistance, the circuit being completed by means of a solid state switch illustrated as a thyristor 23 which is triggered periodically e.g. every 70 seconds, by a short duration pulse from the counter-timer 14 on a lead 24.

The capacitance of capacitor 1 6 is such that the current discharge timer is normally very short, typically 2m secs.

A high impedance potential divider 25 is connected across the battery terminals 11, 1 2 to provide at a tap 26 an output voltage representative of the voltage of the battery.

The battery may be open-circuit or it may be feeding a load or it may be on charge, e.g. on trickle charge. The aforesaid output voltage is thus representative of the actual battery voltage. A differential amplifier 30 provides an output dependent on the difference between the voltages on the taps 20 and 26. This output is sampled and held by a sample and hold circuit 31 which is operated by the timer 14 to hold the voltage difference over a period, referred to as the battery pulsing time, which extends for a duration of about 2.5 secs before the thyristor firing pulse to about 2.5 secs after that firing pulse.The sample voltage is held on a capacitor 32 and provides a reference voltage (ref) for a comparator 33 which compares this reference voltage with the voltage on a tap 34 of a high impedance potential divider 35 connected across the aforementioned resistor 22. The voltage (VR) at this tap 34 is a measure of the current passing through the resistor 22.

When the thyristor 23 is fired to discharge the capacitor 1 6 through the battery, the resulting peak current I is given by í = VcVB r+ R when Vc is the voltage on the capacitor VB is the open circuit voltage of the battery, r is the internal resistance of the bat tery, and R is the resistance of resistor 22.

Therefore r = V,-V, -R I But I is proportional to VR.

Hence the internal resistance is indicated by the ratio of V ~VB VR Vre, is scaled to be proportional to V,-V, and hence the output of the comparator 33 is a measure of the internal resistance and is independent of VCVB over the range of interest.

An adjustable potential divider 36 is provided for scaling the magnitude of the difference voltage, that is to say the reference voltage Vref for the comparator 33. This voltage is adjusted so that the reference voltage V,ef is less than the tapped voltage VR from potential divider 35 when the internal resistance of the battery is sufficiently low.

The timer 14 provides a switching pulse for switching on a light emitting diode 37 in an indicator unit, this pulse being applied by a logic unit 38 controlled from the timer unit, to start a short time, e.g. 2.5 seconds, before the firing of the thyristor. The output from the comparator 33 is applied to the logic unit 38 and, provided the battery resistance is sufficiently low, this output operates to switch off the light emitting diode. Thus if the check is satisfactory, the light is shown for 2.5 seconds, thereby indicating that a check has been made. If the internal resistance of the battery is too high however the output from the comparator 33 fails to switch off the light emitting diode 37 which thus remains illuminated.The state of the light emitting diode is checked a short time (e.g. 2.5 seconds) after it would normally have been switched off by the logic unit 38 and, if the light is still on, a cold-relay 39 is operated to energise an alarm 40.

It will be seen that, if the battery resistance increases, the light emitting diode 35 will fail to switch off and a fault condition is indicated.

The amount by which the Vref signal input to the comparator 33 is less than the input VR representative of the peak current determines how high the battery resistance will rise before the alarm is operated. This amount can be adjusted by the aforementioned potential divider 36 which is set to give an output Vret less than VR.

A Zener diode clamp 41 limits the increase in VR in the event of the battery voltage falling and hence the increase in Vref produced by the fall in the battery voltage causes an alarm condition to be indicated. For a 110 V bat tery, this Zener diode clamp might be set to give an alarm if the voltage falls below 110 V.

A switch 42 enables the magnitudes of the input resistances for the differential amplifier 30 to be increased and hence enables a fault condition to be simulated for test purposes. A sample and hold circuit 43 controlled by the timer 14, is provided to sample the peak voltage VR each time the capacitor 1 6 is discharged and to hold this voltage until the next discharge, the output being indicated on a meter display 44.

The logic unit 38 also monitors the regular operation of the light emitting diode 37 which is normally operated for about 2.5 seconds in each cycle. The operating pulse is sampled and held on a comparator which discharges very slowly such that, in the absence of the operating pulses, the voltage on this capacitor will gradually fall as operating, for example, after a delay of 10 minutes, the alarm 40.

The alarm, provided it is independently energised, will therefore operate if the power supply for the monitoring apparatus fails.

The battery is protected from a low resistance fault on the battery tester by an output fuse 46; fuse failure will cause an output alarm. Transient suppressor diodes may be provided across thyristor 23 to protect the battery tester output from high voltage transient interference, the diodes having a hold-off voltage large enough to prevent a spark if the battery is connected with capacitor 1 6 discharged.

With the above described apparatus, the light emitting diode is periodically energized to give a positive indication that the battery is being monitored. It will remain illuminated in the event of the internal resistance of the battery increasing to a predetermined adjustable level or if the battery voltage falls below a predetermined level or if no current pulse is applied to the battery. In each of these cases, the alarm is operated. The alarm is also operated in the event of any failure of the power supplies to the apparatus.

Claims (11)

1. A method of testing a cell or battery comprising the steps of periodically charging a capacitor to a voltage higher than the voltage of the cell or battery and subsequently discharging the capacitor through a circuit comprising the cell or battery and a resistor and indicating if the peak current is less than a predetermined amount dependent on the magnitude of the difference between the voltage to which the capacitor is charged and the voltage of the cell or battery.

2. A method as claimed in claim 1 wherein, for indicating if the peak current is less than a predetermined amount, the voltage across said resistor during discharge of the capacitor or a voltage proportional thereto is compared with the difference between voltage applied to said capacitor and the open circuit voltage of the battery or cell or a voltage proportional thereto.

3. Apparatus for testing a cell or battery comprising a capacitor, terminals for connecting the cell or battery in series with a resistor and a switch, means for charging the capacitor to a potential higher than the voltage of a cell or battery to be tested, time-controlled means for periodically operating said switch to discharge the capacitor through said resistor and cell or battery, and means for determining if the peak discharge current is below a predetermined magnitude dependent on the difference between the voltage to which the capacitor is charged and the voltage of the cell.

4. Apparatus as claimed in claim 3 wherein the means for determining if the peak current is below a predetermined magnitude comprising a sample and hold circuit controlled by said timer and responsive to the difference between the voltage to which the capacitor is charged and the voltage of a battery or cell connected to said terminal with the voltage developed across said resistor or a part thereof.

5. Apparatus as claimed in claim 4 and having means including a further sample and hold circuit, for indicating the peak current flowing in the cell during the period of pulse application.

6. Apparatus as claimed in claim 4 and having a two-state indicator operated by said timer shortly before the time when the capacitor is discharged and reset if the comparator shows that the peak current is of sufficient magnitude.

7. Apparatus as claimed in claim 6 wherein the two-state indicator is a lightemitting diode.

8. Apparatus as claimed in claim 4 to 7 wherein, in the comparator, the voltage across said resistor or a voltage related thereto is compared with a reference voltage of smaller magnitude, derived from the difference between the voltage to which the capacitor is charged and the open-circuit voltage of the battery or cell.

9. Apparatus as claimed in any of claims 4 to 8 and having an alarm operated by said comparator.

10. Apparatus as claimed in claim 9 and having means operative to monitor the state of said two-state indicator, or a control signal therefore, at a predetermined time after each operation of said switch to discharge the capacitor, which means energizes the alarm if the two-state device has not been reset.

11. Apparatus as claimed in any of claims 4 to 10 and having a voltage limiter for limiting the voltage applied to the comparator from said resistor.

1 2. A method of testing a cell or battery substantially as hereinbefore described with reference to the accompanying drawing.

1 3. Apparatus for testing a cell or battery substantially as hereinbefore described with reference to the accompanying drawing.