GB2130735A - Battery testing apparatus - Google Patents

Abstract

An apparatus for testing a battery 1 comprises a source of predetermined voltage 3 having its positive terminal connected through a capacitor 7 and resistor 5 to a terminal Y and its negative terminal connected via a switch 11 and fuse 40 to terminal X. A battery to be tested 1 is connected across terminals X and Y and upon closure of switch 11 the capacitor 7 is charged up to the predetermined voltage in excess of the open circuit voltage of the battery 1. A timing pulse from a timer 13 passes through an open gate 14 to cause a switch 12 to go short circuit thereby shorting the capacitor 7 through the battery 1. Discharge of current flow through the battery 1 is monitored by a sample and hold unit 4 detecting the voltage across resistor 5. An alarm means 34 can be provided which is responsive to a comparison between the sensed discharge current and a predetermined threshold current level. A further sample and hold unit 16 senses a voltage difference between the discharge voltage across the battery 1 and the open circuit voltage of the battery 1. A divide 19 divides the voltage difference by sensed discharge current to produce a value proportional to internal resistance of the battery 1 which can be displayed on a meter 21. Means are also provided to detect incorrect connection of battery 1 to the battery testing apparatus and means to discharge capacitor 7 when battery 1 is disconnected from the apparatus. <IMAGE>

Description

SPECIFICATION Battery testing apparatus This invention relates to an apparatus for testing batteries or cells.

It is desirable to check the condition of batteries at regular intervals, in particular when the perform as standby batteries. Any electrochemical and structural deterioration that may occur needs to be discovered. It is known that a measurement of internal resistance of a battery is a useful indication of the battery condition, and a method and apparatus for periodically monitoring this internal resistance is disclosed in our British Patent Application 2046923A. Nevertheless, the apparatus and method described therein is limited to monitoring a battery at a predetermined voltage unless the apparatus voltage employed is altered.

It is an object of the invention to provide an apparatus and method of battery testing substantially obviating the above-mentioned limitation.

According to one aspect of the invention there is provided an apparatus for testing a battery or cell comprising a source of a predetermined voltage, a capacitor and terminals to connect the apparatus to a battery under test, the source, capacitor and terminals being arranged such that the source is connected in series with the capacitor and the battery under test so that the capacitor is charged to the predetermined voltage in excess of the open circuit voltage of said battery under test; the apparatus also including means to discharge the capacitor across the battery under test, means to sense discharge current through the battery under test and means responsive to the sensed discharge current to provide an indication of the state of the battery under test.

In this way it is possible to charge up the capacitor to a predetermined voltage in excess of the voltage on any battery to be tested.

Consequently it is not necessary to alter the source of predetermined voltage when testing a battery having a different voltage to a prior tested one.

In a preferred embodiment the apparatus includes a sensor capable of detecting when the battery to be tested is disconnected from the apparatus, and in response causes discharge of the capacitor; and also includes a polarity detector capable of de-activating the means for discharging the capacitor in response to reverse connection of the battery to be tested to the apparatus.

Reference is made by way of example only to the accompanying drawing, the single figure of which illustrates an embodiment of an apparatus for testing a battery in accordance with the invention.

Referring to the figure, an apparatus to test a battery is connected by terminals X and Y to positive and negative terminals respectively of a battery 1 to be tested. Terminal X is linked through a fuse 40 to a diode by means of a line 6, the diode is connected by a switch 11 to a negative terminal of a power supply 3 of a predetermined voltage, the supply being illustrated as a 12 volt battery for example only.

Terminal Y is connected to a point B which is joined to a sample and hold unit 4 and also to a resistor 5. The resistor 5 is connected through a point A, which is connected to a capacitor 7 by a line 15, the point A being maintained at a zero volts reference potential by a line 8. The capacitor is linked by a line 9 to a resitor 10 joined to the positive terminal of the supply 3.

When the switch 11 is closed capacitor 7 is charged up to a voltage 12 volts in excess of the voltage on battery 1. A switch 12, for example a thyristor, is !Inked across line 6 and line 9 and when the capacitor is fully charged the switch is closed by application to the switch of a timing pulse from a timer 13, the timing pulse having passed through an open gate 14. Consequently the capacitor 7 is discharged through the battery under test. At the instant of discharge, a sample and hold unit 4 takes a sample of the voltage V at the point B relative to point A. The sample is representative of the current flowing through resistor 5, which is an effective measure of the current through the battery (I) at the instant of discharge. The sample is taken for typically 70ys following the discharge initiation.

A further sample and hold unit 1 6 is AC coupled to a high impedance potential divider comprising series linked'resistors 17 and 18, which are connected across point A and line 6.

The sample voltage taken by the sample and hold unit 1 6 is the increase in voltage at the instant of discharge, thus giving an effective measure of the voltage difference V between the voltage across battery 1 and the voltage on the capacitor 7 at the instant of discharge of the capacitor. Therefore the quotient V (K+Q) I where K is the resistance of connections between the battery testing apparatus and battery 1 and Q is the internal resistance of the battery under test.

Sampling V during the period of discharge allows a dynamic value of VTl to be established.

Therefore the outputs of units 4 and 16 are read into a dividing means such as divider 19 and the result of the above-mentioned quotient relayed into a further sample and hold unit 20 under the control of timing pulses from timer 13. These same pulses are arranged to control the sampling by units 4 and 1 6. The output from the sample and hold unit 20 can be employed to drive any display means dependent on the application of the apparatus, for example a meter 21 can be employed to indicate a value of internal resistance and hence give an indication of the state of the battery under test. A scaling unit 22 can be interposed between the meter and unit 20 in order to allow calibration adjustments of the meter, referred to hereinafter.

Initial calibration of the apparatus is required in order to obtain direct values of 9 since the above mentioned quotient includes values of resistances in the circuit other than the battery under test, for example the connection resistance K. To calibrate the apparatus the terminals X and Y are shorted by a short heavy duty link (not shown) and current from an external supply (not shown), typically 100 mA, is applied to the terminal X. The voltage across terminal X and the line 8 as directly measured, divided by the current flowing from the supply gives an indication of the internal apparatus resistance R,.Removing only the external supply, the apparatus by itself is switched on by closing switch 11 and the voltage appearing between the output of a unit 20 and the line 8 is measured by a multimeter and the divider is adjusted until the multimeter reading corresponds to the value of R,. Thus the apparatus is calibrated with respect to its own internal resistance. A resistor of known value is then connected across terminals X and Y to replace the heavy duty link. Before switching on, meter 21 is set to zero and after closing switch 11 the range control in scaler 22 is adjusted until meter 21 reads a value equal to the resistor known valve.

The resistance of any leads can be calibrated for by connecting together the leads and adjusting the zero shift in scaler 22.

In a preferred apparatus a component chain 23 spans line 6 and line 1 5 such that a voltage at a point C therein can, when applied to gate 14, close the gate inhibiting timing pulses from timer 1 3. The voltage at C appears, only in the case when the battery 1 to be tested is connected with incorrect polarity, the chain therefore acts as a polarity sensor. The detailed construction of such a component chain will be apparent to a person skilled in the art, an example is illustrated in the Figure.

A further preferred feature of the apparatus is to include a high impedance potential divider chain 24 between the line 6 and line 15, a voltage from which an be employed to activate a switch S2 thereby shorting capacitor 7 through a resistor 25. The resistances in chain 24 can be adjusted such that activation of the switch 52 occurs only when terminals X and Y are open circuit. The divider chain therefore detects when the battery 1 is removed from the apparatus. The detailed construction of divider chain 24 will be apparent to a person skilled in the art. It will be apparent that all leads discharging the capacitor 7 should be constructed to be capable of handling such a discharge, and preferably leads joining at points A and B are soldered respectively at one location.

Clearly the testing apparatus described herein above is particularly suitable in applications wherein the apparatus is portable. In operation, the apparatus is activated by switch 11, calibrated as described herein above and a reading of internal resistance of a battery connected to terminals X and Y is taken by closure of switch S1. In practice switch S1 can be closed periodically by timing pulses applied to gate 14 which can be varied in accordance with requirements. A period of 40 secs for the timing pulses from timer 13 produces a suitable periodic discharge. Cleariy visual indicators of the test cycle can be employed to indicate activation of the cycle, initiation of the discharge and indication of a negative comparison with a reference value of the output of unit 20.

In a more permanent testing apparatus it is preferred to incorporate the components described within box 30. This box includes a comparator 31 to compare the output of the divider 1 9 which is, stored in sample and hold unit 20, with a reference value on line 32. When the divider output no longer exceeds the reference value logic circuit 33 is no longer instructed by the comparator to maintain alarm 34 inactive. it will be apparent to a person skilled in the art that the above type of comparison with a reference value can be incorporated at different positions within the apparatus circuitry and an alarm signal derived therefrom. For example, the sensed discharge current contained within sample and hold unit 1 6 could be compared with a reference discharge current.

The power supply 3 of a permanent testing apparatus is preferably derived from a 50Hz mains supply This allows synchronization of the apparatus testing to the mains phase, thereby reducing meter reading variations resulting from phase shift between values of V and I which may occur if the battery under test is being pulsed or is on charge.

By employing an apparatus in accordance with the invention it is possibie with typically available circuitry to test batteries having a voltage range between a single cell and 300 volts, since the apparatus uses the battery under test to charge up the capacitor providing the discharge current.

This flexibility allows the construction of a simple and robust apparatus, which is portable and can include an alarm means.

Claims (11)

Claims

1. An apparatus for testing a battery or cell comprising a source of a predetermined voltage, a capacitor and terminals to connect the apparatus to a battery under test, the source, capacitor and terminals being arranged such that the source is connected in series with the capcitor and the battery under test so that the capacitor is charged to the predetermined voltage in excess of the open circuit voltage of said battery under test; the apparatus also including means to discharge the capacitor across the battery under test, means to sense discharge current through the battery under test and means responsive to the sensed discharge current to provide an indication of the state of the battery under test.

2. An apparatus according to Claim 1 wherein said means to discharge the capacitor across the battery under test comprises a switch responsive to periodic timing pulses from a timing means.

3. An apparatus according to Claim 1 or 2 wherein said means to sense discharge current comprises a resistor in series between the capacitor and battery under test, and a means to sense voltage across the resistor.

4. An apparatus according to any preceding claim wherein said means responsive to sensed discharge current includes an alarm means responsive to a comparison between sensed discharge current and a predetermined threshold current level.

5. An apparatus according to any preceding claim wherein said means responsive to sensed discharge current includes a display means for indicating a value dependent on the level of sensed discharge current.

6. An apparatus according to any one of Claims 1 to 3 including a means to sense a voltage difference between the discharge voltage across the battery under test and the open circuit voltage of said battery under test, and including dividing means capable of dividing said voltage difference by said sensed discharge current to produce a value proportional to internal resistance of said battery under test.

7. An apparatus according to Claim 6 including an alarm means responsive to a comparison between said value and a predetermined threshold value.

8. An apparatus according to Claim 6 or 7 including a meter display means for indicating said value.

9. An apparatus according to any preceding claim including a sensor capable of detecting when said battery under test is disconnected from the apparatus, and in response to disconnection discharging the capacitor.

10. An apparatus according to any preceding claim including a polarity detector capable of detecting reverse connection of the battery to be tested to the apparatus, and in response to reverse connection deactivates said means to discharge the capacitor.

11. An apparatus substantially as herein described with reference to the accompanying drawing.