GB2182155A - Testing batteries - Google Patents

Abstract

A method for determing the state of charge of a secondary battery (13) comprises the application (11, 12) of a brief boost charge and the measurement of the subsequent discharge capacity via a given load and gated circuit (14-17). The circuit permits discharge only at a given voltage, and the permitted voltage is progressively lowered from a high value at which discharge does not occur to a low level at which it does occur. Comparison (15) of the actual discharge current with stored values (16, 17) from batteries at known states of charge allows the state of charge of the tell cell to be determined. <IMAGE>

Description

SPECIFICATION Methods and apparatus for testing secondary batteries This invention relates to methods and apparatus for testing secondary batteries.

U.S. Patent Specification 4,361,809 discloses a battery diagnostic method and apparatus for performing same based on a sequence of a plurality of charge and discharge cycles involving a plurality of voltage measurements used to evaluate five arithm etic functions, three of which are compared with predetermined limits to establish the state of the battery being tested. The other two are compared to determine whether the battery is defective or not. The test sequence takes about 30 seconds.

Adevice acting as a "fuel gauge" is set out in US Patent Specification No. 4,397,787, and is applicable to the testing of Pb/acid batteries.

Another method, based on the constant monitoring of a battery in use is disclosed in US Patent Specification 4,433,294. According to this patent the battery status is tested by a dynamic voltage-current measurement involving applying a varying charging current and measuring the voltage produced across the batterywhilethe current is supplied.

The present invention provides a simple and rapid method for testing the state of secondary batteries and a comparatively simple device for carrying out such testing.

The method of the invention comprises applying to the tested battery a brief and high boost charge and instantly scanning the thus boosted battery to establish at which voltage level the battery is capable of providing a discharge current. The discharge isvia a gated circuit which tests the cell at discrete decreasing voltage levels, establishing which is the highest level at which the cell will dischargethrough a calibrated load. This scan is a very rapid one. It can be effected at decreases of voltage by decrements of 0.01 V, the enture scan being effected within a brief period of time, of the order of one second. The maximum voltage supplied by the battey is compared with data stored in a computer and thus it is possible to obtain immediately an indication of the state ofthe battery.The comparison is made with a previously calibrated nomograph which is stored in a computerized bank.

The method of the invention is applicable to secondary cells such as Ni/Cd cells, Lead/Acid cells and the like.

it has been established by us that there exists a correlation between the state of charge of the battery and the maximum discharge level of the cell. The cell is discharged at the C-Rate (Capacity Rate) and the higher voltage at which the cel I is capable of discharging, the higher its state of charge. The C-Rate is the capacity rate i.e. the current in amps numerically equal to the capacity of the cell or battery measured in A.hours. The discharge is effected immediately after the boost charge, as the voltage of the cell starts to decrease after such charge within a short period of time.

The device used for testing is gated so asto allow at a certain test step discharge only above a certain predetermined voltage, if the voltage of the cell is below this voltage, no current will flow, and the de vicegoes immediatelytothe next lowerstep and so on, until the voltage is reached at which the cell starts supplying current. The voltage can be decreased by predetermined decrements of any predetermined value: in the device tested this way by steps of 0.01 V each. The decrements can also be by different steps; it is alsofeasibleto resortto a gradual decrease and to stop this when a level of discharge is reached.

The voltage levels have been tabulated and a graph as been prepared for each type of cell. This allows an immediate determination of the state of the cell and its degree of charge. A high accuracy can be attained, and the entire testing sequence of a cell (boost charge and stepwise testing) takes a period of time of about 1 to 5 seconds.

The boost current applied at the start of the test applies to the battery a small charge compared with its overall capacity (ofthe order of less than 1 per cent of the total charge). This charge is applied during a brief period of time (of the order of 1 to a few seconds) and when such secondary cells are left after a charge and the open circuit voltage (OCV) is meas ured, itisfoundthatthis remainsata high level with lead batteries for a period of the order of one or more hours, and with Ni/Cd cells the voltage decreases from about 1.45 Vto about 1.35 V during about a day.

If cu rent is drawn from either type of battery, the voltage drop is a very rapid one, of the order of seconds only.

The invention is illustrated, byway of example, with reference to an illustration in the accompanying drawings, which are of a schematical nature and which are not to scale; and in which: Figure lisa block diagram of a test device of the invention; Figure2 is a discharge curve of a Ni/Cd cell; Figure 3 is a 3-dimensional projection of the complete discharge history of a Ni/Cd battery; and Figure4 is a 3-dimensional discharge history of a 50% discharged Ni/Cd battery.

As illustrated in Figure 1 the test device of the invention comprises, in combination, a waveform generator 11 connected to voltage/current amplifier 12, which is connectable to the battery to be tested, and connectableto awaveform analyzer 14which provides one inputto a comparator 15which also receives information from a memory unit 16 and which is connected to a programmer 17,the programmer 17 being connected with a keyboard 18. The comparator is also connected to a display unit 19.

The programmer 17 receives battery information, such as its C-rate and its nominal potential as input from the keyboard 18 and from the memory unit 16.

Atthis stage the test cycle is started by applying to the battery which is being tested a high boost charge, the current and voltage parameters of which are generated by the waveform generator 11 controlled by the programmer 17, and amplified by the amplifier 12. The battery reacts to the applied boost charge according to its state and degree of charge, which are measured and analyzed bythewaveform analyzer 14, and after comparison with the data sto red in the memory unit 16 the results are computed and displayed by the display unit 19.

Atest was carried out with Ni/Cd cells of the 2 A.4 type. The cell was given a power boost charge as follows: A current of 10 Awas applied for 1.0 second.

Thus a charge of known Coulombic quantity is applied to the cell, and this resulted in an OCV of about 1.45 V.

Immediately after the charging, the cell was discharged through the discharge means of the test apparatus. The discharge was via a current sink of 2 A; and it was determined by a gated circuit what vol- tage can be supplied by the tested cell. The gated cir- cuit is such that it tests the battery at a given voltage, and if this is not available from the battery, no current flows. In this case, the voltage is decreased by predetermined steps (of say 0.01 V each), until after a number of such steps, a lower gated voltage is reached, at which the battery is able to supply current at the given voltage. The discharge is effected at the C-Rate. The maximum period oftime of this scan is about 0. 5 to 1 second.

The discharge is programmed to give a distinct discharge load pattern and a very precise high voltage cutout (HVC).

For Ni/Cd cells the HVCwill be about 1.3 V, and during the C-discharge there takes place a very rapid decrease of voltage. When this falls below the critical HVC the circuit will be instantly switched off, and this is recorded in the terms of A-seconds. In practice the device will search for the value of voltage at which currentwill flow dueto an adequately high voltage of the cell. Each HVC corresponds to a certain depth of discharge (DOD) of the tested cell, and thus it is possibleto establish rapidly the degree of charge ofthe tested cell.

At the given discharge, afterthe highest voltage is found at which the cell will be able to discharge via the gated discharge, there is determined the amperesecond value of the discharge, and this value is indicative ofthe degree of charge of the cell.

As shown in Figure 2, the discharge curve of a typical NiCd cell is as indicated. There are shown a number of high power boost charges (HPB) along the discharge curve: when the cell is being tested the degree of discharge is not known, the response ofthe cell to such HPB charges with subsequent discharge via a given load being indicative of the degree of charge of the cell.

Figure 3 is a plot of the complete discharge history of a 100% charged cell, in decrements of 25%, for illustration only. From this it is clearthat at the highest voltage value gated, there exists already a current flow indicating thatthe voltage of the cell is at its upper limit. This is followed byfurther discharges at lower voltage values. The device is adapted to maintain highlyaccuratevalues ofvoltage. In practice measurements are carried out at substantially smal ler decrements of voltage (such as steps of 0.01 V).

For each voltage which is the lowest value atwhich current can flow, there exists a distinct discharge pattern.

As shown in Figure 4, a cell with only 50% charge is tested. Thus, during the first two steps no current will flow th rough the gated circuit and only at 50% DOD currentwill startflowing, this being indicative of the degree of charge of the cell. The entire scanning and measurement is effected with a matterofa second or so,theHPBtaking about 1 second.

Similar measurements were made with iead/acid batteries and the results are indicative of the degree of charge ofthe tested battery. This measurement is of special value for batteries of the sealed type where no access exists for measuring electrolyte density.

Claims (11)

1. A method for determining the state of a secondary battery as to its charge and condition, which comprises: applying a brief high boost charge, ata predetermined voltage, immediately testing the discharge capability of the cell via a given load and via a gated circuit which permits discharge only at a given voltage, decreasing the voltage until a value is reached at which the cell discharge via the gate circuit, and determining the status of the cell bycomparison with data accumulated from cells ofthistype atvariousdegreesofcharging.

2. A method according to claim 1, wherein the voltage is decreased by successive steps 0.01 to 0.05 Apart.

3. A method according to claim 1 or 2, wherein the power boost is applied at a voltage exceeding Vgad2svoltage of the cell fora duration offrom 1 to 5 seconds.

4. A method according to anyone of claims 1 to 3, wherein the boost charge is at a voltage of between 2 Cto 10 C.

5. A method of testing a secondary cell substanti ally as hereinbefore described with reference to the accompanying drawings.

6. A device for the evaluation ofthe degree of charge of a secondary battery which comprises: means for applying a brief high power boost charge at a predetermined voltage; means for scanning the possibility of discharge from said cell at decreasing voltage until a voltage is reached at which the cell is capable of providing an electric current; means for comparing the obtained value with pre viously stored values of representative cells with var ious percentages of charging; and means for evaluating the data obtained so as to indicate the charge of the cell.

7. A device according to claim 6 wherein the electric current to be provided is expressed in amperes, numerically equal to the capacity of the cell expressed in ampere hours.

8. A device according to claim 6 or7, comprising a waveform generator charging the cell via a voltage/ current amplifier.

9. A device according to anyone of ciaims 6 to 8, wherein the response of the boosted cell is applied to a waveform analyzer and to a comparator, means being provided forfeeding the comparator with data stored in a memory device.

10. Adevice according to anyone of claims 6to9, where means are provided for scanning the cell through a gated circuit at decrements of voltage at steps of from 0.01 to 0.05V each.

11. Adevicefortesting a secondary cell substantially as herein before described with reference to Figure 1 ofthe accompany drawings.