GB2269019A

GB2269019A - Testing batteries

Info

Publication number: GB2269019A
Application number: GB9212485A
Authority: GB
Inventors: David John Collins
Original assignee: Emerson Electric UK Ltd
Current assignee: Emerson Electric UK Ltd
Priority date: 1992-06-12
Filing date: 1992-06-12
Publication date: 1994-01-26
Anticipated expiration: 2012-06-12
Also published as: EP0645018A1; WO1993025917A1; GB2269019B; GB9212485D0

Abstract

A method of testing a battery (10) comprises applying (32,26) a mains frequency AC test voltage to the battery to generate an AC current in the battery and measuring the impedance of at least one cell, or a quantity related thereto e.g. the voltage across the cell. The current through the battery is measured by the voltage across a resistor (28), DC drain is prevented by a blocking capacitor (30). A voltmeter (40 Fig 2 not shown) may be connected alternately across the resistor (28) and the battery cells.

Description

A Method and Apparatus for Battery Testing.

This invention relates to a method and apparatus for testing the condition of a battery having a plurality of cells in series.

It is common in various systems such as for example a mainframe computer installation to provide a back-up power supply in the form of a storage (e.g. lead-acid) battery which automatically is switched-in in the event of a mains power supply failure.

At regular intervals such batteries must be tested to ensure that they are in good condition and will perform if required.

The conventional method for checking such batteries, which may comprise up to two hundred individual cells, is to perform a discharge test in which the battery is connected to a load bank of relatively low resistance but high power-dissipation capacity. The heavy discharge current produced by the battery in such a test is a general indication of the overall condition of the battery, but does not reveal the condition of each cell, the failure of any one of which disables the battery. Furthermore considerable preparatory work is necessary to bring the load-bank on-site and to connect it to the battery. In addition the test generates considerable heat and the battery requires recharging after the test.

An object of the present invention is to provide a method and apparatus for testing multi-cell batteries in which the inconvenience and expense of a discharge test is avoided. Another object is to provide a method and apparatus with which it is possible to obtain an indication of the condition of each individual cell of the battery.

Thus, in one aspect the invention provides applying a mains-frequency AC test voltage across the battery to generate an AC current therein without drawing a DC current from the battery, measuring the impedance of at least one cell of the battery (or a quantity related thereto) and utilising the i.mpedance or the related quantity as an indication of the condition of the at least one cell.

In another aspect the invention provides apparatus for use in the aforesaid method comprising means for deriving from an AC mains supply an AC test voltage of the same frequency, means for applying the test voltage to the battery to generate an AC current therein without drawing a DC current from the battery and means for measuring the impedance of the cell or a quantity related thereto.

The AC voltage across the at least one cell may be the related quantity.

The voltage may be compared with a corresponding voltage across at least one other cell to indicate the condition of the at least one cell.

The AC current through the battery may be measured to provide with the AC voltage across the at least one cell a measure of the impedance thereof as an indication of the condition of the at least one cell.

Preferably the AC current is measured by measuring an AC voltage across a resistance by means of the same voltmeter as is used for measuring the AC voltage across the at least one cell.

The means for deriving the AC test voltage may comprise a transformer.

A blocking capacitor may be employed to prevent a DC current being drawn from the battery. There may be a plurality of blocking capacitors, at least one of which may be switched into or out of the circuit.

The invention will now be described with reference to the accompanying drawings, wherein Figure 1 shows diagrammatically one embodiment of the invention, and Figure 2 shows a modification to the embodiment of Figure 1.

A standby battery 10 comprises a large number, perhaps more than two hundred, individual cells in series, and has external terminals 14, 16, the DC potential across which may be up to 450v.

Apparatus according to the invention comprises a transformer 20, the primary winding 22 of which in operation is connected to the AC mains supply.

The secondary winding 24 is connected at one end to a terminal 26.

At the other end it is connected via a resistor 28 and DC blocking capacitor 30 to another terminal 32. Measurement points 34,36 are provided at each end of the resistor 28 so that the AC current through it may be measured by applying voltmeter to measure the AC voltage drop across it.

In operation the terminals 26,32 are respectively connected to the battery terminals 14,16 to apply a mains frequency AC voltage. The DC blocking capacitor 30 prevents discharge of the battery through the transformer secondary 24.

The AC current through the resistor is measured indirectly by means of a voltmeter applied at 34,36. This same voltmeter is then used to measure the AC voltage across each cell of the battery. The AC voltage divided by the AC current indicates the nominal impedance of the cell; at mains frequency (50/60Hz) quadrature components of the impedance are negligible and the impedance is for all practical purposes resistive.

The impedance of the cells may then be compared, whereby any cells with impedance different from the norm are identified and may be checked for incipient failure.

The impedance of each cell may also be compared its value measured on a previous occasion, and any deteriorating trend identified in good time for remedial action.

The apparatus may be used over a wide range of cell sizes, which may vary in impedance from fraction of a milliohm to approaching one hundred milliohms. The blocking capacitor 30 represents the major component of the total impedance of the series circuit consisting of the capacitor and the battery, and consequently effectively controls the current in the circuit. The current typically is chosen to be in the range of about one amp to several amps, depending on the size of the battery, to give a convenient voltage reading without causing significant heating of the cells during the test. Small-capacity batteries (e.g. a few amp hours) have a lower thermal capacity than large ones and thus for them a test current at the lower end of the range is preferred.

The capacitor 30 may usefully be constituted by two or more switchable capacitors which can be individually switched into or out of the circuit to provide a range-change facility enabling the current to be chosen appropriately for the particular test e.g. so that it is approximately the same as in an previous test of the same battery.

The method may be simplified, provided the current through the battery is kept constant. Then as between individual cells, or (subject to metering accuracy) between successive tests of the same cell on different occasions, the AC voltage across the cell is proportional to impedance and may be interpreted as indicative of the condition of the cell.

The apparatus usefully may be packaged such that the voltmeter (40, Fig.2) may be switched between the measurement points 34,36 and a pair of wander probes 42,44 used for testing each cell of the battery as already described. The switching is shown symbolically as achieved by a double-pole double-terminal mechanical switch 46, but other more elegant arrangements can readily be envisaged.

For example, if a digital voltmeter is employed then the incorporation of simple logic circuitry would enable an automated test routine to be adopted, in which following the application of the probes to each cell the voltage is measured, then the current is measured automatically, the impedance calculated and displayed on a VDU (e.g. a LCD) and/or stored in a memory, in which case a comparison with a previous reading could also be displayed. Additionally a hard-copy print-out could be provided e.g. for retention by the operator of the system.

It will be appreciated that the invention permits the battery to be tested in-situ and without disconnection of the cell-to-cell interconnections of the battery, enabling testing to be performed (e.g.

with extended probes 42,44) in inaccessible locations.

Claims

1. A method of testing a battery having a plurality of series cells comprising applying a mains-frequency AC test voltage across the battery to generate an AC current therein without drawing a DC current from the battery, measuring the impedance of at least one cell of the battery (or a quantity related thereto) and utilising the impedance or the related quantity as an indication of the condition of the at least one cell.

2. A method as claimed in Claim 1, wherein the AC voltage across the at least one cell is the related quantity.

3. A method as claimed in Claim 2, wherein the voltage is compared with a corresponding voltage across at least one other cell to indicate the condition of the at least one cell.

4. A method as claimed in Claim 2 or Claim 3, wherein the AC current through the battery is measured whereby to provide with the AC voltage across the at least one cell a measure of the impedance thereof as an indication of the condition of the at least one cell.

5. A method as claimed in Claim 4, wherein the AC current is measured by measuring an AC voltage across a resistance by means of the same voltmeter as is used for measuring the AC voltage across the at least one cell.

6. Apparatus for use in the method of Claim 1, comprising means for deriving from an AC mains supply an AC test voltage of the same frequency, means for applying the test voltage to the battery to generate an AC current therein without drawing a DC current from the battery and means for measuring the impedance of the cell or a quantity related thereto.

7. Apparatus as claimed in Claim 6, wherein the means for deriving the AC test voltage comprises a transformer.

8. Apparatus as claimed in Claim 6 or 7, comprising a blocking capacitor to prevent a DC current being drawn from the battery.

9. Apparatus as claimed in Claim 8, wherein there are a plurality of blocking capacitors, at least one of which may be switched into or out of the circuit.

10. Apparatus as claimed in any of Claims 6 to 9, comprising means for measuring the AC current through the battery.

11. Apparatus as claimed in Claim 10, wherein the means for measuring the AC current comprises a resistive element and means for

11. Apparatus as claimed in Claim 10, wherein the means for measuring the AC current comprises a resistive element and means for connecting a voltmeter to measure the AC voltage across the resistive element said voltmeter also being connectable to measure the AC voltage across the at least one cell.

12. A method of or apparatus for testing a battery substantially as herein described with reference to the accompanying drawings. Amendments to the claims have been filed as follows Claims

1. A method of testing a battery having a multiplicity of series cells comprising deriving a mains-frequency AC test voltage from a means supply, applying said test voltage across the battery to generate an AC current therein without drawing a DC current from the battery, measuring individually the impedance of at least one of the cells of the battery (or a quantity related thereto) and utilising the impedance or the related quantity as an indication of the condition of the at least one cell.

6. Apparatus for use in the method of Claim 1, comprising means for deriving from an AC mains supply an AC test voltage of the same frequency, means for applying the test voltage across the battery to generate an AC current therein without drawing a DC current from the battery and means for measuring individually the impedance of the at least one of the cells of the battery or a quantity related thereto, and means for indicating the value of said impedance or related quality.