Electricity: measuring and testing – Electrolyte properties – Using a battery testing device
Patent
1995-08-25
1998-02-10
Karlsen, Ernest F.
Electricity: measuring and testing
Electrolyte properties
Using a battery testing device
324650, 324436, 340636, 320 48, G01R 3136
Patent
active
057173362
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a method and apparatus for determining the charge condition of an electrochemical cell. It is often of importance to know the charge condition of an electrochemical cell. For rechargeable or secondary cells, the general performance is affected by the history of charge and discharge cycles. Particularly, the lifetime of a lead acid cell is so affected to a very substantial degree. In order to manage charge and discharge cycles in a fashion which ensures better performance, it is however necessary to ascertain when charging is required, which involves acquiring knowledge of the prevailing state of charge of the cell.
Measurements of cell resistance or impedance may be made in order to assess state of charge; see for example U.S. Pat. Nos. 4,697,134, 3,873,911, 3,676,990 and United Kingdom Patent Specification 2088159. Generally, these measurements concentrate on DC resistance or impedance at the mid frequency region, where the difference in absolute values of impedance for fully charged and partially charged batteries is very small. This has meant that, to determine with any degree of certainty the state of charge of a battery, the impedance of that particular battery needs to be measured in advance at various states of charge and compared with the presently prevailing measurement. Because of impedance variations from battery to battery, every battery needs to be so characterised initially. Also, as the battery approaches the fully charged state, the change in impedance becomes small so that it is very difficult to measure state of charge above 75% charged.
SUMMARY OF THE INVENTION
In one aspect, the invention provides a method for determining whether an electrochemical cell is substantially fully charged, wherein the determination is made by ascertaining the rate of change of the impedance of the cell with changing frequency, at low frequency.
The invention also provides a method for determining the state of charge of an electrochemical cell, comprising comparing the impedance of the cell at two or more frequencies.
The invention also provides a method for determining the state of charge of an electrochemical cell, comprising comparing the impedance of the cell at a medium frequency, with the impedance at high frequency.
The high frequency may be about 100 kHz or in the range 30 kHz to 500 kHz.
The low frequency may be about 0.03 Hz or in the range 0.003 to 1 Hz.
The medium frequency may be about 300 Hz or in the range 10 to 1000 Hz.
In another aspect, the invention provides a method for determining whether an electrochemical cell is substantially fully charged, wherein the determination is made by ascertaining the rate of change of the impedance of the cell with changing frequency, at frequencies at which the cell exhibits a greater impedance when charged than when not charged.
The invention still further provides a method for determining the state of charge of an electrochemical cell, comprising comparing the impedance of the cell at a frequency at which the cell exhibits decreased impedance when charged, with the impedance at a higher frequency at which the impedance is substantially independent of the state of charge.
The invention also provides apparatus for carrying out the methods comprising means for applying to an electrochemical cell an AC signal of differing frequencies and means for measuring the impedance of the cell at those frequencies.
Separate signals for separate differing frequencies may be applied sequentially or signals have differing frequency components may be applied, or both techniques may be applied together.
In another aspect, the invention provides apparatus for generating signals representative of resistive and reactive components of an impedance, comprising AC generator means for generating an AC current for passing through the impedance, and a synchronous demodulating means couplable across the impedance for demodulating AC signal generated across the impedance pursuant to passing of said current through the impedance, said demodulating means being
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Basell Malcolm Charles
Hawkins John Maurice
Dreger Walter H.
Elcorp Pty. Ltd.
Karlsen Ernest F.
Solis Jose M.
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