Capacity-fade of nickel-based batteries correlates, in part, with a rise in internal resistance. NiCd and NiMH share similarities with lead- and lithium-based batteries in that the internal resistance remains low at first and then rapidly increases towards the end of life. Measuring resistance could serve as a simple rapid-test method to detect end-of-life but this would not provide dependable state-of-health (SoH) information(See BU-208: Cycling Performance)
QuickTest™ (by Cadex) goes deeper than mere resistance measurements and fuses data from six variables. These are capacity, internal resistance, self-discharge, charge acceptance, discharge capabilities and mobility of electrolyte. A trend-learning algorithm combines the data to provide a dependable state-of-health (SoH) reading in percentage. The system uses battery-specific matrices that are stored in the test device. Figure 1 illustrates a simplified structure of the algorithm.
Multiple variables are fed to the micro controller, “‘fuzzified” and processed by parallel logic. The data is averaged and weighted according to battery application.
As QuickTest™ includes the internal resistance of a battery pack, the welding joints between the cells must be taken into consideration, especially with packs of 10 cells or more. Although seemingly insignificant in terms of added resistance, the mechanical linkage behaves differently to the chemical cell and this causes an unwanted error. The linkage error is not seen on a conventional discharge test or when doing only a resistance check but it interferes with higher end rapid-test methods involving excitation signals that look at many variables.
In addition to the resistance of interlinking cells, each cell in a multi-cell pack responds differently to the excitation signals. As the pack ages, these characteristics begin to diverge; QuickTest™ looks at the average of all cells combined. QuickTest™ only works effectively with nickel-based batteries, Li-ion and lead acid systems uses different rapid-test technologies(See BU-907: Testing Lithium-based Batteries)
References
[1] J. Tinnemeyer, "Multiple model systems and methods for testing electrochemical systems". US Patent US6778913B2, 17 08 2004.
Last Updated: 4-Nov-2021
Batteries In A Portable World
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On June 24, 2019, Vinod Keerthy wrote:
Good Article. Aside, may I know standard procedure to measure internal resistance of a cell. Is there any standard (IEC/IEEE) ?
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