System noise as a signal source for impedance measurements on battery strings

Alternating-current impedance measurements provide useful information about the characteristics of electrochemical systems, such as batteries, and are envisioned as the basis for indirect methods of determining battery capacity. Impedance measurements rely on recording the response of the battery to an applied current or voltage test signal. However, this type of measurement can be difficult with large batteries connected to operating equipment. High background noise levels, caused by time-varying changes in the battery load, interfere with the impedance measurement. Instead of attempting to eliminate background noise, the authors have used the noise as a test signal, in conjunction with Fourier transform (FT) signal processing. The current and voltage passing through cells in an operating telecommunications lead-acid battery string were determined with a simple, three-terminal connection. Impedance calculations were performed with a commercial FT signal analyzer. The results indicate that completely passive (no externally applied test signal, and not requiring discharge or disconnection of the string) impedance measurements of batteries connected to operating equipment are feasible