Abstract: An apparatus for measuring charge state of a battery having multiple cells includes a transmitter that is connected to the battery to apply a test signal across the battery. This test signal is preferably a continuous square wave signal having a frequency less than 3 Hz. The test signal alternates between a voltage adequate to charge the battery and a lower voltage, and the charging voltage is retained for a time sufficient to allow a polarization voltage to develop across individual cells. A receiver is connected to the cell to monitor the polarization voltage across a cell as it develops in response to the test signal. This polarization voltage can be monitored for example using a DC voltmeter, or by displaying the voltage versus time waveform. In either case an accurate indication of the state of the charge of the cell is provided.

Abstract: A testing device measures the impedance of secondary cells that form a battery, such as a lead acid battery, while the battery is in a float charge condition and connected to an active electrical load. The impedance measurement is made at a frequency selected to be different from those frequencies otherwise present in the charger-load circuit. A first application of the testing device monitors the battery for a change in impedance that can signal a developing defect in one or more individual cells or intercell connections that can prevent the battery from delivering its stored energy to the load. In a second application, the testing device is used to compare the impedance of individual cells and electrical connections to locate faulty components.