It is often desirable to determine a battery’s internal resistance to evaluate its condition or suitability for an application. Accurate battery resistance determination is complicated by inherent capacitive terms which corrupt results taken with AC-based milliohmmeters operating in the kHz range. Figure 1, a very simplistic battery model, shows a resistive divider with a partial shunt capacitive term. This capacitive term introduces error in AC-based measurement. Additionally, the battery’s unloaded internal resistance may significantly differ from its loaded value. As such, a realistic determination of internal resistance must be made under loaded conditions at or near DC.
Figure 2’s circuit meets these requirements, permitting accurate internal resistance determination of batteries up to 13 V over a range of 0.001 Ω to 1.000 Ω. A1, Q1 and associated components form a closed loop current sink which loads the battery via Q1’s drain. The 1N5821 provides reverse battery protection. The voltage across the 0.1 Ω resistor, and hence the battery load, is determined by A1’s “+” input voltage. This potential is alternately switched, via S1, between 0.110 V and 0.010 V derived from the 2.5 V reference driven resistor string. S1’s 0.5 Hz square wave switching drive comes from the CD4040 frequency divider. The result of this action is a 100 mA biased 1 A 0.5 Hz square wave load applied to the battery. The battery’s internal resistance causes a 0.5 Hz amplitude modulated square wave to appear at the Kelvin-sensed S2-S3-A2 synchronous demodulator. The demodulator DC output is buffered by chopper stabilized A2 which provides the circuit output. A2’s internal 1 kHz clock, level shifted by Q2, drives the CD4040 frequency divider. One divider output supplies the 0.5 Hz square wave; a second 500 Hz output activates a charge pump, providing a –7 V potential to A2. This arrangement allows A2 output swing to zero volts.
The circuit pulls 230 μA from its 9 V battery power supply, permitting about 3000 hours battery life. Other specifications include operation down to 4 V with less than 1 mV (0.001 Ω) output variation, 3% accuracy and battery-under-test range of 0.9 V to 13 V. Finally, note that battery discharge current and repetition rate are easily varied from the values given, permitting observation of battery resistance under a variety of conditions.