(experimental and as yet untested)

This is a differential buffer for connecting cheap voltmeters (for example: https://amazon.com/CenryKay-4-5-30v-Digital-Display-Voltmeter/dp/B08PB5NDMC) to the individual sticks of a Honda hybrid car’s IMA battery pack. The battery pack of a Honda hybrid car has 120 NiMH D-cells arranged in 20 sticks of 6 cells each. The cable running from the battery pack to the BCM (Battery Condition Monitor) has voltage sense taps for every two sticks. We tried attaching ten of the cheap voltmeters to these sense lines. The voltmeters have only two wires, which are used for both sensing the voltage and powering the meter. The problem is that it seems the taps are not tied directly to the battery – there must be some sort of current limiting resistor in each wire. The current drain of the voltmeters is enough that it changes the voltage read by the BCM, which throws a hissy fit and shuts down the hybrid assist system.

Here are some pictures from our first attempt: https://www.insightcentral.net/threads/yet-another-ima-problem-post.130739/page-2

This experiment uses a high impedance differential amplifier to read the voltage across each pair of sticks, and translate that to a ground referenced voltage with enough drive to run each voltmeter. The maximum voltage that might come out of the battery is less than 200 volts with respect to ground. The maximum differential across each pair of sticks is around 19 volts. The buffer that reads the highest stick pair might see inputs at full charge of 184 and 165, with a difference of 19 volts. With these inputs, the buffer would output 19 volts, referenced to vehicle ground, with a drive of around 25 ma to drive the voltmeter.