A battery charger generally needs to know the voltage on the battery being charged, among other things. In order for my design to work right, I’ll need to know when the battery is full, when it needs current-limited charging, and when it needs a boost because the motor is running. So I strapped a $5 digital voltmeter to the headset to monitor the battery voltage under various conditions.  Here is what I found.

Those readings were taken with the old, weak batteries. I should redo the experiment now that fresh batteries have been installed. But this data gives me encouragement that my power transfer circuit can work by monitoring battery voltage alone.