How does my ecig know the individual battery state of two batteries in series?

Question

My ecig / vape takes two 18650 batteries in series and has two indicators for the battery charge status. They often differ, and if I swap the batteries around the indicators swap charge levels.

My high school physics memory says two batteries in series are cumulative, so how does the on-board electronics know the two batteries are at different charge levels and which is which?

In: 0

Answer 1

There will be a wire to the point between the two batteries, which the small microcontroller can measure the voltage of.