Battery potential energy versus mass

[rcgldr]

I'm asking this question on behalf of another person who doesn't know english well.

What he wants to know is the relationship between energy extracted from a battery and the corresponding reduction in mass of the battery, and if it would be possible to measure the difference with some type of precise instrumentation rather than than it being an abstract calculation.

Asume that all energy outputs, including electrical and heat related losses are considered in the relationship between reduction of potential energy and reduction of mass.

[vanesch]

Yes, there should be a diminuation in mass of the battery when it discharges, according to relativity. As to be able to measure this, I don't think this is going to be possible, as the change is too small.

120 Ahr at 12V (a serious car lead battery) corresponds to about 5 MJ, which corresponds (mc2) to ~60 nanogram.
I don't think you can see a difference of 60 nanogram on the weight of a car battery of say, 20 kg. You'd need a precision of about 12 digits to start seeing the difference.