[ebarker]

easy enough.

A cubic foot of water is about 64 lbs. A battery is about 15 lbs.

Rough order of magnitude math cause I'm too lazy to do it right:

Lets say a battery weighs about the same as a fourth cubic foot of water.

Therefore if the battery is larger than a fourth cubic foot, it floats, if less than a fourth cubic foot, it sinks.

Archimedes principle.

Thanks Archy