[paul.fr]

see this battery
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Notice how i have not used it yet, when i do and it becomes 'dead' will it weight less than it did when new? Also does the opposite work for rechargeable batteries, that is, they weigh more when they are fully charged?

[syhprum]

There will be some change in the mass of batteries due to the chemical reactions that cause the generation of power.
They may for instance absorb Oxygen and increase in weight.
In a completely sealed system there would be a tiny loss of mass due to e=mc^2 but it would be very hard to measure.

[lightarrow]

There will be some change in the mass of batteries due to the chemical reactions that cause the generation of power.
They may for instance absorb Oxygen and increase in weight.
In a completely sealed system there would be a tiny loss of mass due to e=mc^2 but it would be very hard to measure.

Exactly!