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Acid specific gravity variation affect lead acid battery weight?
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Circlotron:
So we charge up our wet cell lead acid battery and when it is fully charged the specific gravity of the acid as measured by our trusty old school hydrometer has increased from a previous lower value. Does this mean that the overall weight of the battery has increased, or has this extra weight / mass been transferred from the lead plates into the acid?
Ian.M:
E = m c2

You haven't supplied enough energy to significantly (or even measurably*) increase  the total mass so where do you think the extra electrolyte density comes from? 
Hint:  consider the existence of SLAs, which obviously rule out significant gas exchange with the atmosphere then ask your self; What parts of the battery are in contact with the electrolyte and change during charging and discharging?

* Work it out and you'll find fully charging a 200AH 12V lead acid battery theoretically makes it about 100 picogrammes heavier.  That's several orders of magnitude below detectability on the best laboratory balances in the world!

Edit: Equation corrected.
ebastler:

--- Quote from: Ian.M on February 06, 2024, 07:02:49 am ---E = M C2

--- End quote ---

Umm... I think Circlotron was referring to a conventional battery, not a fusion reactor.  ::)
IanB:

--- Quote from: ebastler on February 06, 2024, 07:07:19 am ---Umm... I think Circlotron was referring to a conventional battery, not a fusion reactor.  ::)

--- End quote ---

No. You charge a battery, you store energy inside it. By mass-energy equivalence, the stored energy will increase the mass of the battery by an undetectable amount, but the physics is still a real thing.
mendip_discovery:
I remeber there was a thing about AA batteries etc having extra weight when charged that you could notice a difference. Never tried it myself but it does make sense that it would heavier but not sure if it is enough to easily measure.
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