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Does a fully charged battery weigh more than a discharged one?

Given every thing else being equal, does a fully charged battery weigh more than a discharged one?
Andrew David Stiff from West Midlands (Age 45-54)

Actually asking on behalf of my son who asked this a few years ago when he was about 10 (he’s now 19). Does a rechargeable battery weigh more charged than un-charged?
Andrew from Berkshire (Age 45-54 )


3 Responses

  1. Don’t believe so. A battery is stored and separated charge. It is not a big pile of electrons that flow out when discharging. When the battery is discharging the electrons flow out through one terminal through your device and back in the other terminal(positive current as we measure it flows in the opposite direction; electrons are negatively charged).

    It is the same when discharging or charging a large capacitor. The capacitor stores separated charge and makes this separation available as a voltage at one of its terminals. The device only has access to the charge by allowing the return to the opposite terminal.

  2. The above answer by Alaster McDonach is half-correct. It is true that a battery is not like a big store of “electricity” which then flows out as the battery is used (and therefore will weigh less when empty). A better way to think about a battery is like a pump in a water system (think of water going round a central heating system, or around a garden pond with a waterfall). The electricity is like the water, and its motion around the system is caused by the pump, or battery.

    So,why did I say half-correct? Einstein’s special theory of relativity says that mass and energy are equivalent. It is true that there is more energy in a full battery than an empty one. Therefore there must also be more mass (or weight). However, this effect is EXTREMELY small. In reality you would not be able to measure the weight change.

  3. Hi Andrew :).
    As I read the fine answer to your question given above, by Dr. Gordon Love, I recalled a similar application of ‘Einstein’s special theory of relativity’ to the question – “Is a kettle which is full of water heavier after boiling than before (allowing for evaporation)”. Again the answer is “yes” for the reason stated and practically immeasurable for the same reason given but nonetheless it is a certain fact.
    I hope that my submission makes a worthwhile extension to the responses to your question.
    Good Luck,

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