Self discharge (batteries losing charge when not in use) usually happens because of undesirable chemical reactions happening inside the battery. Usually a chemical reaction should only occur when the electrodes on the top and bottom of the battery are connected in a circuit and that is what gives you the electrical charge. So if you have a particularly old battery that has undergone the undesirable reaction the chemicals required to produce the charge have already at least partially reacted, leaving less of those same chemicals to produce a charge when needed.
It’s also quite interesting to look up different battery types to see how quickly they self discharge on average, so for instance a non rechargeable lithium metal battery takes on average 10 years or more to self discharge, whereas a non rechargeable zinc carbon battery only takes 2-3 years on average to self discharge.
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