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.
Self-discharge is a phenomenon in batteries in which internal chemical reactions reduce the stored charge of the battery without any connection between the electrodes or any external circuit. Self-discharge decreases the shelf life of batteries and causes them to initially have less than a full charge when actually put to use.
Self-discharge is a chemical reaction, just as closed-circuit discharge is, and tends to occur more quickly at higher temperatures. Storing batteries at lower temperatures thus reduces the rate of self-discharge and preserves the initial energy stored in the battery. Self-discharge is also thought to be reduced as a passivation layer develops on the electrodes over time.
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