Why do batteries deplete very slowly over time when they’re just sitting in the pack not being used?

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I only recently realised that batteries (like AA, AAA, C, D, etc.) have a best before date when I accidentally bought an out of date pack and couldn’t figure out why it they wouldn’t work! Why do they lose charge, despite not being connected to anything, or one another?

In: Chemistry

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.

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.