Why would a car battery go out if the car hasn’t been used for a long time?
In: Engineering
A chemical battery (like in a car) is two chemicals that want to react with each other separated by a barrier. The chemical reaction that wants to happen involves little energy balls (electrons) going from one chemical in the battery to the other. BUT, because there’s a barrier between the battery’s two components, the energy balls can’t transfer directly – they have to go out one end of the battery, through whatever circuit we have hooked up, and into the other end of the battery to get at the chemical inside that half and complete the reaction.
So why do batteries “leak” or “self-discharge” over time? Well that barrier is a physical thing, and it’s not completely 100% perfect seal at the atomic level. This question compares pretty well with “why does a basketball go flat if you leave it for months after being inflated”. Isn’t it sealed by the valve? Yes, but every seal leaks at least a little and in time it adds up. In the basketball, air pressure forces air out through the valve. In battery, there’s a sort of “chemical pressure” from the reaction trying to happen that eventually leaks through/around the barrier.
Plus in the case of car batteries, one of the two chemicals is strong sulfuric acid, so it can physically eat a hole right through the battery and completely wreck it after a long time rather than just slow self-discharge leaking.
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