“How should I charge my car battery when it’s not being used?” I run into that question occasionally, and being that I worked for a battery charging manufacturer as an electronic tech a couple years back, I feel obliged to respond.
If by ‘not being used’ you mean that the battery is sitting on a shelf (or in an inanimate vehicle), you should get one that is made specifically for deep cycle batteries.
The problem with wet cell batteries (car batteries and the likes) is that they depend on the juice stirring effect caused by driving (the bumps, acceleration, deceleration, etc. shake/stir the electrolytes). If the battery sits for long periods of time (no shaking), the electrolytes stratify (think of how oil and water separate given enough time). Anyway, this damages the battery irreparably (dendrite growth causing shorts between plates). In order to overcome the lack of shaking/stirring, these type of chargers actually periodically overcharge (for a short period of time), causing electrolyte boiling (which stirs the juices).
Side note: Placing the battery on the ground can exacerbate/accelerate this effect because of temperature differences between the air and the ground (some think it’s the ground grounding the battery, but it’s actually the temperature difference). The ground temperature is much slower to change than the air, so always place the battery on something like a piece of wood to partially insulate it from the heat sink.
That’s my two cents.