All secondary batteries (rechargeable) self-discharge over time.

I've read somewhere that SLA is about 1% per day, it might be higher. Lithiums are much less something like 1/10 as much. Nicad and Nickle/Hydrid somewhere in the middle.

The primary batteries have have a chemistry that will slowly lose capacity over time, You've probably got an alkaline cell that has a shelf-life of up to 10 years.

For the SLA in the hangar or trailer, there are "float" chargers that are designed to offset the self-discharge, sometimes called a "trickle charger".

My experience it that most charge-then-trickle charger systems will do quite well for a duration of several months, but eventually the battery is useless, no matter what. Reference the battery back-up systems for emergency lighting in most industrial building.

So my process is this (when I'm using SLA) one battery on the stand-by charger, one on the regular charger, when I fly, take the standby, move the other one to stand-by and when I return put the battery on the regular charger.. This way I'm rotating the usage.

YMMV