I had a battery that was being charged at 17V from my overcharging alternator. One symptom was that the battery had no(!) charge one day. A trickle charge restored the battery but, well, I was scared to leave that battery in the car.
I bought a new battery. The battery dealer was just fine with me keeping the old battery but he said it should be charged every couple of weeks in order for it to be usable. Every couple of weeks seemed like too much of a hassle so I left it there.
I drove hom wondering if I made a mistake.