I'm posting this here, because this is about a battery not connected to a vehicle.
I'm suspecting that I have a battery that won't hold a charge, even though it's only a few months old. I charged the battery (ground cable from car disconnected) last night and it was at 12.5v after removing the charger. This morning, while still not attached to the vehicle, the voltage was 12.15v. Is a "small" drop like that normal?

 

That's a very bad sign and I'd want to see what's happened in those months - perhaps a drain but not enough recharging but maybe something else. A typical battery needs to be fully charged rather than left part-discharged, or you get the result you're suffering.