Generally batteries also have a tolerance. So if the battery has a 3 hr life and u used it for 2 hours, it's safe to assume it needs a 2/3rds charge. If a full charge takes 9 hours, you can safely set the timer for about 6.5 hours and it might over charge like 5%, which won't kill it, and it'll be full. the danger is if you use one that constantly is set for 9 hours, so you end up overcharging it by 1/3rd instead of like 1/20th.

Problem with a battery pack and getting a dV/dT temperature based charger is, while the thing can detect dV, it can't detect dT w/o a probe. Mmm... probe... *snaps on glove*