Originally Posted by slvoid
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.
Wouldn't you solve this problem by leaving the light on when you get home until it is fully discharged and then recharging it for the exact amount of time suggested. For a commuter, that would mean leaving the light on while you have dinner. Then leaving it charging overnight. In the morning you are charged and ready to go.