how to fully decharge your battery

hi, i got the old ibook battery problem - percentage goes down to about 85% and then the ibook turns off. tried resetting the pmu and recalibrating the battery, nothing did work. i think i remembered i read somewhere it could be helpful if you fully decharge your battery with a lightbulb or something, cause if its in normal use there will always be some energy left in the battery. does anybody know if this could help me, has anybody tried this yet? any help appreciated

 

the battery runs just fine, i don't know 2 or 3 hours. it's just that the status indicator in the menubar can't calculate the remaining time and counts from 100 - 85 like a normal battery would from 100 - 0. so it's not that big of a problem, but it's kinda annoying. i read lots about this problem at the apple.com discussion, but they must have deleted the old threads, anyway this is not a software problem, a new battery would do the trick but i ain't gonna spend 70 bucks just for knowing how much time is left so i thought i'd go the cheap way