you won't want to hear this, but i think you have a lemon of a battery
the battery draining thing is done for nicad and nimh batteries, not liion bateries
draining it to below 50% percent is advised, which you obviously have done
outside of some software problem i have never heard of, it sounds like you got a bad battery
basically, if under warranty, get a new one
if you are past your warranty...again, you will not like to hear this...get a new battery
your problem is the first time i have ever heard of such a thing as a tech, but talk to an apple certified technician like alphatech and see what he has to say...i would say PM him asap
good luck on your ibook battery problem