Power management. Or faulty battery.
Normally, the battery is not supposed to RE-CHARGE if it's not below 96% charge. It's design to prolong battery life.
The problem you are facing is usually caused by YOU not completely discharging the battery at the first use.
I think it's a fixable problem. Just unplug your computer form the wall, and keep it on untill it goes into a sleep mode after the battery is empty. Then wait an hour or so and recharge the battery. If it still charges to 96% only, check out your power management options.
If still occurs, it maybe a faulty battery. (I've seen many of those)
Send it back to apple, and you should get a new one within a couple of weeks.
If you want your batteries to last longer, "empty" them completely atleast once every month. I'd also suggest not to leave the battery in the powerbook if you are planning to be connected for a long time to the wall power outlet.
Following those procedures will most likely prolong the life of your batteries up to 100%.
Cheers.