Don't panic if your iBook battery won't charge. It might (and usually) be the Power Management Unit (PMU) problem. Resetting it should solve in most of cases.
Apple Support provides instruction for various Powerbook and iBook models.
- Turn the machine off
- Hold Shift+Option+Control+Power for 5 seconds
- Turn the machine on
Only side effect is your clock will be reset back to 1970. Easily change it back by using Network Time Server feature.