That does sound a little strange, but batteries can loose their charge eventually. It just seems thats rather a bit too fast. I'll try it on my TiPB tonight and see if it comes anywhere near 78%.
How many batteries do you have? Have you ever let the charge go all the way down (which you should do once in a while). Try letting the charge fade to 0, let it sit over night with no charge and then plug it back into you iBook to recharge. Once you do that, try the same thing again and see if you loose 22% of the charge over night.
