The first iBook Clamshell came with a battery of 3200mAH. New replacement batteries with more powerful cells have 4000mAh, 4400mAh oder 4800mAh or even more. They come from china and one a few have certified quality.
You will get replacemnent batteries for 60$ which are manufactured in China, only a few have a certified quality.
If your new battery won't charge it might be a problem with the internal PRAM capacitor of your iBook.
The iBook has no internal backup battery which saves the contents of RAM when no power is available.
If you still use Mac OS9 you will gain up to 2 hours additional battery life because od the low power consumption of the OS.