Which version of the iBook have you got?
I've had this issue before with the iBook 12" model. When the battery was not in the iBook it worked fine.
When it had the battery in the iBook it wouldn't boot. It turned out to be the ribbon cable from the DC inboard wasn't attached properly to the logic board. It's easily done when you remove the base and shielding. They attached the ribbon cable to the shielding which means that it comes unclipped when the shielding is removed.
This tends to only happen with the 12" iBook. If you have removed the DC inboard, it easy not to have attached the ribbon cable at the logic board end. Those clips on the logic board are very fiddly and often don't sit right when you put the ribbon cable back in.
Sorry I didn't realize you had a working iBook without the battery, other wise I wouldn't have made the other suggestions 🙂