Battery hacking, replacing with power supply

Hi, I have an old Ipad I got at the dump. I took it apart, ripped the dead battery cells out and replaced one of the cells with a lithium battery backup cell I got also at the dump.

Not sure if this is dangerous to only have one cell in there, that’s not the same mah. How does the Ipad determine the battery percentage? It says it’s charging up to 100% then drops down to 50% immediately.

I was thinking of trying to make it into a photo frame, and removing the battery entirely and replacing with a 5 volt power supply that’s regulated to 4.2v.

Has anyone done anything like this?