|That's an old 486 motherboard. The P3 is part of the model number and not a cpu designation:|
Likely the battery is one of the barrel-shaped 3.6 v rechargeable ones, usually located near the keyboard socket. They tend to leak and corrode after a few years so if you locate something that shape that's nasty looking, that's probably it.
The link above shows an external battery connection. They all had those connections but usually came with a rechargeable battery.
A Batteries Plus store will have replacements or Radio Shack or a computer supply store.
Ubuntu Cheeta, Ubuntu !