You should probably list the things you've already tried. Do you know if there were any problems before the incorrect battery was installed?
Without knowing more specifics, I guess my first approach would probably be something like this (steps one through four really don't need to be done in any specific order):
1) Remove all ram and reinsert one stick (don't just remove all but one stick, you actually want to reseat the singe stick that you're going to try to power on with in step 6).
2) Remove the video card and all PCI cards.
3) Disconnect the power cables from all drives.
4) Remove battery.
5)
Only if it has not already been done,
reset the PMU at this point. Resetting the PMU is not something that you want to do more than once between power cycles.
6) Try to start the machine.
Did you get a chime? If yes, start adding things back one at a time starting with the video card. If no chime, next step would be to test the PSU. Do you have a multi-meter?