The Centris 650 **DOES** have a battery on the motherboard. You just need to look more closely. Some older Macs will turn on if this battery is dead, some will NOT. So it's a good idea to replace this battery. It usually has a little black cover on it.

Try borrowing a battery from one of your other Macs and put it in the Centris 650.

You will probably have trouble connecting an ATX supply to the 650's motherboard. The signals and voltages may not be compatible and the supply won't fit properly in the case. I recommend that you do this ONLY as a LAST resort.