Battery won't charge so I can only run on A/C power

these are my computer specs:

Model A1181 / 2 GHz Core 2 Duo processor

(C2D, Late 2006) MacBook 13"

The computer works fine while connected, but even though the battery is fully charged it will not take power from the battery. It won't even recognize it's there. I get a black X. I've used a different working battery and it didn't work neither. No liquid-spills have happened in this unit.

I've been reading around other people with similar problems and someone recommended a new magsafe-DC-In. but, if the computer gets power from the power cable plugged in, doesn't that mean that my MagSafe DC-In Board is working? also, I also read that it could possibly be the battery connector/sleep sensor, but that is usually the case after a liquid-spill, which hasn't happened.

help please!!!