Won't run on known GOOD batteries. Works only on AC

Hi, I have 2 G4 15" 1.67ghz low res laptops. One runs fine and charges and runs on both batteries. The "bad" laptop will ONLY work if it is plugged in, even if one of the good batteries is used. I've tried all the usual things like pram/pmu resets etc but nothing works. The bad laptop also fails to charge either of the batteries although it does"RECOGNIZE" the good batteries in System Profiler.

Does this mean a bad DC & SOUND card is the problem or is there something else that could be at fault?

Thanks!