Battery not detected, what is the exact fault?

Powerbook 12-inch 867 Mhz which I got for a song. No battery, but I soon fixed that by transplanting my other powerbook's good battery into this unit. It was ok for a day but now the battery cannot be detected. It works fine if I move it back to its original unit. Where should I look at next? Everything else works except this no battery detected problem.

I have replaced the DC-DC Board but it didn't fix the problem. It's unlikely the replacement board has the exact same problem. I guess the logicboard is the culprit here. Reminds me of an 800 MHz iBook I still have that works fine except that it won't charge the battery. The battery charges OK if I put it in another iBook. I've replaced the DC-In board and cable with one known to be good, but it didn't help any.