G4 powerbook battery charge problem


My g4 powerbook will not charge the battery and when the battery drains the computer shuts down. I have done all the obvious. I own 3 chargers and 3 batteries. I can charge the battery in another computer place it in the problem mac and again it will operate fine until the battery runs out of gas. If I try to boot the machine without a battery, It trips the charger and I have to unplug it so it will reset. It sounds to me like a hardware problem and I don't mind going inside to replace the problem but don't know what to try and replace?