Inspiron 8100 charging ISSUES

Hello everyone.  

My friend brought over her Inspiron 8100...she said it was "dead". I tore it down and found a loose DC power board. I put it back together and it booted right up. Yay!

She brings it back 2 weeks later saying the battery won't charge. It runs perfectly using the AC adapter, but only with the battery removed. So I began testing...

I removed the battery and plugged in the AC adapter. It booted right up into XP. I then inserted the battery and the laptop changed from adapter power to battery power (this should not happen). The battery low indicator came on since the battery charge was low and it was not getting charged from the adapter. I removed the battery (laptop is still plugged into the wall), and the laptop shuts down (this should also not happen). I press the power button and it boots back into windows under the power of the AC adapter.

Here is what I do know. Just 2 weeks ago the battery held a charge for ~2 hours. I replaced the AC adapter and I know it works. Putting a battery into the system forces it to use Battery power instead of using AC adapter power and charging the battery as it should. The laptop will not charge the battery ever. Not even when its turned off and plugged into the wall. The battery charge indicator light on the laptop blinks green, but battery does not charge and adapter does not get warm. The AC adapter only works when there is no battery in the laptop. For some odd reason, plugging in the battery totally bypasses the AC adapter.

Also, when the laptop is plugged into the wall with no battery, the green light on the adapter is steady. As soon as I put a battery in, that light begins to pulse...bright to dim..continuously. If the laptop is on, the screen pulses also, likes its getting power pulses....strange

I have a pretty good idea what the problem is, but I don't want to lead anyones thinking, so I won't say yet. Here is your chance to shine! What do you guys think the problem is?

Thanks in advance!