Dell 600m NOT powering on

I have a Dell 600m which was working well until yesterday. I unplugged it and while i was working on it (about 5 minutes later) it shut off. I was unable to power it back on. I removed the battery, replaced it and restarted it and again and it started but about 1 minute later it shut down. The computer started normally when it was plugged in. Then I smelled a burning odor from the battery (near the reserve battery). I shut down the computer and then have been unable to power up even with the computer plugged in. I removed the reserve battery and it appears undamaged. However, the computer will not start up even when it is plugged in. Could it simply be the reserve battery? Is the reserve battery needed for start up - even when it has a battery and it is plugged in? Is this likely something more serious? - ie Motherboard. I doubt it is the AC adapter as the green light stays on when it is plugged in. Thank you.