suggestions on troubleshooting my buddies laptop, boot up powercut

so what is going on is when you press the power button it tries to boot up the computer but sounds like it just doesn't have enough juice and will turn itself off after a few seconds sometimes even before i have a chance to get to the bios. once it turns itself off it waits 5 seconds and tries again. I don't get any beep codes or flashing led codes to decipher the problem. The very first thing i did was to check the power adapter with a dmm and it is within specs and also i would get a white light when adapter is in and battery is out. When the adapter is in and the battery is in also i get a solid amber light which would tell me that the battery is just low. I let it charge for over an hr then took out the cmos battery and tested it and it was good at a little over 3v i had a ton of those batteries so i replaced it for shits n giggles, after that didn't work i started taking out modules to narrow down the list of culprits so i took out the optical drive and the same, took out the memory stick and tested it with one of my spare laptops and it passed, i even took out the hdd so all that was installed was 1 stick of memory and that was it and it still couldn't boot up for much longer than to get into the bios and i might have 3 seconds if i was lucky to look at something before it would restart. i held down the power button for over 30 seconds upon taking each module out and the battery out each time to clear any floating electricity from the mobo and nothing. I am out of ideas any suggestions? the only thing that i can think it would be is either the something with the mobo or the battery, however i am pretty sure that any computer will boot up perfectly fine with no main battery in it and just connected to the power adapter?