C640 Battery runs laptop - a/c doesn't

Verify the ac adapter is ok. I found somewhere else on this board a solution to this same problem. You have to take the computer all the way apart to access the back of the mother board. Just below the external power connection is a black rectangular component with 4 connections to the board. I found a cracked solder joint at one of these legs. I re-soldered it and now it is working great again. If I find the post with the component designator on it I will edit this and post a link. You will need good mechanical and soldering skills as well as the dell service manual to get it all apart and repair it if this is the problem.

Three things that could cause this. 1- power supply, check it for the proper output with an ohm meter. The aftermarket power supplies, such as Tagus, don't work well with Dell machine and will also cause this, 2- bad battery or engaged fail safe, try starting with the supply only, 3- failed power management system on the motherboard. Other than an aftermarket supply, this is most commonly caused by the power management system or PM chip. Unfortunately, a failed power management system or chip is common with these, and the motherboard will have to be replaced in order to fix it.