Power Circuit issue, won't run off AC yet DMM tells me board is getting power

[seiferoth10]

Firstly, if it helps any, laptop in question is a Sony Vaio VGN-N250E.

My issue, in bulletpoint format:
0.) Laptop won't turn on when only on AC adapter. It runs when only on battery power, but when on AC adapter and battery inserted at the same time, the battery is not being charged.
1.) The AC adapter works, confirmed via DMM into DC plug side registers ~20V.
2.) The connection from the AC adapter plug to the board is solid, confirmed via DMM on the connector of the motherboard when AC adapter plugged in also shows ~20V.
3.) I'm assuming power is getting to the battery, because the battery prongs on the motherboard register ~5V across them when the AC adapter is plugged in.
4.) The battery itself is a Li-ion rated at 11.1V/4400mAh. It's an old battery, but when fiddling around to get to this point I managed to run it on battery only. I plugged in the AC adapter when it was on, but the battery critical lights kept blinking, and it eventually died.

Any ideas? The laptop won't run on AC adapter only, which is the mind boggler because my DMM confirms it gets power, at least at the mobo plug.

 

[Adamh94]

could be a dc fault. get a new dc cable first to elimate that being the issue, VGN-N series are awful for dc problems.

 

[seiferoth10]

The DC cable is perfectly fine. The solder pads that the DC cable connect to, as well as neighboring solder pads, all show the proper ~20V.

Anyway, I'm pretty sure I found the culprit. There's a few SMD components that look like they're done for, and they're all in the power circuit (labeled with a P before their label).

New question, is there a way to wire an AC adapter directly to the battery prongs on the motherboard? Of course I'd have to match the battery voltage and current, but that's what I was thinking as a possible solution.

 

[AidanRoberts]

This should always be possible, but there may be manufacturer settings to prevent such tampering. I would be very careful as well because you could end up frying your motherboard and everything else even with the proper current. If your battery connections are not working properly you could get in big trouble. Have you tried another battery?