Precision M4700 powers off when removing AC adapter, even with functional battery

Hi all,

I have a ~2012 M4700 that has a history of battery issues. I'm trying to resurrect it after moving on from it a while ago.

TL;DR: I have Ubuntu 21.10 running on it quite well, except that when I unplug the AC adapter the machine shuts off after ~5 seconds. This happens in the BIOS as well. The battery is currently at 100%, and if I boot the machine up with just the battery, and no AC adapter, it runs fine. Plugging in the AC adapter also causes no issues. The charger and battery are after-market (as I attempted to debug the original battery issues). The original battery won't charge at all, although the original adapter does seem to charge the new battery. Perhaps there is a charging circuit hardware issue, where unplugging the adapter is unintentionally tripping the power off signal? Pressing the power button would normally prompt me to turn the machine off, but that is not currently happening.

The BIOS is A17. I've been attempting to update that with the instructions here , but I've run into an issue there as well, so I'm looking for a way to avoid that. I'd reinstall Windows if it allowed me to update the BIOS more easily, but I don't have a separate Windows machine to create the boot stick (I think that's how it works?). I did just install Boot Camp on my Mac so there may be a path forward there.

Any advice welcome, and apologies if I've broken any forum rules.