E560 randomly going to sleep or shutting off completely

[tpmccusker]

Hey all, long time lurker, first time poster.
I've had a fairly nice used E560 for a little while now. When I got it there was no battery, RAM or HDD. I installed some RAM and an SSD and set off, but it would randomly go to sleep or hibernate during use. I'd be using it at any point and then it would make the Windows 10 low battery noise and either go to sleep (and not wake up) or crash completely...despite not having a battery. It's as though it's behaving like it's battery is completely dead and it's shutting down due to low power.

I bought a decent battery, and it still does it whether or not it's plugged in. It will do it with light web browsing or heavy CPU benchmarks. I've adjusted BIOS settings, Windows power management settings, reset everything to default, reinstalled Windows, grabbed the BIOS updates and tried with and without firmware installed.
The only other hints I have is that in the Windows Event Log it seems to talk about the speed of the various CPU cores being limited by system firmware right before it happens.
There's also mention of a "Critical Battery Trigger met" despite there not being a battery half of the time, and a single error about the Embedded Controller not responding and to check for an updated BIOS.

Very excited to hear of any possible solutions, I appreciate all of you!

[tpmccusker]

Doing some more digging, I've found errors about the Embedded Controller returning data when none was requested, and that "The BIOS may be trying to access the EC without synchronizing with the Operating System".

Very strange.