Battery Issue or Power Management IC?

[fxc]

Hi all.
For over two years, I've had a great time using my PinePhone as a daily driver under postmarketOS edge. Up to this point, I've been able to overcome just about any issue. However, I've suddenly run into a hardware problem, which I can't seem to troubleshoot...
Here are the symptoms:

1. The phone will start to boot, (green LED lights), but about 1 second into the ToW boot loader the LED just goes dark.
   
2. Plugging in a USB-C cable from my labtop results in a boot loop; the phone will boot until the OS splash screen flickers for a brief moment before repeating.
   
3. Plugging in a slow USB-1.2 charger allows the phone to boot normally. However, I noticed that my battery status indicated a 100% charge state. (I have always set the OS to undercharge to no more than 90%...)
   
4. The phone also boots normally on a slow USB-1.2 charger with the battery removed. The battery status indicated a 0% charge state, in this case.
   
5. In either case #3 or #4, all radios are disabled within the OS.
   

Given the above, I just assumed that I somehow had drained battery too far...
Except the following seems to conflict with this:

1. I booted from a SDcard Jumpdrive using a USB-1.2 power source. It boots fine to the Jumpdrive splash.
   
2. I then let it sit for hours attempting to slowly bring the phone battery back to life.
   
3. This Jumpdrive charge method did not seem to work. Pulling the charging cable immediately kills the phone, and attempting to boot from the battery has the same symptoms as above.
   
4. Measuring the voltage across the battery terminals shows 4.2V, though... This indicates a fully charged lithium-ion battery under no-load, correct?
   

So, at this point, I am at a total loss...
It would appear that I've a fully charged battery. However, the symptoms are the same as a drained battery.
I could really appreciate any suggestions that could help.

The symptoms do indeed describe a drained battery. Your point one from the second list is not a contradiction to that, Jumpdrive is one of the few things actually booting with a drained battery. The battery measuring 4.2V is strange. Maybe it would be a good idea to buy a compatible battery from Amazon and check if it works with it, otherwise you could always send the battery back.