I'll just ignore Zwerg01's comments here, as it's the usual trolling and you guys have already reacted to it
What happened:
The charger chip has an emergency battery shutdown. The temperature, when this happens is currently set to 39C.
This means: if the battery temperature reaches 39C, the charger will report an error and stop charging it.
Now, when I was visiting my parents this weekend, I charged it while it was idling around. The charger chip reported an error (red blinking LED), even though the battery felt pretty cool.
So I thought, that either that temperature limit is set too low or the chip uses the wrong temperature sensor: not the one in the battery (which was still cool) but the one on the charger chip (which is inside the case, together with the OMAP, where 39C are reached pretty fast).
After some discussion, I knew it was the battery temperature - and that a blinking LED simply means the charger chip reported some error with the battery. Doesn't have to be the temperature (and surely wasn't in this case, as trying again to charge while it was on worked without any issues and the battery reached 37C - and that really is pretty warm already).
You can make a register dump and see what error the chip reports, but I didn't know how.
So probably something simple after which you reset the error and continue charging. The currently very basic version of the driver can't do that yet.
So, all in all, nothing is wrong here.
The only thing to debate is whether the set 39C is too low (ideal operating temperatur for a Lithium battery is 15 - 45C), so we could raise it to 45C.
But then again, the warmer a battery gets while being charged, the faster it dies.
Remember how the GPD Win 1 eats batteries for breakfast?
They die within a year if you charge them while playing high-performance games, which really heats up the battery a lot.
Keeping it at 39C should at least ensure your battery survives for longer periods of times - and when we have a thermal management system running, it should handle that anyways and try to keep the temperatures lower.
I hope that explains all the confusion and makes more sense than the (once again wrongly assumed) stuff Zwerg01 posted.