You keep ignoring the good advice.
And you contradict yourself: "The laptop only turns off when the load (CPU and or GPU) becomes too high." and "But if it's completely (100%) charged, it works well, even under heavy load."
So please decide and to not troll.
The laptop only turns off when the load (CPU and or GPU) becomes too high.
(Apparently with a high battery load)
If i reduce the brightness and reduce the CPU performance to <50%, it will work until about 15-20% remaining battery capacity.
If I limit the CPU performance more than <50%, it works even up to 5% remaining battery capacity.
The problem is not with de board itself but with the soldered in connector there... It simply does not connect well over time... It just worns out.
Your problem may be the battery. Do you have the capability to "load test the battery"?
How quickly does the battery drain its' power?