I found a workaround to boot up the phone though, but I'd like to be able to know what's left of the battery anyhow!
To boot up my device I have to disconnect the battery and connect the charger. That way I'm able to boot up download mode. Once I'm there I reconnect the battery and choose start. The phone boots just fine and I don't experience any issues at all.
It all happened when I switched out my cracked display for a new.
I even tried replacing the battery, as well as the flex cable connecting the USB port to the motherboard.
The only thing I could imagine be wrong would be some hardware sensor failing, but then again I don't see how that could have happened.
Is that sensor on the motherboard?
A thing I haven't yet tried is a factory reset. Would that have any impact?