@Emperor if you recently upgraded your phones firmware, that might be the problem.
When i first transformed my mx4 pro from chinese to international i had the same problem.
Every time it went below 40%, it dropped down to 0 in 1-2 minutes.
Sometimes it wouldnt even charge over 90%
Try depleting the battery to 0% until the phone turns itself off and let it charge turned off for 2-3 hours.
Do that 5-6 times, and if it doesnt fix itself, try another firmware or buy a replacement battery.
Batteries cost 5-8 euros
You might need a battery replacement. Don't think you can fix it through hardware.
Please dont post everywhere when you dont know what you are talking about, just because you want to increase the post counter on your profile