Phone works on power but not on battery
My sister owns a Note 2 which is now our of warranty. I've rooted it, flashed a TWRP recovery and gave it some custom ROM love. It has been working perfectly for months.
However, recently, a weird problem has occurred. The device works in charging mode but when the charger is removed, it runs on battery for a few minutes then dies off right away. She has to sit next to a wall plug to use the device.
Now, obviously, my first guess would be to replace the battery. Here's the surprise - the problem is not with the battery. They (third party service centre) took out the battery and insert it into another device, and that device worked perfectly fine. They are going to charge me just to take the device and diagnose it - let alone to fix it. So, I came here to seek some help in case someone knows actually what is the issue.
I have not flashed a new ROM or anything yet because the juice that it holds is pretty risky to play with. But of course, if it is needed, I have no problem doing the troubleshooting.
Any help is greatly appreciated.