I had the same problem.
My phne will charge with the generic battery till 40% then it normally jumps to full.
When using the phone, and I get to about 70%, it shuts down and I have to battery pull. Battery life is terrible, 3 or 4 hours when in use.
I have 2 batteries, a new "generic" one and an original one which is pregnant/bloated.
My problem has now been sorted.
1) Let my generic battery drain till phone switched off.
2) Plugged into PC with USB cable to charge a little, just below 20%.
3) Did a security wipe on the handset.
4) Plugged into PC again and did a wipe with BBSAK. Did this twice just to make sure I got all the suckers.
5) Installed OS barebone. Installed 6.0 Bundle 2534 (v22.214.171.1240, Platform 126.96.36.199). This is the latest OS for MTN South Africa, the network I'm on.
6) Its been a few days now, and haven't had problems.
I am going to try re-install the latest OS available soon, just going to make it run as it is for a few days.
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