Chosen Solution
My iPhone 6 plus suddenly turned off so I got a new battery, with the new battery in, it turned on again, not long later the phone turned off again and the brand new battery is now dead, I have tried it also in another iphone and it is ruined, any ideas?
It appears there is an issue on your logic board that is shorting out the gas gauge board in your batteries. What happened to your phone just prior to it suddenly shutting off? Were there any drops or impacts, contact with water or repair attempts? It could just be a sudden failure on the logic board but these types of issues typically have some history. At any rate, this will most likely require a micro-soldering repair.
I was afraid of that lol it’s a second hand phone but it was working fine before and there was no drops or water, could it be the lightning cable as I repaired that a week earlier
