iPhone 6 Battery life

iPhone 6, have replaced battery 3 times, even different APN’s, battery standby time less than 48 hours, have factory reset phone and also installed new OS still the same, any suggestions ?

have noticed when batteries are flat, the phone shuts down and cannot be started unless the physical battery is unplugged and plugged in again then the charge symbol appears and charges normally.

am guessing its the charging IC