i have an iphone 12, which i’ve had since it’s came out. the battery health is at 77 percent. recently it has been turning itself off and then when it turns back on (in less than a minute) the whole battery is drained. last night it was at 28 and turned off and then it was at 10 when i turned it on. i plugged it in and it turned off again. then when it turned on (within 2 minutes) it was at 50. tonight it happens again, phone is on 48, turns off and then turns back on and is at 10%. how can i fix this?