Chosen Solution
Yesterday I have replaced my iPhone 8 Plus’ battery with an iFixit kit. The repair went smoothly, but I have realised an annoying issue: Yesterday, I have used the device normally, and around 15% the device shut down. When I plugged to the charger, it showed 11%. Then I charged the device to 100% to start the calibration. Today, after I have plugged off the device around 9 o’clock, and around 8 pm, the device turned off again with 50% charge. Then I plugged the device to my Macbook, the iPhone booted back, and I launched the CoconutBattery. According to iOS, the iPhone’s battery was only at 12%, but the CoconutBattery showed 46%. Should I try DFU mode and do a fresh install of iOS 15? It will solve the issue? Or the battery is bad?
There is something wrong with the gas gauge circuit. This circuit tells the CPU all about the battery health and charge. When one of its components fails it will produce the symptoms you are seeing. If you can I would put the old battery in again and see if the problem is the same as the battery is the first thing you need to rule out. You also need to make sure the connecter on the logic board is clean and the battery connector is seated properly. The most common cause of this on the 8 series after a battery replacement is nudged components. There are a number of tiny components around the connector that if nudged will produce this problem. You will need to examine around the connector with some magnification.
