Chosen Solution
I have replaced the battery in my iPhone SE with two new batteries, both unable to work out how much charge there is (sometimes stays at 1% and changes when I plug/unplug the charger) The first battery I used made the phone restart (even while charging) every 3 mins or so, the battery I’m currently using is staying around 58% and is telling me to service in battery health. Both batteries were from eBay and I’ve tried basic troubleshooting like a software reset, updates and checking that the connectors are pressed in properly.
Do a thorough visual inspection, preferably with magnification, of the area surrounding the battery connector. There are several tiny SMD components just above the connector. Of particular importance is FL2400, hghlighted in red. If it is missing (which can happen when you pry the connector off), then the battery state does not get communicated to the CPU and this can also cause charging issues and fluctuating readings.
This can be repaired via a micro-soldering repair.
