I have had this around three years and bought at a refurbed. It's the older version of the SE. Issue is it seems rather confused about the charge state of it's battery sometimes, maybe every few weeks. Unless I am out and about, it sits on a charging stand of my own design, just a stand into which I fitted one of those magnetic charge connectors - lower it towards the stand and the magnet tugs it to make a connection. That part seems to work fine.
I picked it up this morning to use, it was showing its usual 100$ charge, I started to use it and it shut down with low battery. Put it on the stand, back instantly too 100%, picked it up and it shut down again. Repeated and all worked fine with 100% charge showing.
When out and about, I can return home with it still showing near 100% after several hours. Other times, the battery level can drop quite quickly.
Battery diagnostics show it as being a 90% good battery. Suspecting a poor connection to the battery, I have tried banging the phone and cannot reproduce any issues.
Might it be a firmware issue?