My phone is Xperia SP, it's on CM 12.1, unofficial build (because the official one is buggy due to bad kernel). It has been bought second-hand, so I'm not sure about its history (the phone was in a good state).
So, the problems started gradually, but the symptom is this: when I connect the phone to the charger, the phone goes into it's charging state, the battery icon shows the charging symbol, the LED shines red, but the phone doesn't actually charge, and the battery percent actually actually descends. I would usually just detach the phone, and reconnect it, and it would actually start charging. However, this happened more often as the time passed, to the point where it is completely empty, and it cannot be turned turned on, or charged. When I connect it, the LED goes red (the usual charging behaviour), but then it goes into booting the system, instead of going into "offline charging" mode. It fails to boot the system, because the battery is empty. It then turns off, returns to the red LED state, and from time to time it boots into recovery, which is quite unusual, since by default one has to press volume buttons to enter recovery. While in recovery, usually it would display the battery percentage, but now the place which should show the battery percent is empty.
Are these the symptoms of an faulty battery, or a more severe issue? Did anyone experience anything similar?