Do you suspect from your phone battery? Testing today's mobile batteries is very easy: Lay it over a flat surface and make it to spin.
Does it spin freely? Replace it as soon as possible. Your battery is defective and can cause several problems to you, like explode or burn. Fortunately the first sign on a defective / old / aging li-ion battery is swelling. And a swollen battery spins nicely!
My one year old phone worked fine until I noticed it got turned off on heavy loads, for example while video recording. I thought it was a software problem but I also noticed a much shorter battery life, and surprisingly, a much shorter charging time.
Another clue pointing to the battery is its strange behavior: I start to record a video and the phone turned off itself after a while. Then I boot the phone and it warns of low battery level, something like 3%-5%. Then I plug the charger for only one minute, and after unplug it, battery displays around 60% of charge.
This behavior is not new to me. This is the way a high internal resistance battery works. And high internal resistance in li-ion batteries means defective battery. Once I removed the phone's cover I noticed the battery was sightly swollen. After replacing it with a new one, phone worked again fine as usual.
So, my advice is check your phone battery from time to time, It will avoid unpleasant surprises.
Miguel A. Vallejo, EA4EOZ