I have a Samsung Galaxy S6, Stock ROM, running Android 5.1.1.
Last night, as I was browsing on my phone before I fell asleep, my battery dropped from 19% to 15% in an instant, engaging power save mode. It was concerning that the battery dropped like that, but I didn't think much of it. When I woke up this morning, I was using it again, starting at 9%. It then dropped instantly to 5%, engaging the Ultra power save.
I have never had my battery drop like this. I purchased the phone in November of 2015.
Attached are two screenshots, one from the Battery menu, and another from GSAM Battery Monitor.
Upon looking further at the stock settings graph, it seems to be dropping at other times too. Is this a sign of a failing battery? Should I bring the device in to my carrier for service?