First of all, I know this question is duplicated, (this is the original question: Battery Popup at 25% ) but I can't comment or add any reply and I have some information that could be useful to identify the problem.
This is the Pop-Up:
The original answer states that it can be a default system pop up, but my system is in spanish and the pop up is in english. The pop up stays on top of the screen and there is no way to hide it, not even by the recent apps button nor the home button. Even locking the screen keeps the pop up visible. The only way I found to get rid of this is with a soft reset. In fact, the pop up looks completly different from the system default notification shown with the battery at 15%
My device is a Samsung Galaxy S6 with Android Nougat (7.0)
I think it can be some kind of malware, but I've run some security apps (AVG, Dr.Web, ESET) and none of them found anything.
I've not installed any apps recently and nothing outside Google Play
Any help will be much appreciated.