so on two separate occasions now, my phone has shut off and an image of a gray battery with a white loading circle shows up. Both times, this is happened when my phone is plugged into a car charger. The car charger is a generic plug with a USB port and I am using the charging cable that came included with the Galaxy S4. In both situations my battery charge we well above 50%. Both times I had push the button to turn off my phone screen and about five minutes later when I go to look at my phone again I click the Start button and my screen doesn't go on. Then after a few seconds that gray battery shows up and I have to do a long press to turn on my phone. I'm not sure if it is related to it being plugged into the car but I thought I'd include that because it's the common denominator.
Sounds like your phone is turning off while charging for some reason. Could be your third party charger. It may not have the correct output voltage for the phone or may be faulty in some other way.
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