I should have mentioned. This happens when the phone is plugged in and charging.
When the phone is fully charged, I unplug it and have a different problem: the battery drains unbelievably fast.
I unplug the phone, get on the internet for about two minutes, and the batter charge is 87%.
When the phone is plugged in, and I try to use it, the screen does not stay on for 15 seconds, or anything close to that. I might get three or four seconds, certainly no more than that.
I am not a great phone enthusiast. Before I got this S5 I had an old Motorolla Bravo. The Bravo may be old, and low end, But I can still use it for about 3 days before I have to charge it. With this S5, I doubt I could get though a single day, certainly not if I use it much.