Lately, for the past couple of months, the battery in my 18-months-old iPhone 5S has been acting strange to say it at least. It takes about 15 minutes of intense use (listening to music + browsing) for the damn thing to go from 100% to 80%. I do not know if that is normal or not. The real issue is that it can go from 38% to 1% in a split second. Then when I charge it again, it sometimes jumps back to the 38% – sometimes it does not. I have no idea what has happened to it. Any suggestions, guys? Is this a well-known issue?