I just bought a used iPhone 5S for a friend. The battery doesn't tend to last a whole day even with minimal usage. Coconut battery says that the battery is at 83% with almost 1000 cycles. It's running iOS 11.3. Everything works well on the phone, there is no sign of water damage on the SIM card slot indicator. I'd think getting the battery replaced should be a good idea. However, when I go to Settings > Battery, in the list of apps using the most battery percentage, "Phone" has "weak signal" written below it. So do a few other apps like Safari and Contacts. I assume this means that cell signal is weak, causing excessive battery drain. However, I highly doubt the signal is actually weak, because we live in an area of very good signal. Also we're on the same network so if they had weak signal, so should I, at least every once in a while. I have to add that when browsing in Safari and doing some other things, the back of the phone gets hot around the Apple logo. Not too hot, just hotter than the old iPhone 4S or my iPhone SE would ever get under normal use. Do aging batteries tend to cause more heat? Or does the iPhone 5S simply produce a lot of heat under iOS 11.3? Can anyone with the same setup confirm? So here's my dilemma: is an 83% battery with almost 1000 cycles so bad that it should be replaced? Or is there some sort of antenna problem that is causing the battery to drain more than normal, and replacing the battery wouldn't really solve the issue? It's a used phone so spending too much money on it isn't worth it. What would you recommend doing? Thanks! PS: Please don't tell me to just buy a new phone, iPhone 6/6S/7/8/X are all way too expensive for us even used.