I bought a used iphone 4s on ebay that was excellent condition about 1.5 years ago. It was great. Then recently i noticed my battery would go out really fast. Such as when you don't even use it, it would go from 100% all the way to 50 percent when i didn't do anything on it. I got a battery replacement and my battery now is much better. However i still have the issue where the moment it goes to 18%, it would shut off. I would then have to recharge it. Does anyone here have this issue and know how to fix it? Surely this isn't a battery issue since i got the battery replaced?