Ok here's the breakdown: I just bought an iPhone 5s from a girl on the Internet. It wasn't in perfect condition but nothing was wrong to make it not run good. I was only on the phone for about 20 minutes and it was at full charge when I got it. It shut off at 20 minutes from dying so quickly. Then today it did the same thing. The battery percentage drops insanely fast and charges in weird leaps sometimes too. I assumed that I would have to buy another battery which I can't really do at the moment. I called the Apple Store but the guy that answered was a little impolite and seemed like he was just trying to get me off the phone already. He said I would need to buy a new battery for it.
What I'm wondering is if this might just be temporary? The person I bought it from told me she replaced the battery in August of '15 and hasn't used the phone since November '15 (roughly six months of non-use). Could it just be that the phone hasn't ran in a while and that it just needs to be drained and charged a few times? Or is a new battery the only solution?
P.S. I just spent the money on this new phone, and I don't really have enough money for a new battery from Apple which I was told is about $100. Are the self-help battery kits on amazon and such reliable if needed?