Help S3 Battery problem?

[Jonny34511]

So a couple of weeks ago when I was on vacation, I started noticing my S3 was charging very slowly, like I plugged it in for one hour, with the phone off, and when I turned it on, the battery had only charged 1%. So when I got home I was thinking I should look into this problem, but then it started charging fine. Then about a week ago, I had it plugged in the entire night, and when I woke up, I had the green LED light, telling me I was 100% charged, and when it was plugged in, it said 100% battery. Then I unplugged it, and it was at 40%, exactly how it was when I plugged it in that night before I went to sleep. And then last night the exact same thing happened. Should I get a replacement battery or is this a problem with the phone itself? I never dropped the phone or anything like that, and I also checked to make sure the battery wasn't loose.