iPhone 3G with a 3gs battery!?!?

Hello help please, today i received an iphone 3g that does not hold charge, the person who gave it to me said that the battery was new but when I plugged the phone in the battery stayed at 5%.When I opened the phone I checked and I saw that the battery is actually for Iphone 3gs do you think that this is causing the issue?? I read that 3gs batteries dont work on 3g, at the same time i read the opposite some people say it works perfectly???