U620 Battery Life Problems

A couple of months ago, I bought a Samsung U620 on eBay. The seller was a legitimate Verizon dealer, and the phone arrived sealed in the original packaging. I was able to activate it with no problem, and it worked perfectly for a couple of months.

For the last few weeks, however, the battery life has been very poor. Right now, the battery lasts an average of less than a day and a half. I've been trying to think of reasons for this. The phone isn't analog-capable, and lately I've used it only in the city where there is good service. Bluetooth is turned off. I haven't used it an unusual amount, maybe 10-20 minutes of talk time a day, and I don't use it for any kind of EVDO internet for my computer or anything.

Apart from the battery, the phone works fine. I'm pretty much out of ideas. I'm beginning to think the problem might just be a bad battery or something. Has anyone else had this sort of a problem with this phone. It's still well within Samsung's warranty period, I was thinking of just sending it in for a warranty repair. Does anyone have any experience with Samsung's warranty service? I'm just wondering if it's likely that I will actually get it fixed, or just a bunch of excuses for why it's not their problem.

Thanks for your help