Well, you can always replace the battery. A Seido replacement isn't all that expensive. I think mine was... 17 bucks? On Amazon.
As for a factory restore, it will return the phone to a like-new state which means all of your data will be gone. Now, for your contacts, calendar and mail, they'll be restored when you re-log into Google through your phone.
If you're worried about your apps, I'm pretty sure you can use Titanium Backup (in the market) to backup your apps to your SD Card (this can, and will, save all the data you put into the apps such as log in information and other game saves), then use the same app again to restore them after the factory reset. You'll have to redownload it from the market.
There's a few options to try and get it back on your own. If you these don't work, then my only other advice is to go a Sprint store and see if they can help you.