If it's a newish battery (miles don't matter; how many months old is the battery?), then it should still last a while, if it doesn't, then it could be a bad alternator. If you are in doubt, take it to advance auto parts and have them test your battery for free. Then you can figure it out from there. Drive it occasionally (like once or twice every two weeks at minimum), and if it doesn't die again, then you're set.