My battery died while I was at work the other day for no reason (no headlights on, no dome lights on, no door ajar, etc) and after getting a jump start, I took the car to Wal-mart to get a new battery. Here they told me my car just needed to be charged. The next day, I got in my car to go to work and it didn't start again. Does this sound like the alternator? How much does it cost to replace?