Yesterday I charged my battery, put it back in and cranded up the car. While it was running the HU read 14V. After about five minutes the car just shut off. The HU read that the batter now had 10V. I left it alone for about 20 min and tried to lock the door remotely but nothing happened. There was no electrical power in the car at all the lights, radio, etc. didn't work. I don't have a mechanic and I'm afraid the stealership might try and sell me a part that I don't need. What could be some of the causes for this issue?