V50 2.0D engine does not start

Hello to each other, I just bought the Volvo, V50 with the 136PS 2.0D Bj. 2004 with 158,000km. Test drive and transfer drive to my home was running like smeared, even under load there were no problems. Now the car was standing in front of my house for about a week, because I only got an appointment at the registration office this Friday. When I showed it a few times to my friends, the engine also did not cause any problems. He only indicated for a few days that the battery is almost empty, so I had let it hang on to the charger since then. Today the DVDs arrived for the Navi. I wanted to try them again, because he still showed me that the battery is almost empty and that I did not let me into the Navi because of the power pair mode, so I thought I just started the car and make it fast with the running engine. So now I’ve let it start again again, because it’s still normal and after about 5 seconds just went off. I tried to let it start again, but it really started again.