1998 Volvo V70 Please Help :)

This winter my car's battery died one cold night and the car wouldn't start. I got a jump and took it home and it ran fine for a few weeks. Then the service light started to come on and the battery died again. So I ordered the kit from CarMD. Com to see if the computer could tell me what was wrong. The car's computer did not send a code to the CarMD unit so I was back where I started. Then the battery died completely so I bought a new battery thinking that the battery was too old. ( I recently purchased the car)
so I put in the new battery and it worked for about a week, then I was about to leave work and the battery had died again. I work at uhaul and we have some battery chargers for the trucks, I left the charger on the battery overnight so I could at least drive it home. The charger had a green light (it's a new battery it should be fine) but when I turned the key the electricals went out immediately.

i don't know what to do, I don't make much money and I am still paying for the car. Please help me and make my life a little easier :p
thank you