Dying battery
 

Hopefully someone can help me with this. My 1982 Volvo 245 with the D24 engine just keeps dying. Allow me to elaborate, while I was at basic training my friend borrowed my car for a week and he left the lights on so the battery was completely dead when he was going to take for another drive now he got the battery charged back up and the car started right away but when I got home I noticed it started taking longer and longer to start and my voltage was dropping and my headlights were completely off(I was driving at night). I thought maybe it was just the battery so I bought another brand new battery and that worked maybe 3 days before it went back to the same issue. I had the alternator tested and it's working just fine. Can anyone help me!?!

was the alternator tested in the car, or was it removed and bench tested?

the test in the car is to read the battery voltage, then start the car and read it again, it should be at LEAST 1 volt higher, ideally 13.8-14.2 volts