1990 240 DL. Car History - Unknown, I just got the car and it's been sitting for a few years in a garage, but I was told that prior to that it was a daily driver and ran fine.

Had to put a dying battery (it's from another car, it holds a charge but dies after a few starts) in the car to drive it home. No engine warning lights, ran fine over 15 mile drive. Today I was cleaning out the air ducts which involved leaving it running and the fans going for ~10 minutes. I did this, turned the fans off, let the car run for a minute before turning the car off. Went to start it 10 minutes later and it's dead. No big deal, I knew the battery was bad and this would happen. Waited a few hours, went to start it, car starts... BUT, now the Check Engine and SRS warning lights are on and the car died after running for a few minutes.

I haven't put a new battery in yet, but I'm wondering if the Check Engine and SRS warning lights are just because of the battery, or did some cleared codes just show up again and i'm looking at some bigger issues with this car?

 

Yes, both are likely due to a low battery. You are lucky because you can clear both codes easily without any software.

Start by putting a new fresh battery in the car. Clean the terminals on the car, too.