battery problem w/ 450sl

Had recently replaced my battery in 80' 450sl (approx. 3 months ago) Car started fine, until recently, when hadn't used for approx 2 weeks. Went to start and wouldn't turn over. Jumped and started fine. Drove for approx 5 minutes. When went to start an hour later, needed to be jumped again. I know the battery was good when it went in. What could be causing this drain on the battery. Could it be the alterator or is it just that I needed to keep the car running for longer than 5 minutes to recharge.