Why does my new battery keep dying with a good alternator?

My son is away at school so I know I need to start his car to keep the battery from dying. I start it and let it idle for 30 minutes a week. But after a few months the battery dies. Does idling it not regenerate the battery...do I have to drive it? Is 30 min a week not enough. I have had the battery replaced 3 times in the last year and half. The current one died after just 2 months. My alternator was checked out as being ok. Help