A Harbor Freight float charger is about $10 and is a lot cheaper than idling for 30 minutes. I keep my Crossfire batteries on them for the winter (removed from car only because my storage facility does not have electricity) and the 36-month batteries are lasting 5-6+ years. HF gets a bad rap but most small electronic devices come from China anyway.

I keep a good CenTech float charger on my Maserati almost all the time since I tend to only drive it a couple times a week in the summer and it is stored for the winter.