Re: don't wait 20 mins !
Yup. Although you would be shocked how long it takes to charge a car battery at idle. Alternators do not put out anywhere close to full rated power at idle, and they taper off current rapidly as the battery charges. In my camper my experience is going from 50% to 80% charge on a 40 Ah battery takes close to an hour of idling. Getting that last 20% takes a solid two hours of driving at highway speeds.