F16man 15 minutes at idle will not charge your battery.

most all car/truck alternators do not produce rated power till above 3000 or so car rpm.

you can buy some aftermarket alternators from monster power but they are about 400 dollars and up but they are designed to generate 80-90% rated power at idle-1100 rpm.

Harbor frieght has a small 800 watt generator for 130 dollars. plug it into a 30-40 dollar car battery charger and then hook that up to your deep cycle battery and have a great field charging set up for around 200-250 dollars.