Following this advice will ruin your battery for you...Lead Acid batteries do not last long when deeply discharged... Do you really think you can recharge a big battery in 15 minutes? If you are going to use a Lead or gel battery as a power supply, you need to properly re-charge the battery with a battery charger before flying each day. It can be done, but not by hooking to the auto's 12volt system for brief periods of time.
ORIGINAL: f16man
Wire the battery into your charge circut on your vehicle , so when the battery charge is low just start up the truck for 15 min. and your good to go, most all cars and trucks now have at least a 60 amp alternator and I know my truck has 120 amp.