Local track has power but local flying field does not, I use one of these batteries to power a quad charger at the flying field and can charge many packs with it. In my personal experience a car battery will drop voltage fast and cause a lot of chargers to error out, batteries made for constant, heavy draw are better for a charger. Wheelchair packs are great and come in even larger sizes.

UPG UB12350 Sealed Lead Acid Battery For Trident Front Wheel Drive