Being a field geologist, I have successfully used my 4runner battery to charge a set of four mavic batteries with the car charger and block. I built a fused auxiliary charger circuit connected directly to the battery, bypassing the car circuitry. My rig also charged the controller and two of those cell phone chargers that keep my controller at 100% while I am hiking from site to site. The next morning (cold), the 4runner started immediately.
My last field trip was cut short because I tried to charge the iPad using one of those common little bucks that charge cell phones. Just got a 24W version to charge the iPad next trip.
After a not-unpleasant hike out of the volcanoes to a nearby highway (17 km) three years ago, I got a Weego to jump start my car and recently found that it fits in my case and charges the iPad while I walk.