I was doing a big job last weekend that required more flight time than my 3 battery packs would allow and needed a way to charge in the car. I tried purchasing a power inverter for charging the batteries using my standard wall charger, but for some reason it wouldn't work, even though it was a pretty decent one. Has anyone else had this issue? I ran across DJI's Car Charger, it looks really nice but $85 seems a little steep to me. How do you guys charge on the go?