FYI: charging karma batteries isn't officially supported by gopro (I saw it posted somewhere). Its probably for a liability clause. I don't know what the risk/likelihood a battery catching fire is. (And if the battery catches on fire while driving or you get into an accident... Well, you get my point...)
I've used "150w" inverter to charge my karma batteries in a parked car. My inverter was chirping most of the time indicating a high power draw. My inverter is something I got from Radio Shack a number of years ago to power my laptop when I need to do on call support while camping. I don't have the manual, and the labeling on the box is minimal. I think the "150w" is for "peak wattage", not "continuous". I'm guessing the continuous rating is around 100w. I've hooked up my charger with a Karma battery +remote while connected to a Killawatt meter and saw the draw was 93w. So, you can see, you may need to get a "120-150w continous" rated inverter.
Side note: after charging 2 batteries in 3h, I ended up needing to get my car jump started.(I didn't have enough juice to get the starter to engage). So, keep your focus and don't kill your car battery out in the middle of nowhere.