The car charger charged a bit faster than the stock AC charger. However the car charger is quite fussy about the input voltage. When the alternator is not running, with the charger drawing 6A the car wiring voltage drop can cause the voltage at the charger to be close to 11 volts.
I thought the DJI charger only works when the engine is running?
This is true, the DJI charger requires 13.1v, so unless a vehicle is running (or within the first 10 mins or so after turning off engine, before it settles to high 12v reading) it will behave erratically.
I found it would sort of 1/2 duty cycle, flash a while, then cut off, return flash, etc etc . . . but it would never charge a battery from near drained to full, even leaving it for many hours hours.
This is not an issue if driving most of the day, you have say min 3 batteries, and you can charge as you go.
The 12v charger is quite a lot faster at charging batteries than the 240v (or 110v in those countries with 110).
One thing you can do is get a step converter for the vehicle, it provides reliable boosted voltage.
I bought one similar in look to this (ebay search your region) that converts 12v to 13.8v, and it works great at any time.
NEW Voltage Booster Power DC Converter Step Up Regulator 12V to 13.8V 20A 276W | eBay
Not exactly same one as mine, as mine is 15 amp, couldn't find one readily on ebay now.
The 20 amp one would be ok to use I guess, just use the appropriate inline fuse when wiring it up.
Bear in mind I have a 110amp/hr 2nd (agm) battery in my 4WD that I can use for running my fridge, led lights, and charging phone, mavic batteries and other small items.
A good main battery would be fine for charging one at days end when parked up, as long as it is kept sensible, not charging 4 on a hub etc.
I purchased a heavy duty female and male cigarette lighter plug, some suitably heavy (same dia as converter in / out wiring) dual core wiring, a fuse holder (with 15 amp fuse), and some shrink tube in red and black, and put this together . . .
It will work with the DJI 4 battery charging hub fine, but I have found with my 4 batteries I just don't need it when touring.
The normal single battery lead above is ample for my daily needs.
As said you might be fine just charging up as you are driving, using the DJI 12v charger, but it is a heck of an isoloated place to be without reliable charging if you find it isn't working for you.
Hope that helps.