Car camping in NZ for 6 wks. Need advice.

[SoloTraveler]

I'm going car-camping in New Zealand for 6 weeks. Most campgrounds have electricity only in the restrooms and service areas. In 2012, I car-camped alone for 3 months with a deep marine battery, charged up with battery charger plugged into my "cigarette-lighter". I've got the NZ plug adapter so I can plug in when 220V is available since my cpap can use 220V. But the rest of the time I will need a good battery that can somehow be charged by the car while we drive. I would appreciate any knowledge/suggestions on this subject. I want to buy the battery/charger system there, not try to take it on the plane.