seems there are a few different opinions on this subject, my 2 bobs worth, my RANGER has an episode last year running the bushman fridge and radio for 4 hours started the mongrel 3 times then when I was ready to go , NOTHING !!!!!! dash lights flickered , what the f is the battery flat, light bulb moment, I have 2 x75 amp hour agm's in the back, hooked one up and still nothing, then a Pajero man pitched in, hooked up his smart cables and still nothing even when his engine valve bounced, so I called the NRMA in Bathurst, perhaps I had terminal electrical meltdown, the technician found me , the bloke hooked up the smallest 40amh battery and away the Ranger went broom broom bot was I happy, question time tested my age batteries full charge, the Pajero hookup , no explanation, you will be fully charged by the time you get home further advice, the alternator reads the battery voltage and for the first 10 minutes the regulator allows a max charge then it returns to a trickle charge, the Ranger has 4 computers ,i think, the main computer kicks in to protect the battery from fully draining so the electronics are preserved. I had a battery charger but did not want to disconnect the vehicle battery , the advice ,,, carry a small 12 volt battery and connected in parallel with the main battery , before you disconnect the main battery, this will keep the computers running ,,, simple ehhh , don't ask about why the secondary battery not installed, there no room in the engine bay in the Ranger, you have to stuff it in the back , so I use my CHRISTIE gen/charge invented and made by a genius from horsley park specifically for camping and battery charging, no sun , no worries.
all the pro advice I have recieved is consistent , the vehicle alternator is not capable of fully charging a flat battery , as it takes about 10 hours on constant charge, so you drive for 5 hours and camp for 4, at the end of a week your battery is flat , dual batteries is another sales "you gotta have" thing i like it simple my car battery is for the car my camping batteries are for camping and the Christie generator/charge will and does charge flat lead acid batteries in 1 hour without damaging the battery, now lead acid batteries are as cheep as chips, i can buy 4 of those for the price of an agm