In today’s world, car charging systems use "alternators" to charge the car battery. A car battery has different internals to the common AGM battery found in caravan’s.
The strength of a electrical current ( a amp) is abbreviation for amperage.
Alternators charge lead acid batteries by putting amps back into them. For fuel economy purposes, vehicle manufacturers design alternators so they reduce the amps the more charged they become. As a result car batteries are often never %100 charged and at most they only ever get %80 charged.
Trying to charge a caravan AGM battery back to %100 using only a alternator is not a efficient method. This is why when towing a caravan and wanting to charge a battery we use DC DC chargers to assist. They work by taking a reduced supply of amps from the alternator (EG: 10 amps) and ramping that up to a constant 30 amps.
That constant 30 amps allows 30 amps of electricity to go back into the battery per hour until it reaches %100 charged. Much better than the %80 charged like a alternator on its own would do.
A %80 charge of a set of 240ah batteries = roughly 50 amps short. This is a lot when relying on power from batteries to be off grid to keep the beer cold.
CTEK DC/DC Charger
Charging batteries takes time. The more amps you can put back into it the quicker it charges (but not too much that you overheat the batteries).
Example: Our caravan down to %50 flat means its 180amps short of being %100 charged.
180a / 15a = 12 hours to charge at 15 amps or 6 hours at 30 amps.
This could mean the difference between running a generator for 12 hours to charge batteries, or 6 hours to achieve the same thing dependent on the charger.
Now’s a good time to mention you’ll read a lot about people complaining about other travelers that use generators in public areas. I partially agree with those that complain to a certain extent. But everyone's situation is different. Some may use a generator for medical reasons others may use them purely cause they can't afford a solar set up. Each to their own i say but at the end of the day it's all about doing what you have too, as long as you're considerate.
Our DCDC charger set up in the back of the ute