Originally Posted by lt4vette
When I get my Overland CRD (its actually been allocated to a ship!), then one of the first things to do is fit a second battery. From my reading so far, it seems the options are in the wheel well - small battery - or in a separate tied down box on the floor in the back. It also seems to be highly recommended to use a DC-DC converter, partly to ensure the charging system doesn't get confused, and partly to ensure the right voltage for,say, a fridge. I have yet to determine whether a separate battery isolator is required. Any further suggestions would be greatly appreciated.
I use a DC-DC converter in my Camper Trailer, It's a CTEK D250S-dual. Works very well, though it did die just before warranty was up so it got replaced.
Most DC-DC chargers should shut-off when the input voltage drops below a certain value. This provides battery isolation and should be sufficient unless you want the threshold voltage to be higher. For the D250S-dual it is:
The charger starts charging the target battery when the supply voltage exceeds 13.1V for 5 sec (engine on).
The charger stops charging the target battery when the supply voltage drops below 12.8V for 10 sec (engine off).
Of course, my second battery, along with the D250S-dual are in the trailer(you want the DC-DC charger as close to the battery as possible) but electrically, it's the same, except my cable run is longer hence there is an even greater need for a DC-DC charger.
I did see one site talking about the D250S-dual needing a relay when used with a variable alternator. Might need to do some more research. I don't have my car yet so I have not tested it.