Interesting comments.. I was under the impression that the BC-1220 and similar DC to DC chargers still charge at the 12 - 14volts but are able to increase current to the battery so that your alternator doesn't need to work so hard.?
Nope, doesn't magically create current. What it does do is boost the voltage coming in to the correct charging voltage for the type of battery it is doing. The only way it can do that is to "rob" current.
If the inputs to the charger areat 10V(wiring is far too thin) and it is getting 14.1Amps, or 141Watts, then the theorectical maximun it can actually out put is 10Amps at 14.1volts(correct battery charging voltage?) They are all about bringing the voltage being applied to the correct level.
In truth, they would just suck more current from the alternator(make it work harder). If it is available. So you are not going to get maximum possible charging if you have all the spotties running. Hint, if turning on an extra spotty dims the others, then your battery recharger is going to be getting less current that the battery needs.
i.e. I am installing a dual battery under the hood, and a dual battery in my camper trailer. So there will be two x BC 1220's taking up the current from the alternator and boosting it so all four batteries at least get some juice. Otherwise, they just wouldn't charge properly at all..
If you have a voltage dependant isolator, your twin starter batteries will be "properly recharged first and only then should the alternator surplus be given to the BC1220's.
What is the capacity of your alternator? At full bore, they should only take 40 amps maximum (bulk stage)
Considering I will be running two fridges I need as much power into them when the car is running as I can get..
What sort of fridge? Fridge or freezer?
Floating around are typical consumption figures for the various types of fridge/freezers. Easy to do some basic calcs and work out how long you need to drive for.