keiron wrote:

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Hi. I am fitting up my new tug, Isuzu MU-X, and will be fitting a 12volt fridge in the rear behind drivers seat. I am also want the tug to charge the Caravan battery whilst we are moving. there is solar on top of caravan but none on the Isuzu. I am seeking some advice and ideas on fitting DCDC charger(s), and where they need to be located.

The Isuzu has a Smart Alternator.

I am unsure as to how many dcdc units I need. I understand they should, ideally, be located as close to the battery as practible. Does this one (1) close to each battery???

I am confused and need some enlightened direction. KB

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I have a similar setup. Modern car with smart alternator, fridge in boot of car and camper trailer towed behind, and here's how I tackled it.

I have 16mm2 wire running from car start battery via an ignition triggered relay to back of car where it branches, one branch to an Anderson socket inside the boot which the fridge plugs into (better for constant current draw than a cigarette socket), and the other branch goes out to an Anderson at the tow hitch. The Anderson from the camper plugs into this and 13.5mm2 wire runs to a DC/DC charger inside the camper to charge the lithium battery.

When car is running the relay closes and fridge takes about 5 amps, and the DC/DC charger pulls up to 32 amps for charging the camper battery, so a max total draw of about 37 amps, there is a 50 amp fuse at the starter battery. 

The battery in the camper benefits from a DC/DC charger in two ways. First it compensates for volt drop. Measured voltage levels from my system show 14.6 at the alternator, but only 13.5 by the time it gets to the camper trailer which is not enough to fully charge my battery, so the DC/DC charger bumps it back up to the required voltage. Secondly, the camper battery benefits from a decent three stage charging profile (CC, CV, float) to charge it efficiently and effectively, and the DC/DC charger will provide this.

I don't have permanent solar on my camper trailer, but did on my caravan which used the same setup, and my DC/DC charger (Redarc Manager 30) has dual inputs for car and solar, and will first use all it can get from the panel on the roof to charge the battery, then top up from the car.

The only downside of this system is that if the car ignition is off, the fridge isn't getting power, and you have a fully charged battery sitting in the trailer not being used. So, as a final feature I added a manual switch in the camper which bypasses the DC/DC charger and backfeeds the Anderson on the car through the same wiring. This allows me to keep the fridge running off the camper battery when I break a journey for lunch or even stop overnight on a long trip. 

This setup has worked well for me for seven years now.

EDIT: I should add that my fridge is only a temporary feature in the car for when I go on camping trips. When the car engine is running it is fed by the alternator, if the engine is off for a lunch stop or even an overnight stop and the camper is still attached I can flick a switch and backfeed the fridge from the camper trailer battery. When I get to my camping destination the fridge comes out of the car and goes in the camper, and is powered by the camper's battery and fold out solar.

If your fridge is intended as a semi-permanent fixture in your car, ie. you want it powered even when the car ignition is off and the caravan isn't connected, then you will need a different setup. The most common way of doing this is a traditional dual battery setup with the starter battery reserved for starting the vehicle but connected to an aux battery dedicated to powering the fridge, and a DC/DC charger between the two to manage the charging of the second battery and isolate the two when there's no charging source. Including the caravan, this would mean you have three batteries in total and two DC/DC chargers.