Practical dual battery system

What is the ultimate goal with a dual battery system?  To provide reliable power to our 12-volt accessories, which in most cases is a fridge and some lighting. And to do this without running down the charge in our starting battery.

We recently set up a system in a D40 Navara, the customer’s requirements were simple, power his fridge.
After discussing how the vehicle will be used it became clear that an elaborate setup was not really needed and would only add to the cost and see no real benefits over a simple Voltage sensitive relay (VSR) based system.

Our systems are always designed with proper protection and cable sizes that can support the expected current draw with minimal voltage drop.
So we begin at the cranking battery, we need a suitable fuse and for this, we favour the Midifuse, simple to mount and provides sturdy cable connection with its bolt down fuse.  We like to keep the unfused cable very short, a tab was TIG welded to the battery clamp to mount the fuse and a short 6 B&S cable was made to connect it to the main battery.
A simple Projecta 100amp VSR was mounted in the engine bay and a 6 B&S single core cable was run with an existing wiring harness to the rear of the vehicle.
For the ground side, we don’t use a full-length ground cable. We prefer to the use the body for the ground, to this end we replace the stock Battery negative (B-) terminal with a Projecta 3 post distribution terminal. The reason for using the body is simple, there is less voltage drop.
We then install a second cable from B- to the body and from B- to the engine. This is mostly because we see a lot starting/charging issues related to high resistance in the factory cabling of many vehicles, so this is a good opportunity to put in place a cheap solution before any real issue surfaces.

Dual battery installation using a Projecta voltage sensitive relay and Midifuse

At the rear of the vehicle, the customer wanted the auxiliary battery mounted in the tray. A battery box was bolted down and three holes drilled through the bottom of the box and through the tray floor for cable routing. These were later sealed to prevent water ingress.
Properly fusing a dual battery installation means fusing the charge cable at both ends. A second Midifuse holder was mounted to the underside of the battery box lid. Three post distribution terminals were used again for the auxiliary battery.

Projecta distribution terminals with midifuse and blade fuse in battery box

We now have a very simple and cost-effective way to keep our auxiliary battery charged and it is properly protected with fuses at both ends.
The remaining task is to provide a power outlet for the fridge.
For this, we used a blade terminal holder mounted in the lid of the battery box. We don’t use any pre-wired connectors as this means making butt splices in the wiring and introduces unnecessary potential failure points.  From the fuse, we ran back down the charge cable to the base of the B pillar where there was a convenient hole in the sill to route the cable.
A twin outlet was installed on the B pillar trim which has a single Acc socket and a twin USB outlet.
Twin USB and accessory outlet at the base of the B pillar
It is by no means an elaborate system. It is, however, a safe and reliable dual battery system that is easily upgraded with a DC-DC charger and further power outlets should the owner want to in the future.

The products used in this install can be found at these link.