Adam runs you through the process of fitting a dual battery system.

My wife owns a nice Prado, a perfectly good 4WD, yet it only seems to get used dropping the kids to and from school. I have been working on her for ages now asking her to let me upgrade and modify it and the usual response I get is either, ‘no Adam it’s my 4WD not yours,’ or, ‘as long as I can’t see it then go ahead.’ These parameters, as well as the fact that we recently suffered a flat battery due to the kids leaving the interior light on, led me to the idea of installing a dual battery system with jump starting capabilities.


I decided to go with the tried and trusted Redarc SBI12 solenoid, which I keep in stock at all times. The SBI12 will do a great job of charging another battery, but you will need to check your charging system as some new 4WDs have a variable voltage alternator which will not allow the SBI12 to work efficiently. In this case you will need to go for the upgraded Redarc BCDC range. Alternatively if you are charging more that one extra battery it would be of benefit to go for the BCDC charger.
We fitted up a Roadsafe battery tray, used cable and cable lugs that I have in stock and sourced a Narva fuse block that also has an earth return so that the wiring could be kept neat and tidy. I decided on an Exide 95A/hr Stowaway deep cycle battery. If my Engel uses one amp per hour once cooled to refrigerate and I run some LED lights, then the Exide 95 A/hr battery should do a good job of lasting about 3 days before it needs a charge again.
Once I chose this set up, I did a quick scout around the engine bay and laid everything out in a rough configuration to plan where I would place it all. Very quickly I figured out that mounting the fuse box might be a problem unless I made up brackets, but a favourite of mine is to fit them on top of the battery, which gives me easy access to all the fuses and keeps everything really neat and tidy.
First, I fitted up the battery tray. The Prado already has a space designed to accommodate it, so it was just a case of relocating the air conditioning mount and bolting the tray in place. Then I fitted the battery in place to make sure that it sat securely and looked good.
I fitted up the Redarc SBI12 solenoid, made sure that I mounted it into a proper screw hole, as the black wire needs to be mounted into a good earth. It is a really good idea to clean off some of the paint to make sure that you get a good earth for this wire. From there I ran my wire from the positive of my starting battery to the terminal of the solenoid that has the sensor connected to it. Within the wire there should be fitted a fusible link. Redarc do offer fusible links, part numbers FK40, FK60 and also FK100 which are 40amp, 60amp, 100amp accordingly.
The fusible links are very important.  If a main wire shorts out the link will blow and this protects your vehicle against damage and possible fire. You will have to decide on which amperage rating you require as the 40 amp fuse may not be enough if you want to jump start via the solenoid off the auxiliary battery. A large diameter cable is also necessary if you want to be able to jump start off the auxiliary battery.
Making up your own cable is very easy to do, you just bare the wires and fit up the correct size cable lugs. The lugs should be crimped in position with a lug-crimping tool, and then soldered. At home, soldering may be the only option.
Next I ran a wire from the other terminal to the positive terminal of the auxiliary battery. Again a fusible link is recommended to fit onto this cable as well. Once I ran all the wires I made sure that conduit was fitted to all the cables for protection, not only for the cables, but also to protect them from shorting. A good earth wire and connection is very important. I made up the earth cables in black and ran one to the factory earth wire, to the side of the battery and ran another to the motor just to be sure.
After securing all cables, running conduit and making sure that everything was secure and double-checked the dual battery system was installed. Finally, I started the vehicle and checked that the solenoid was switching over and charging the auxiliary battery.




You have two choices, both of which involve using the blue wire on the solenoid. The first is to get out of the vehicle and connect the blue wire to power. This will switch the solenoid and connect the two batteries together, allowing you to jump off the auxiliary. If you are really clever you can run the blue wire back inside the cabin to a push button switch and then back to power, using an inline fuse. With a set up like this, you will be able to jump start your vehicle from the driver’s seat.
If you are running power sockets throughout the vehicle to power 12V accessories, then a favourite method of mine is to fit up a separate fuse box. Narva makes a fuse block that also has an earth return. Fuse boxes are a great addition to any electrical upgrade and will give you a great base to start from. It will make all additional wiring simple to access for troubleshooting but it will also make it really easy to add accessories in the future.
The Narva fuse box is very simple in operation. You run a main power wire to the block and from there all you need to do is add a fuse in the block, which will activate a circuit.  You can then run your power wires from the other side of the fuse. As I suggested, it also has provision for an earth return so you can run all your wires down one length of conduit and back to the fuse box to keep everything neat and tidy.
If you are fitting up a dual battery system at home, my first suggestion for ease of fitment would be to purchase a full kit. Something like a Redarc SBI12KIT or even a Projecta DBC150K. These kits are designed to provide you with everything that you require for fitment. They include cable, the cable lugs and even battery terminal covers to make your job much neater and easier. Detailed fitting instructions are available within the kits.
Now that I have figured out how to hide this from my wife, I will have to try to hide a front bull bar and driving lights from her too!

All parts mentioned are as follows and can be found on our website
Redarc SBI12 or for ease of fitment Redarc SBI12KIT
Exide battery Stowaway ST27DC95
Narva Fuse box 54450BL
Roadsafe Battery tray BT009


Top two: The Roadsafe battery tray; Exide Stowaway ST27DC95.

Second and third row, left to right: Redarc SBI12 isolator; large diametre cable is necessary for jumpstarting from the auxiliary battery; the black wire is mounted into a scre hole, to ensure a good earth connection; battery in place, now the fusebox needs wiring up; The Narva 54450BL fusebox in place.

Next time you’re out and about, make sure you do a quick channel scan and if you find me give me a shout. Over and Out!

Qualified motor mechanic Adam Adler has spent half his life under the bonnet of a 4WD and has worked for some of the top accessory companies and workshops. He knows what it takes to get your vehicle out there and back home in one piece. He runs the online aftermarket store

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