Power Distribution: Your Guide to Powering Your Off-Road Accessories

Power Distribution: Your Guide to Powering Your Off-Road Accessories

by Patrick Rich

Picture this, you've spent the day reaching solitude, off the beaten path and you are now standing back, face illuminated by the glow of the fire, quitely soaking it all.  What a day.  Only this isn't the warmth of the campfire, this is your vehicle, your pride and joy, burning itself down to the wheels and taking everything with it, including your food and shelter and leaving you without any way to get home.  

This scenario, while uncommon is a real possibility and it can happen and has happened to travelers like us in the past.  More common?  The heart dropping "click" of a dead battery from leaving something on overnight or while you were away from camp: You don't want either of these. The good news is that the common source behind both of these problems can be easily avoided.

Your electrical add ons are there to make your trip better, but they can easily turn against you without proper setup and installation.  In this segment we are going to talk about the major issues with power distribution and how to avoid problems in your setup. This guide is meant as a broad overview of DIY wiring, but also serves as education if you are looking for options for a qualified upfitter to put into your rig.  

In real estate the saying is “location, location, location”. In overland power distribution, we say Isolate, isolate, isolate. Isolation is the key and I’m not just repeating myself here, there are in fact 3 lessons or rules with power distribution based on isolation. Learn them, love them.

Rule the first - Isolate from the factory wiring.  This is true whether you are installing 100 lights, 2 winches, 2 fridges, etc, or a single additional reading light.  Leave the factory wiring alone.  The factory system was engineered by a team of highly qualified people over a period of years and tested to do a job and to do ONLY that job.  Wire is weight and cost, as such the wiring harness is designed to be no more robust than it needs to be. Not only that, it’s a complex system with carefully designed circuits that function with known values for voltage, resistance and returns.  Tapping into factory wiring to run an accessory can cause malfunctions, or worse, overheating of wires leading to fires. 


There is one exception to this rule I will cover later. 

Rule two - Isolate your loads. A load is any electrical device you want to add; From lights to the radios.  Wire each load individually, don’t bundle loads together on a common power source.  Fuse each load separately. That is, wire your CB on its own fused circuit separate from your ditch lights which should be on their own fused circuit. The reason that you do this is to reduce the loads on your wires and to ensure than a fault in one place doesn’t chain to other places, making diagnosis much easier. More importantly, a fuse per load isolates the failure and prevents wires or fuse blocks from overheating and starting a fire.

Adding one or two additional loads can be done with inline fuse holders right off the battery, but more than that and you should start looking into a separate fuse panel to manage these new loads.  This avoids both the "rats nest" of wires problem and makes diagnosing faults easier.

My Blue Sea dual buss fuse block installed.

There are various paths you can take at this point from basic fuse blocks from trusted folks like Blue Sea Systems or more integrated DIY systems like “Bussman” boxes and semi complete systems from PAINLESS wiring all the way up to electronically switched and managed systems like ARB Linx or sPODBe sure to isolate this new panel, or buss, from the battery by a separate, larger fuse or circuit breaker to avoid internal faults in the block from causing issues. 

100 A circuit breaker

Whichever route you go, keep in mind these principles:

  • Your loads should all be isolated from the factory wiring
  • Your loads should all be isolated from one another and individually fused
  • Your fuses should all be isolated from the battery by a fuse or circuit breaker.  

The final rule - Isolate your power supply. Beyond isolating your loads, this isolates your entire load group from the factory system. There are a few ways to do this and while there is no right way, there certainly is a strong preference among seasoned and well-equipped travelers.  

Ignition hot relay.  From the factory, when you turn your car off there are fewer ways to run your battery down as most loads are cutoff from the battery. i.e. you can’t run the radio or seat heaters with the key out. The easiest way to add a new load is to connect your fused load or fuse box directly to the battery.  In this case all loads would be active all the time, or battery hot, leading to possible drains leading to a dead battery. To make your new loads act like your factory system you need a relay.  

A relay is a switch that is triggered electrically by a current that energies a coil and pulls open or closed a circuit electromagnetically.  By adding a simple high amp, normally open relay between your battery and fuse box, you can isolate your loads from your battery when the car is off and no current is applied to the relay coil.  The relay is closed when a current is applied to the coil, in this case from a circuit that also comes to life when you turn the car on, or ignition hot.  An example of an ignition hot load might be a cigarette lighter socket; only active when then car is on.  By tapping into this ignition hot power source, you can draw a very small amount of current from your factory system, to direct a very large amount current directly from the battery and isolate it in the same manner.

Isolate your trigger wire by fuses.

When I said earlier there was one exception to rule one, this is it. In order to piggyback off your car’s ignition switch you need to tap into an ignition hot circuit as a trigger for your relay.  In this case you still isolate this tap by fuse to protect the wire to the relay.  This won’t affect something that is meant to carry a lot of current like a cigarette lighter, but it might affect something with low current demands, choose your trigger carefully.  

The disadvantage of this is that some loads are desirable to have power when the car isn’t running, a fridge for example.  There is the possibility of running both ignition hot and battery hot circuits separately, but it does complicate your wiring. This is the route I’ve taken in my personal vehicle.

Blue Sea dual buss fuse block with Buss A battery-hot and Buss B ignition hot through a relay.

Battery isolator. This connects loads as battery-hot but only if certain criteria are met which prevent the battery from draining excessively. Isolators are relays that function with some kind of built-in logic. With voltage sensing relays, if the battery voltage falls below a certain threshold for a given period of time the relay will open the contacts and disconnect the loads prevent further discharge. The relay is closed again by various means; a manual switch, a cycling of the lights or just turning the car on.  

This is a good, relatively low-cost safety net for those who want to keep a single battery system. 

If your only battery-hot load has a low voltage disconnect built-in, an ARB fridge for example, this may be sufficient to not need having a battery isolator as the load will automatically disconnect with low voltage at the fridge.  

  • Isolated house battery.  The best way to make sure your loads never drain your start battery, is to never connect your loadsto your start battery.  This requires a second power source that is isolated from your start battery, also known as a house battery.  The benefit of a house battery is that your factory electrical system is wholly unaffected and anything you add on is on its own isolated power supply.  The trouble, as you already be noticing, is that a house battery would have to be connected to the vehicles systems in order to recharge itself when the vehicle is running.  So how do you have a dependent, isolated system? Battery isolation.  
  • Charge relay – a charge relay is similar to a battery isolator in that it closes a circuit and allows power to flow under one set of circumstances and opens and isolates in another. In the simplest systems when the vehicle is started the circuit is closed and both batteries charge at the same rate, when the vehicle is turned off the circuit opens and isolates the house battery from the start battery. Not ideal for batteries of different sizes, states of charge or even battery chemistries. More advanced systems will monitor voltage levels of the start battery and only close the circuit when it determines the start battery is up to float voltage, sending all the current to the second battery.  This still doesn’t solve the battery mismatch issue, but it is more equitable to the batteries. Some systems also allow for manual linking of the batteries to use the house battery to supply the start battery if the start battery has drained.  An example of this kind of system is the Redarc isolator kit
    Red Arc BMS
  • DC/DC chargers –An alternator can’t accurately manage dual batteries separately.  While batteries of different states of charge, size or even chemistry will have different voltage and current needs at any given time the alternator will see any 2 batteries connected directly as a single 12v load and attempt to reach the voltage needs of the combined batteries as a single battery charging both batteries at less than ideal rates and levels of completeness. The solution is a DC/DC charger. DC/DC chargers monitor your start and house batteries state of charge and use digital circuitry to tailor the voltage and current needs of the house battery for the fastest, and most complete charge. Because they are chargers independent of the alternator, they also act as a battery isolator when the vehicle is off and only supply current when the vehicle is on.  This allows your house battery to be charged independent of the starter battery as if is being charged by a digital charger in your garage.  You can run any battery chemistry, or size as is supported by your charger and you will always be sure that your start battery will be maintained exactly as it would with the factory system. If you have more complicated needs like multiple input sources or complicated battery chemistries, a BMS (Battery Management System) like the one from Redarc may be worth considering

I said at the beginning of this section that there is one system modern travelers’ preference, and this is it.  DC/DC chargers solve all the battery isolations problems in a single, easy to manage system.  REDARC, CTEK, and others are leaders in this space.  

A few more thoughts.

Northstar X2 AGM

Choose a high quality battery that is compatible with your setup. I personally use an Odyssey AGM battery and have been nothing but impressed. After researching I found they offered the reputation, density and compatibility that I required in my battery. There are other great AGM batteries as well as newer LiPo4 or other advanced chemistry batteries. If you are planning a dual battery setup without a DC/DC charger stick to a single battery chemistry, either have your second battery be wet cell or upgrade your start battery to AGM to go with an AGM second battery.  If upsizing your battery be aware of the additional weight and size as the bigger batteries are quite hefty, especially if you are adding a second battery to your engine compartment.  

Finally, if you do your own wiring DIY make sure you are sizing your wire and fusing according to established standards.  ABYC is a good reference standard to use.  Any wire that will be in the engine compartment should have a thermal rating of 105 C or higher and should be sized up compared to colder temp standards.  Use high quality connectors and crimp with high quality crimping tools.  Use heat shrink on your terminal ends and make sure to shield any wire that could rub against something in a braid or loom, especially around sharp bends. Lastly - draw a diagram of your wiring and label your work.  Trust me, you won't remember how you did it not long after you've done it and you will want a map or at least a color coded or labeled guide.    


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