Auxiliary Electrical Systems – Introduction

In our previous articles on vehicle electrical systems we talked a lot about the OEM components and some basic upgrades dealing with adding things like lights and associated switches. This article series is going to focus on auxiliary electrical systems, their components, and ultimately designing and installing an auxiliary electrical system into your vehicle — or trailer.

Previous articles of relevance:

• Electrical Upgrades: Round 1 – Go!
• Electrical Upgrades: Round 2 – Odyssey AGM Battery
• Vehicle Electrical Systems – Basics
• Vehicle Electrical Systems – Automotive Battery Basics
• Vehicle Electrical Systems – Power Distribution
• Vehicle Electrical Systems – Switch Install Tips

Batteries – Dual Batteries vs House Batteries

I want to start off with a point of clarification on the difference between a “Dual Battery System” and a “House Battery System.” Dual battery systems are a way to supplement the starting battery under the hood of your vehicle. These systems can be very basic – simply slaving the two batteries together in parallel. A parallel connection between two batteries keeps the voltage the same, but doubles the amperage. A more elaborate dual battery system not only links the two batteries in parallel, but it also isolates the two batteries from each other. That means if you have a parasitic draw to the main starting battery a solenoid can be activated (manually or automatically) to isolate the two batteries; so if the main battery goes dead there is still a reserve battery with a full charge. Dual batteries are great when you have a high-current draw like a winch or run a lot of auxiliary lights or powerful car stereo. The extra battery, in theory, helps mitigate current draw on a single battery by splitting the discharge demands across two batteries. Dual batteries can also help mitigate the current draw on the alternator by doubling the reserve capacity of the batteries.

Where many vehicle builders go wrong, in my opinion, is thinking that a dual battery system with an automatic isolating solenoid is the best solution to power “house” accessories like a 12v fridge or camp lights while the vehicle is not running. While you can connect accessories to their respective batteries (things like a which, lights, stereo, etc to the starting battery and things like camp lights, fans, and 12v fridge to the auxiliary battery) and use the isolating solenoid to keep the camp accessories from draining the starting battery, this system is not ideal for a few reasons.

First, under-hood dual battery systems are limited to using starting batteries. While there are some AGM deep-cycle starting batteries (like the Odyssey one we use in the LS LJ) they only have a 50% depth of discharge. This means the 100 Amp-Hour battery under my hood can only safely and routinely supply 50 Amp-Hours of power before it needs to be recharge. Repeated discharges beyond this rate will shorten the life of the battery considerably. I saw this a lot with people coming into the four-wheel-drive shop I managed with two year old AGM batteries that were pretty much toast. These types of batteries are fine for normal starting duty, and the occasional deep discharge while winching, but beyond that they are not well suited for “house” duties for things like fridges.

Second, under-hood dual battery systems do not adequately charge deep-cycle batteries. Even the smartest of modern smart alternators will not adequately charge and maintain a deep-cycle battery. Alternators are designed to charge standard automotive flooded lead-acid batteries which are a lot more tolerant of voltage irregularities. These flooded lead-acid batteries also have a quicker recharge rate due to their shallower depth of discharge rating. That means a standard car alternator can recharge the starting battery a lot quicker than an AGM deep-cycle battery — even a starting capable deep cycle battery — however it cannot fully recharge a deep-cycle battery which requires higher voltage and higher currant rates than their flooded brethren. Another component to keeping a deep-cycle battery healthy is conditioning the battery and keeping it fully topped off with a float charge. Again, the smartest of smart alternators in modern cars do not condition or float-charge starting batteries. That’s why I always recommend someone using a battery charger and conditioner at least once every few months for about 24-48 hours to keep their AGM deep-cycle starting battery charged.

Third, under-hood dual battery systems require both batteries to be exact matches in brand, size, capacity, and age. When running batteries in series or parallel both batteries need to be identical twins. Not just the same brand, but the same size, with the same capacity AAANNNDDD they need to be the same age. If you pair a new battery to an old battery in either series or parallel you will shorten the life of both batteries and run the risk of current imbalances leading to premature failure of both batteries.

Lastly, when you hook different accessories to separate batteries you create a load imbalance between the two batteries. This means when the two batteries are separated by the solenoid one battery will discharge more than the other. Consequently this means the two batteries need to recharge different amounts for different amounts of time. This imbalance shortens the life of both batteries. This is why batteries in most under-hood dual battery systems only last 2-3 years — which is about a third of what those batteries should normally last.

So what’s the difference with a “house battery” system compared to a “dual battery” system you ask? With a house battery you have a fully independent battery that is not directly connected to the starting battery. This means the four main issues facing dual battery systems we mentioned above are negated. You can run a different sized battery with a different chemistry, like say a deep cycle lithium battery which has a higher depth of discharge rate to the tune of 90-95% compared to the 50% discharge rating of a standard AGM deep-cycle battery. It also means you can run a much larger battery. While a starting battery may only have 50 Amp-Hours of usable power, you can often find lithium batteries with usable power ratings well north of 100, 200, or even 300 Amp-Hours. For people running larger 12v fridges and multiple other accessories like fans, lights, and phone chargers while camping this increased capacity can make or break how cold your food is the morning after a good night’s sleep. Lastly, if you build a proper auxiliary power system you can incorporate a good battery charger and conditioner that can be supplied by multiple power sources like the vehicle’s alternator, solar, or even 120v shore power.

With all this in mind we will be focusing on setting up an auxiliary power system that utilize a “house battery” rather than an under-hood “dual battery” system. The same methodology can be used when setting up a stand-alone battery system on a trailer. At a later date I’ll get around to talking a little more in-depth about the pros and cons of a standard dual-battery systems (I promise).

Components of an Auxiliary Electrical System

Your vehicle’s electrical system can be broken down into four major components:

1. Power Generation = Alternator
2. Power Storage = Battery
3. Power Distribution = Fuse Boxes
4. Power Consumption = Systems and Accessories

Your vehicle generates power via its alternator. As the main crank spins it turns a belt which turns a pulley on the alternator, which in terns generates power through accessing a compact black hole. Okay, not really, but I also don’t want to get mired in the weeds here with how alternators work, but just know as the alternator spins the combination of wires and magnets generates power.

Your vehicle stores electrical energy in its battery. This lead-acid battery (whether flooded, gel, or AGM) chemically stores the power generated by the alternator for future use. What’s that future use, well, starting the vehicle. The alternator can supply all the power a vehicle needs — but only while it’s running. Once the vehicle stops the alternator no longer produced power. In order for a vehicle to start the ignition creates a circuit between the battery and the starter motor. The starter motor using the stored power in the battery to spin the vehicles crank and get it running. Once running the starter disengages, the battery starts recharging off the alternator, and you’re good-to-go. The downside is that a standard starting battery has a very low depth-of-discharge and cannot supply a lot of power for very long which is why if you leave your headlights on (like I am famous for doing because I pulled the dome light fuse in my Jeep which also happens to be the same circuit as the door chimes — and the headlight chime) your battery is “dead” after only a few hours with a relatively low charge. Even if the battery is not “dead-dead” and only “mostly-dead” it’s still too dead to engage the starter.

Your vehicle distributes power through a network of wires, relays, and switches. Most modern vehicles have two fuse-boxes. One is under the hood for powertrain/drivetrain systems and external things like headlights and turn signals. The second is often found on the firewall of the vehicle usually up under, in, or behind the dash, or hidden behind a kick-panel, and it distributes power to internal systems like HVAC, stereo, power locks, power windows, power seats, etc. Older vehicles with less powered systems may only have one fuse box, but even some “dumb” vehicles like my Jeeps (2004 and 2006 respectively) have two fuse boxes despite the lack of accessories like power seats, power door locks, or powered windows.

Lastly, and as you can already infer, the final component of power consumption is the various accessories I just mentioned. Everything that uses electrical power in your vehicle falls under this final category. This is where things can get super crazy because every switch, every light, every sensors, every-little-thing in a modern vehicle requires power. The more accessories the more wiring. Modern vehicles can have north of 5,000 feet of wire in them. A mile. A literal mile of tiny wires criss-crossing your vehicle’s chassis to make those little under-seat lights glow, and make sure your license plate has a little light on it. Mind-blowing when you think about it.

An auxiliary power system will be comprised of those four same components, albeit with a slight twist. OEM vehicle electrical systems are often designed on a razor’s edge with barely — just barely — enough to supply everything the vehicle needs in stock form. The moment you start throwing on additional accessories means that system is no longer in balance and you run the risk of overloading some or all of the stock wiring and various components. Sure you can get a heavy-duty alternator and throw in a higher capacity battery, but as we alluded to above, that’s not ideal.

In the adventure travel world there is one power generation solution that reigns supreme above others: solar power. It’s almost become cliche these days to see overland adventure vehicles with roof or hood mounted solar panels or a campsite with portable “deploy on demand” solar panels. I’ve made use of such a panel for a number of years. When I set up camp I can pop out a 150 watt trifold solar panel from my friends at Overland Solar. It’s served me well for a number of years. When it’s needed it can be set up and faced toward the sun. When it’s not needed it can be stowed in my trailer out of the way. Solar is a great way to generate power without the need for gasoline to either power a generator or run the vehicle itself. That said, solar isn’t the only way to generate power. You can also get power from the vehicle’s alternator via “DC-to-DC charging” or from “the grid” which people refer to as “shore power.” More on those later.

Storing auxiliary power is as simple as having a stand-alone battery connected to whatever your power source is. This battery, being wholly separate from your vehicle’s OEM electrical system, supplies power to only those accessories you connect to it. This means any additional accessories will not be taxing the fragile balance of your vehicle’s OEM electrical system or run the risk of draining the relatively low capacity starting battery. If for some reason your disco rave party lights, triple-zone fridge/freezer/deep-freeze fridge, electric blender, microwave, and 12v heated blanket all drain the auxiliary battery it hasn’t drained the starting battery of your rig.

Lastly, because this auxiliary power system is separate from your vehicle’s OEM electrical system, it needs its own system of fuses, switches, wires, and even relays to work properly. This is often where a lot of people get overwhelmed and discouraged. As we mentioned in our vehicle switch install tips article two main things will make all the difference: first, get the right quality tools; second, take your time and work methodically. One thing I might add to those tips is that you shouldn’t be afraid of waste. I’m not saying you should careless or reckless with your materials. Just know it’s better to run your wires long and cut the shorter after than to measure once, cut the wires, run them, and then find out you’re a few feet short (ask me how I learned this lesson). I will also say having a label maker is a great investment. While you can do the whole masking-tape-and-sharpie-marker method of labeling wires, they never last. These days you can buy printable heat-shrink tubing that works in label makers. This means you can type out in clear easily readable text what the wire is for, slide it onto the wire, heat it so it shrinks tight, then for good measure put some clear heat-shrink tubing over it. When running multiple wires over long distances either in a vehicle or a trailer it’s nice to know what wire belongs to what circuit without having to manually trace the wire back to your fuse box.

Conclusion

At this point I am going to wrap up this introduction to auxiliary electrical systems. We know what the four components are (generation, storage, distribution, & consumption). In the next article we will discuss how to spec out a system to fit our needs and how to choose the appropriately rated components for our needs. We will also do some comparing and contrasting of some features and benefits of different types of components. After that we will work on installing said components into a vehicle (the girlfriend’s van) as well as look at an existing stand alone system (the one in the Poor Man’s Teardrop).

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