Vehicle Electrical System – Basics

Vehicle electrical systems can be somewhat scary for the uninitiated. I remember how nervous I was when I installed my first car stereo amp when I was in college. Now, many years and a lot more experience later, I honestly can’t say I’m any less nervous when doing vehicle electrical systems. When you hear about a vehicle fire it’s usually electrical. The last thing any of us wants to see is out adventure vehicle go up in flames, especially because of something we did.

Before going too far into this rabbit hole I figured I’d start with the basics. Now, don’t worry, I won’t be getting too technical throwing out crazy terms like voltage, amperage, resistance just yet. In this article we’re just going to focus on the basics components of your vehicle’s electrical system and what you might want to upgrade based on a few common modifications people add to their 4×4 vehicle.


The three main components to your vehicle’s electrical system are the battery, alternator, and the distribution system. The battery and alternator are pretty straight forward. The distribution system is the collective wires, fuses, relays, and switches that snake throughout your vehicle.

Almost every production vehicle on the road has a finely balanced electrical system that usually doesn’t leave room for any additional accessories (often called loads). With few exceptions the moment you add something new to the system you most likely have to upgrade everything else. There are some vehicle packages available from the dealer with things like dual alternators on full-size diesel trucks (usually called a camper or ambulance package) or high-amp alternators that are sometimes part of a towing package (to power trailer lights and trickle charge a trailer’s battery while it’s being towed). Some newer vehicles do come with some built in auxiliary switches (like the new Jeep JL Wranglers), but such options are not normally standard equipment.

We are going to look at three common modifications that people make to their 4×4 vehicle. We will look at off-road lighting, winches, and fridges and then see which of our vehicle’s electrical components we should up grade. However, let’s first look at if there is anything we should upgrade while the vehicle is stock.

Here is a basic matrix to refer to as we work our way down the list.

Stock Upgrades

As mentioned, most vehicles come from the factory with a finely tuned electrical system. Automotive engineers (much smarter than people like me) spend years researching the best combination of components for a vehicle to not only run, but run reliably for years. That said, I’m sure most of us have heard horror stories about vehicles with electrical gremlins. So not all vehicles are created equal. There are also many variables with things like age, mileage, and how used/abused the vehicle is.

The first thing to look at is the vehicle’s battery. This is an easy upgrade for me to justify right off the bat. Most 4×4’s are sold with a standard automotive battery that uses a combination of lead plates suspended in a liquid acid bath. The chemical reaction between the acid and the lead plates allow the battery to store energy, discharge that energy, and then be recharged. The downside is that those plates floating in a liquid acid bath are very susceptible to vibration and impact damage. So when you’re bouncing around off-pavement there is a chance the battery will short out internally.

Here is a new Odyssey Extreme PC1500 AGM battery installed in the Jeep.
Full writeup on the battery here.

A nice upgrade for any 4×4 vehicle is switching to an Absorbed Glass Mat (AGM) style battery. Unlike their liquid acid counterparts, the acid in an AGM battery is in the form of a paste. This means not only is the battery more resilient off-road when it comes to vibrations, it also means there is no liquid acid to pour out of it. This means on hills and off-camper situation you’re less likely to have battery acid leaking out under your hood.

Another big advantage of an AGM battery is that they are a deep-cycle battery. This means they have a higher depth-of-discharge percentage compared to a standard battery. Most standard car batteries can only be discharged roughly 20%. An average deep cycle AGM battery can go down to 50% meaning it can run accessories a little longer when the engine is not running. Anyone who has accidentally left their headlights on over night or gone to a drive-in-movie knows the pains of a dead battery.

There is a cost for switching to an AGM battery. They usually run 2-3 times the cost of a standard automotive lead-acid battery. However they usually last longer, perform better, and hold up more. So even a stock 4×4 can benefit from switching to an AGM battery.

For the most part a newer vehicle in stock form won’t need to upgrade the alternator. However an older vehicle with higher miles may need the alternator replaced. Over time the internal components of the alternator wear down and the overall performance of the alternator goes down.

Here you can see what 10 year old stock battery cables look like.
The ground wasn’t much better and was causing a trickle drain on the system.
The new cables and new battery had her running right as rain.

As far as the distribution part of the vehicle’s electrical system, there is most likely no need to mess with it. However, a high mileage older vehicle may see some deterioration in the wiring (as evident on my Jeep when I first got it).

Adding Off-Road Lighting

One of the cheapest and most popular upgrade to any 4×4 is off-road lighting. From light bars and light pods to chassis lights everyone loves adding extra lighting. The question is, do you need to upgrade any of your vehicle’s electrical systems when adding lights.

If you’ve already upgraded the vehicle’s battery to an AGM then there is no real need to touch the vehicle’s battery. Most of the time you’ll be using the lighting the engine will be running meaning it’s the alternator not the battery that will be powering the lights.

In that case, do you need to upgrade the alternator? If you’re going to stick with modern LED lights the answer is no. LED’s draw very little power and you’d need to at a lot of lighting before the collective load is more than your vehicle’s alternator can handle. However, like previously mentioned, if you have an older high-mileage vehicle an old alternator may not be working at 100% capacity. So be mindful of that.

The major place to upgrade your vehicle’s electrical system will be on the distribution side. As mentioned most vehicles are not equipped from the factory to accept additional electrical loads like lights. With only a few exceptions there are pre-wired switches and relays for you to tap into.

Here you can see the pair of Rigid Industries LED lights installed on the bumper.
The low current draw of the LED’s means they have minimal impact on the vehicle’s electrical system.
Full writeup here.

This is where an aftermarket power management system like a Switch Pros, sPod, or Trigger system is worth the investment. These systems are a combination of a centralized relay box and a wired control module. This allows you to add multiple accessories light lights without having to wire them all independantly. Some systems also give you features like remote control via BlueTooth, and ways to dim, flash, and sync lights together.

There is of course a cost for aftermarket systems like this, and if you’re only going to install one or two pairs of lights then it may not be worth the investment. However if you’re going to add multiple lights and/or other electrical accessories then making the investment into a power management system early on is worth it. If not, most reputable brands of off-road lights coming with the necessary switch, wires, and relay to hook them up. Just be mindful when you run the wires.


Another popular upgrade for many 4×4 enthusiasts is a winch. While a winch is great for pulling you out of a pretty nasty situation, they do put a tremendous strain on your vehicle’s electrical system. Use a winch too hard too long and you’re liable to cook your vehicle’s battery.

Winches are great for getting you unstuck.
The winch I use is a Warn 9.5 XP rated at 9,500 pounds.
At max draw the 6 horse power motor pulls an amazing 482 Amps!!!
Can you see why you’d need to upgrade batteries?
Intro to winching article here.

The best battery upgrade for heavy duty winching is a dual battery system. This splits the load of the winch between the two batteries. This is important even while the vehicle is running because a winch will draw a lot more power than the vehicle’s alternator can put out while running the rest of the vehicle’s electrical system. That’s why it’s best to winch in short bursts of no more than 30 seconds rather than long sustained pulls lasting multiple minutes. A dual battery system gives the vehicle’s electrical system more reserve power and helps split the load so one battery isn’t baring the brunt of the entire load.

Upgrading the vehicle’s alternator is a good idea if you have the option to. Some vehicles came from the factory with an optional high-amp alternator. So if your vehicle isn’t already equipped with one you can often switch it out for one. There are also some companies that make high-performance alternators with a higher amp rating than what was available from the factory. A last option is going to someone who rebuilds alternators and having one rebuilt with a higher amp rating. Sadly the people that do that are becoming fewer and further between these days.

As far as the electrical distribution system there isn’t anything to upgrade when it comes to the winch. A winch will be connected directly to the battery (or ideally batteries in a dual-battery system) thus bypassing the vehicle’s factory distribution system. The wires connecting the winch are also much larger and carry a much higher current than the aftermarket distribution systems mentioned earlier.

12v Fridge

The final example electrical accessory we’re going to look at is a 12 volt fridge. 12v fridges are a major game changer for any long-term overland adventure. They’re even a game changer for short-term trips or events like tailgating or even for road-trips. However the demands of a 12v fridge can often exceed a vehicle’s stock electrical system.

Whether or not you need to upgrade the vehicle’s battery is a question of whether or not you want the fridge to keep running while the vehicle is off and, if so, for how long. As mentioned a standard automotive battery only has a shallow 20% depth of discharge. Meaning the fridge will only run for a few hours if you’re lucky, and certainly not overnight. A deep cycle battery would be a good upgrade, but again you run the risk of draining the same battery you need to start the vehicle.

A dual-battery system would be a nice upgrade, however a traditional off-road dual battery system like you would use for winching is not the best option. These systems only isolate one battery from the other when the discharge rate gets to a certain point. It then preserves one battery as a starting battery letting the other battery drain. The major downside to these systems is that they aren’t very “smart” nor do they isolate the accessories you’ve added from your vehicle’s main system.

A much better option for running a 12v fridge in a vehicle is a stand-alone “house battery.” A house battery is separate from your vehicle’s electrical system meaning it will only power the things you connect to it, and other vehicle systems (radio, lights, etc) won’t draw from it. This mean this battery has one job to do. This means the house battery will last longer. The question is, how do you charge it?

One way to charge a house battery is with an external source like solar power, a generator, or even a shore-connection to 120v household power. Another option is to use a DC-DC charger that is connected to your vehicle’s electrical system. This is a one-way connection that means the vehicle can charge the house battery while it’s running (using the vehicle’s alternator as its power source) but the vehicle won’t drain the house battery.

Here is the wiring layout for the REDARC Manager30.
You can see the only direct connection is to vehicle battery.
All other loads are to the house battery.
This same setup would work in a trailer.

DC-DC chargers come in many forms and range in overall output as well as additional features. Companies like Renogy, REDARC, and Victron are just a few that offer multiple options. Some more advanced DC charging systems, like the REDARC Manager30, also incorporate things like solar and shore power. Although these all-in-one systems are the more expensive option. However when you add up the various individual cost of each system, the all-in-one option isn’t a bad deal.

As far as the vehicle’s alternator, a 12v fridge alone doesn’t draw that much power once it’s up and running. Just a few amps. This load would go mostly unnoticed by the vehicle’s alternator while it’s running. Most factory auxiliary power ports (some of which are still referred to as ‘cigarette lighter’ ports) can sustain small amp loads like a fridge. However it’s always a good idea to check to make sure the load of the fridge will not exceed what the circuit is rated for. Some auxiliary ports are also only live when the ignition is running, others are live even off the battery. To complicate matters even worse some vehicles have the option to switch between the two depending on the position of the fuse in the fuse box.


For the most part the vehicle’s factory electrical system is robust enough to survive a few mods. The biggest variable is how hard are you going to work the system and how long. That is the biggest indicator of if, and how far, you need to upgrade the battery and alternator. The number of accessories (lights, chargers, radio, amps, inverter, fridge, etc) will dictate if it’s worth investing in a power management system. That’s why have a plan for you build is a good idea.

For more on planning a build:

Stay tuned for a deeper look into vehicle electrical systems and some specifics of when it comes to how all these components interact.

Thank you to Corporate Partners REDARC Electronics, Odyssey Batteries, Rigid Industries, Warn Inc, and others for their support over the years which helps make articles like this possible.