Before you begin repairing or modifying your overland vehicle's electrical system, you must understand some basic electrical theory and familiarize yourself with electrical circuit components, principles, and design. Not understanding these important basics can not only lead to poorly or non-functioning electrical components, but lead to a devastating vehicle fire or personal injury. We're not saying this to try and be some sort of nanny to adults, but the fact remains that nothing we are doing or are interested in with overlanding is worth risking your life or the lives of others. There are lots of quick how-to blogs and forum posts out there to tell you how to install some particular light or accessory the quickest and cheapest way, but they may not be giving you good advice or telling you the why behind component choices and installations.
The Electrical Gospel
The first thing you need to know is Ohm's Law. You should have learned this in high school science, but maybe you were texting your classmates instead of paying attention. The bottom line is that if you don't understand this simple equation, you really shouldn't be hooking up extra lights, switches, winches, or any other addition to your overland vehicle. I don't like referencing Wikipedia, but it actually has a well written overview of Ohm's law. Let's look at what this means to us.
I=V/R or V=IR or R=V/I
I= current V= volts R= resistance
We are primarily concerned with low voltage Direct Current when dealing with electricity in automotive applications as opposed to high voltage Alternating Current like is used in buildings. From here on out, we will be discussing electrical components and principles dealing with Direct Current, or DC. Even though these automotive circuits can be complex, we can still use Ohm's Law to deal with repairs and changes.
We don't need to go into that much detail here, I would highly recommend that you fully educate yourself on electrical theory in order to make the best decisions. The question is likely still nagging you- "why does this matter? My lights came with a harness and instructions..." Well, unless you purchase a manufacturer produced harness specifically designed for your vehicle, that harness may not be 100% what you need, and you still need to know how to properly connect this harness to your vehicle. First let's make sure we all understand what we are talking about in the formulas above. We will use the "hydraulic analogy" of water flowing through pipes to understand Direct Current in our overland vehicle's electrical circuits.
CURRENT- Current is the total amount of electricity flowing to a given point, such as a light or electrical motor. In other words, the total amount of "water" that can possibly flow through the pipes at a given time. This is VERY important to know because it will determine how big your pipe (wires) need to be. This is a function of how much water pressure (volts) you have and the need for water to complete the path (circuit resistance).
VOLTS- Volts can be thought of as water pressure. More voltage, more pressure. Vehicles typically use what is called "low voltage" systems. This means it's pretty much anything below 100 volts. Most automobiles that would be an overland vehicle are 12 volt systems. Exceptions are some medium duty trucks and heavy duty trucks that use 24 volt systems, or antique vehicles that may use 6 volt systems. Your voltage is determined primarily by the battery type on the vehicle. The charging system is then designed to maintain voltage of the batteries on the vehicle as originally designed.
RESISTANCE- Resistance is the load, or water drain on our circuit of pipes. Think of it as though you have water outlets on your pipes to do all sorts of things, but you still need to pump enough water to make it back to where the reservoir is. If not, all of the things you are trying to do with the water slow down or go dry. In electrical circuits, loads are what we are trying to do with our water. On our overland vehicle, this can be running lights, using radios, spooling a winch, running the heat or air conditioner, operating a 12 volt refrigerator, starting the engine, or a host of other things. We will talk about Watts and Ohms when discussing resistance.
The Gospel in Daily Life
Of course, we want to know how to apply this information to our daily lives and understand why it's important. Current, or Amperes (amps), will be your guiding light in choosing and installing everything between the battery and your new lights. But first, let's use our water analogy to understand why we should take the time to make sure we are installing things properly.
Your vehicle from the factory has a well-matched set of electrical plumbing to make everything work together nicely for average day to day use with a little extra built in. But, by golly, we just HAVE TO HAVE more stuff on our vehicle; otherwise it just isn't a proper expo rig! Let's say this truck has got a few years on it before it starts overlanding. What kind of shape is the battery in? Is it still the original, factory battery? Does it have an expensive AGM battery you just replaced? That battery is your extra bucket of juice to run things when the engine isn't running and turning your alternator. The alternator is your water pump. It is providing fresh, new juice to the bucket when your engine is running in the middle r.p.m.'s.
Something to keep in mind is that most alternators aren't "recharging" your battery when sitting at idle speed- you need to run the engine at highway r.p.m. to actually produce enough electricity to replenish the battery after a drain.
Well, now we've added some more off road lights that drain our juice whenever we use them because our electrical plumbing wasn't designed to run them from the factory; we installed a radio to talk to our buddies on the trail; there is now a winch on the bumper that can pull more juice than our engine starter when it runs; there is a neat little refrigerator in the back of the truck keeping essentials frosty, and maybe we even have an overland trailer hooked up with it's lights and brakes running all the time. Uh-oh....now there's not enough juice in the bucket to go around and that battery gets empty and wore out quick. Maybe the lights go dim, the fridge shuts off, or the truck won't start in the middle of the Mojave desert. So how do we prevent all or any of this? Well, don't modify your truck, or stick around while we figure out how to do this right.
You need Ohm's Law to figure out the current that your modifications are adding to your electrical system as it is now. You need to keep a running tally to see how much more "juice" you are going to need to produce. Also, you need to know the current for each circuit you add so you choose the correct sized "pipe" and connectors to put it all together. You need to make sure your have a big enough "bucket" for your juice to fill, and you also need a properly sized "pump" to provide the juice you need. Bottom line, if you are adding lights and other electrical accessories to your overland vehicle but aren't addressing the size and quality of your battery and alternator- YOU'RE WRONG.