Understanding the Ins and Outs of Your Electrical System

The electrical system on your average diesel truck is both simple and complex. Many folks think of batteries as “the things that start your truck,” and that’s all the thought they give to them. However, if you delve a little deeper, questions emerge: How many amps do you need, do you really need two batteries, and when do you need an alternator upgrade? There are also other issues to consider, such as charging and maintaining the batteries, cable length and wire gauge when wiring in aftermarket products, and fighting corrosion resistance. More often than not, people who make mistakes with their electrical system do so simply because they didn’t know better.


Before we get started, there are a few common terms we need to go over: voltage, amperage (amps) and ohms (a measure of resistance). Voltage is the electrical “push” through a wire that sends power from one area to another. Its close relative is amperage, which is the strength of that push. If you think about these terms like speed and power, you get the principles. Because electrical systems are governed at a certain voltage (like a speed limiter), the amps decide how much power we can put through the system (namely, the starter). The last major term is ohms, which is a measure of resistance through a wire. Have you noticed how your battery cables are huge compared with the other wiring? That’s because larger cables have less resistance, and the electricity traveling through them generates less heat.

Different Types of Batteries: Lead-Acid, AGM, and Lithium-ION

Before we get into the type of batteries you’ll need, there are two huge factors to consider: temperature and timing. What works at 80 degrees by the beach might not work at -20 degrees in a snowdrift. Similarly, diesels have huge compression ratios at 16:1, so any added engine timing can make the power plant start radically differently. While there are lots of batteries out there, probably 90 percent of the ones you’ll find are either lead-acid or sealed lead-acid. Why? It’s a simple matter of cost versus power; these batteries have the amps to get the job done, yet they are relatively inexpensive.


Your typical diesel battery is fairly large, weighs about 40 pounds (or more), and has around 800 coldcranking amps (CCA). With twin batteries providing the amperage, they’re usually enough to start a diesel in virtually any climate. Interestingly enough, a “12-volt battery” isn’t really 12 volts — it should test at 12.8 volts or more when fully charged.

The other common types of batteries are AGM (Absorbent Glass Mat) and lithium-ion, both of which are comparable to or exceed lead acid in performance and last longer (and are safer and/or lighter). The real kings of the racing world are lithium-ion batteries (like huge laptop batteries), which are capable of well over 1,000 pulse-cranking amps (think a hot start) while weighing less than 10 pounds. Lithium-ion batteries reputedly last the longest, but they’re also the most expensive.

Cables are often a neglected part of battery and charging systems and can get gunked up with corrosion and engine grease. Whatever is wired straight to the battery has its own encased fuse (black square), which can prevent shorts or too many amps from blowing out a circuit.
If you’re running a single battery, it’s best to step up to a large Group 31 battery. This lead acid piece that starts a high-timing 12 volts tested at 1,158 CCA.

So now for the million-dollar question: What type of battery do you need? In our opinion, diesel batteries should have a minimum of 800 cold-cranking amps (CCA) — and that’s if there are two of them. A single battery should have at least 1,000 CCA and a lot of reserve capacity, something like a Group 31 battery. In very cold weather — or if the engine has a ton of timing — we’d recommend two batteries that are 900 to 1,000 CCA. While these ratings might sound like overkill, remember that these recommendations take into account cranking speed and possible hard-start situations.


Your Charging System:Alternators and Cables

A good battery might be able to start your diesel truck, but there’s also the matter of keeping it running. Manufacturers do a pretty good job in this respect, as most diesels have strong alternators from the factory. If we had three pieces of advice here, it would be maintenance, maintenance, and maintenance. Corroded, broken, or loose wires can wreak havoc on your electrical system — including fires! Dangerous electrical issues seem to be most common in later-model Dodges with a lead-battery terminal. Because the computer regulates the voltage, a bad connection on one battery will give a false reading and kick the alternator into overdrive, which can cause the battery to boil, explode, melt, or a combination of all three.

Lithium-ion batteries are on a different level than most other batteries as far as power per weight goes. This 780 CCA battery from Antigravity Batteries weighs less than five pounds! Lithium-ion batteries also require their own specific charger

As for alternator issues, older 7.3L Fords have the most problems, although a redesign has since fixed most trucks with the issue. Overall, alternators in diesels are pretty stout. Duramax engines (even the older ones), for instance, have 105- and 130-amp alternators, which is fine for just about any electrical draw you’ll encounter. There is one other factor, however: Engine rpm. The OEM alternator might make 105 amps at 4,000 rpm, but it might only make 35 amps at idle, which can be problematic for welding rigs, trucks with air tools, or other utility trucks. Fortunately, upgraded units are available that produce more than 100 amps at idle and more than 200 amps at higher engine speeds.

Charging batteries takes time, and the more amp output from the charger, the quicker the battery will be brought back to life. Shops often use small stand-up chargers that can be as high as 80 amps for a booster charge.

One last (but definitely not least) point to think about concerns cables, fuses, and amperage. In most cases, automotive wire should be at least 10- or 12-gauge if it’s connected to a significant draw. Also, the longer the wire, the more resistance it will have, so keep this in mind when wiring in things like lift pumps. A 10-gauge wire that’s good at 30 amps at 7 feet might drop down to only 15 amps (recommended) at 15 feet, so longer wiring must have a larger diameter. Wires to things like the starter might be a 0- to 2-gauge because of the enormous amperage involved.

Sparking an Interest

If there’s one lesson to take away from this article, it should be that your diesel’s electrical system isn’t something that should be ignored or feared. With clean connections, new batteries, and proper wiring, your diesel will be happy for miles down the road.

It’s OK to see a drop in voltage on some diesels when they first fire up. Glow plugs and grid heaters can take a tremendous amount of juice to operate on a cold day. When voltage stabilizes, it should be around 13.5 volts if the battery is charged.
Powermaster Performance has virtually every alternator (and starter) you could need for your diesel, including race alternators and alternators with adjustable voltage for unique battery systems.
If you’re looking for wire and connectors, look no further than your local auto parts store or battery dealer. Our local Interstate Batteries dealer had just about every connection imaginable on hand
It doesn’t take much to start your own electrical workstation. Trust us. When you’re trying to fire up a truck in the middle of nowhere in the dead of winter, your truck’s electrical system will thank you!

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