How to Charge a Diesel Truck with Two Batteries

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A Diesel Truck with Two Batteries

If you’re new to diesel trucks, you might be surprised when you discover the dual batteries under your hood. And if those batteries die, you’ll need to know how to charge a diesel truck with two batteries. But don’t worry, it’s easier than you think!

Before learning how to charge a diesel truck with two batteries, first, take the time to understand a few basics.

By learning how diesel engines differ and why dual batteries are necessary, you’ll be ready to D.I.Y. your battery woes away.

Breaking down the Difference: Diesel vs. Gasoline Engines

Diesel engines and gasoline engines are both internal combustion engines. This type of engine converts chemical energy from fuel into mechanical energy through small combustions.

However, diesel and gasoline engines produce these combustions differently. Diesel engines use a compression-ignition model, whereas gasoline engines rely on spark plugs.

Gasoline engines combine fuel with air and use pistons to compress this mixture. Then, spark plugs ignite it, creating combustion.

Diesel engines, on the other hand, compress the air before the fuel is injected. When compressed, the air becomes hot, provoking combustion.

Today, modern diesel cars and trucks can achieve peak efficiencies of around 45 percent, compared to the gasoline engine’s 20 to 35 percent.

Why Do Diesel Trucks Have Dual Batteries?

The diesel engine’s greater efficiency requires a higher compression ratio than a gasoline engine. Typically, the compression ratio for a diesel engine is between 15 and 20. Gas engines, on the other hand, have a ratio that is closer to 10.

This higher compression ratio increases the diesel engine’s thermal efficiency. However, it also results in a higher resistance load when starting.

More power is required to turn a diesel engine over because of this high resistance load. And that’s why diesel trucks have two batteries.

Your truck’s dual batteries work together to provide the cranking amps necessary to turn your engine over.

Parallel vs. series configurations

When learning how to charge a diesel truck with two batteries, it’s essential to understand the battery configuration.

Diesel trucks use a parallel configuration for their batteries. This means the positive terminals of your dual batteries are connected, as are the negative terminals. A parallel configuration increases the amperage rating without changing the voltage. As a result, more energy is available for turning your engine over.

On the other hand, batteries connected in a series configuration will have an increased voltage, but the amperage rating remains the same. Never use a series configuration in your truck — this would destroy your electronics and could start a fire!

How to Charge a Diesel Truck with Two Batteries

So, are you still wondering how to charge a diesel truck with two batteries?

You have two options: Either use a battery charger set to a low-amperage charge or jump-start your truck and drive it until your alternator recharges your battery.

How to charge a diesel truck with two batteries using a battery charger

To charge your truck’s dual batteries, you’ll need a wrench and two battery chargers.

Begin by disconnecting the negative terminal on your first battery, followed by the positive terminal. Then repeat with your second battery.

Next, connect your first battery charger’s positive clamp to the positive terminal of the first battery. Then, attach the negative clamp to the negative terminal. Again, repeat this process with your second battery and second battery charger.

Set both chargers to low-amperage charges. Using a lower amperage will lengthen the charge time, but this will also allow you to charge your batteries completely.

After two hours, turn off your chargers and check if your truck starts. If it doesn’t, reattach the chargers using the above procedure and let the battery charge for an additional hour.

Finally, reinstall your battery terminals. Always begin the reinstallation process with the negative terminal.

How to charge a diesel truck with two batteries with a jump start

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If you don’t have time to spare, jump-starting your truck might be your best option.

For this process, you’ll need two jumper cables and a running vehicle, which we’ll call the donor vehicle. If a donor vehicle isn’t available, a jump starter with a power bank will do the trick.

Start by turning on both vehicles’ heaters and turning off all other electrical devices. This will ensure their electrical systems aren’t damaged. Turn off the engine of the donor vehicle and put both vehicles in “park” or “neutral” with the parking brake engaged.

If you notice one of your truck’s dual batteries has thicker cables, that’s the battery you should attach your jumper cables to. But if both batteries have cables of the same thickness, you may use either battery for the jump start.

Next, clamp one jumper cable to the battery’s positive terminal. Then, connect the other end to your donor vehicle.

Once the first cable is in place, it’s time to attach the second one. Start at the donor vehicle’s negative terminal, then attach the other end of the second cable to a solid metal part on your truck — this will ground it.

Start the engine of the donor vehicle, then start your truck’s engine. Allow both engines to run for a minute or two, then turn off the donor vehicle’s engine.

Remove the cable from your ground point, then remove the cable from the positive terminals of both vehicles.

Finally, remove the cable from the negative terminal on the donor vehicle. Be sure to drive your truck for 15 to 20 minutes so your alternator can finish charging your battery.

When do you need a new battery?

Eventually, you will need to replace your batteries. If your battery is cracked, leaking, or dry, you’ll need a replacement. And even if your battery looks fine, you should replace it every three years.

Wondering which battery is best for your diesel truck? Check out our handy guide.

How to Charge a Diesel Truck with Two Batteries: Now You Know

That’s all there is to it! It’s an easy D.I.Y. skill, and learning how to charge a diesel truck with two batteries will pay dividends. You won’t have to worry about getting stranded with a dead battery if you know how to do a jump start!

If you frequently make short trips in your truck (say, a 10-minute commute to work), you may be draining your battery.

This is because the alternator requires at least 15 minutes to charge a battery fully. In this case, you may want to consider giving your batteries a charge every so often.

Do you prefer to use a battery charger or to jump-start your truck when your batteries die? Comment below and let us know!

Featured Photo by Matthew T Rader on Unsplash

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