Why Do Diesels Have Two Batteries – Why Not Just One?

Why Do Diesels Have Two Batteries – Why Not Just One?

Many people do not yet know that many diesel trucks out there have two batteries. If you are one of those who are not aware of this fact, you may be wondering why diesels have two batteries.

In this post, we aim to answer just that question as well as take a closer look at batteries for diesel trucks and how you can ensure your diesel truck always performs at its optimum.

For easier navigation throughout our post, please use our table of contents below.

Let’s jump straight in.

Truck Dual Battery Setup

When you open the hood of the diesel truck for the first time, you might be in awe when you discover that they have two batteries. Most other cars have only one battery. Why do diesel have two batteries? The reason for this is to allow the diesel trucks to crank at higher amps.

Simply put, diesel trucks unlike gas-powered cars require a high resistance for them to start successfully. Putting this in layman’s terms, a diesel engine requires more power to start compared to a gas engine. In fact, the diesel truck will require almost double the energy to start.

If you want to provide your vehicle with a much heavier electrical load, then you may want to consider a dual battery setup. This will also ensure you are never left behind with a hard starter battery.

Dual battery setups are often common in trucks and SUVS, especially those that are set for camping, off-roading, and Overlanding. Often times in such instances there is a demand for more electricity such as for running a fridge.

While some people tend to think that a dual battery setup is difficult, the truth is that it is really quite simple to do. All you have to do is make a few modifications and you are all set.

What You Will Need

  • A dual battery kit
  • A second battery
  • Drill bits
  • Crimping tool
  • Marker pen
  • Diagonal pliers
  • Heat gun

How To Set Up A Dual Battery For Your Truck

dual bettery setup

Why Do Diesels Have Two Batteries, Set Up Process #1: Mounting The Second Battery

You will need to begin by choosing an ideal mounting position that is going to accommodate the secondary battery. You may find that you need a cheap battery tray. The good thing is that these are easy to come by.

The most important thing at this stage is to ensure that the battery contains a wire that is long enough to reach the secondary battery.

Why Do Diesels Have Two Batteries, Set Up Process #2: Disconnecting The First Battery

Before moving on to do anything else, it is important that you disconnect the first battery’s negative terminal.

Why Do Diesels Have Two Batteries, Set Up Process #3: Setting Up The Voltage Sensitive Relay VSR

The VSR or the voltage-sensitive relay is the computer that will control both batteries. It is important that you set this up in a place where you can easily access it.

You also want to ensure that it doesn’t come into contact with hot or any moving parts. Thirdly, make sure that the VSR is close to the main battery.

The VSR will feature a mounting plate. You can use the marker to mark exactly where you will need to drill four holes. Use short screws to secure both the top right and bottom left of the mounting plate.

Next, pick the red cable and ensure there are lugs at both ends of the cable. The red cable should have sufficient length to begin from the mounting plate center and reach the positive terminal in the first battery.

Ensure that this red cable doesn’t come into contact with any moving parts or any hot parts.

Why Do Diesels Have Two Batteries, Set Up Process #4: Cut The Cables To Length

Now that you have the sufficient length of the red cable, cut it out to the required length. You will need to repeat the process with the other positive terminal of the secondary battery.

Why Do Diesels Have Two Batteries, Set Up Process #5: Fitting The Lugs

You will need to strip out up to 15 millimeters of the plastic insulating and fit the lugs. Then take the crimping tool and crimp them in place.

Take the heat shrink and side it above the lug and use a heat gun to ensure a waterproof sealing.

Why Do Diesels Have Two Batteries, Set Up Process #6: Connect The VSR Earth

It is crucial that you earth the black wire which is connected to the VSR. The earth also needs to be grounded securely. You can use a bolt where the engine has been earthed and secure.

Why Do Diesels Have Two Batteries, Set Up Process #7: Attach The Main Battery To The VSR

The cable that runs from the main battery will go to the VSR terminal and will be marked using a red dot. It will also contain the words positive sense battery.

On the other hand, the cable that comes from the auxiliary battery will run and will connect to the battery terminal that has been marked positive second battery.

Why Do Diesels Have Two Batteries, Set Up Process #8: Ground The Secondary Battery

It is important to note that the secondary battery also requires solid ground. Start by connecting the 24-inch earth cable that is black in color and connect it to a bolt that has been used to earth the engine

Why Do Diesels Have Two Batteries, Set Up Process #9: Placing The VSR Onto The Mounting Plate

You will need to use a couple of longer screws to attach the VSR to the mounting plate. The positive red cable can easily be attached to both the first and the second battery.

Why Do Diesels Have Two Batteries, Set Up Process #10: Connect The Negative Leads

Make sure that the negative terminals are connected and secured to the first battery as well as the secondary battery. You can make use of cable ties to do this.

With everything connected, of course, you want to find out if everything works. You can do this with a multimeter.

Start the car and use the multimeter to take a reading from the major battery.  If you find that the voltage is climbing more than 13.3 volts, then it’s only a matter of time before the relay clicks and the red light turns on.

This will show you that you have successfully connected the auxiliary battery.

Turn off the engine. Take the multimeter and get a reading from the main battery. The relay should be able to disconnect when the voltage is under 12.8.

Turning on the headlights can help fasten the process. You will know that you have made a successful connection when everything works according to the above test.

Diesel Batteries

There are different types of diesel batteries that you can find for your truck on the market.

Diesel batteries

Why Do Diesels Have Two Batteries, Type #1 – Absorbable Gas Battery

There are two major functions that this type of battery is able to perform. The first is that it can offer large cold cranking amps as well as be able to keep your car’s electrical system running for a considerable duration of time.

One of the main ways that the battery efficiency can be improved is through the use of an electrolyte formula on the metal plates. By placing the battery formula close to the metal connection, the conductivity of the car’s battery increases significantly.

The AGM batteries will charge significantly slower than other types of batteries and are designed to last long. It is also possible to recycle these types of batteries and plus they are an eco-friendly option. The AGM batteries are also a good choice for cold environments and climates. This is because they can still retain a charge even in the cold weather.

Why Do Diesels Have Two Batteries, Type #2 – Lead-Based Batteries

These are similar in appearance to the AGM batteries but function in a completely different way. Lead-based batteries are common in diesel trucks.

It uses an acid formula that blends with the lead in the batteries and allows this to store the charge. There are two types of lead batteries. These are wet cell batteries and gel cell lead batteries.

The wet cell type of batteries features a compartment that allows them to hold the electrolytes. To ensure an extra kick you can pour in distilled water provided the battery is sealed.

The second type of lead-based battery is the gel cell battery. This contains a viscous liquid that holds up the electrolytes. Such types of batteries are good in hot climates. However, they do not perform well in cold climates as the gel will freeze.

Why Do Diesels Have Two Batteries – Considerations When Buying A Truck Battery

Don’t worry if you are not aware of the best type of battery for your vehicle. Most people do not know.

Let’s take a look at a few factors to consider as well as the different types of batteries that you may want to consider.

1. Cold Cranking Amps

The CCA (cold cranking amps) means our batteries output in amperes. This is either measured in minus eighteen degrees Celsius or zero degrees Fahrenheit. It measures the number of amperes that the battery is able to provide in a span of 30 seconds.

You can also find out the amperage hours simply by taking this figure and diving it with 7.25. Now you will be able to know how long the battery will last during standard usage.

Simply put, the higher the figure the more suited and efficient the battery is in cold climates.

2. Battery Reserves

This is another important factor to consider when buying a car battery. The reserve capacity of the battery is the amount of time that the battery will last while outputting 2.5 amps. Let’s assume you left the car stereo on while switching off the car only to come back and find that the car battery is died and won’t start the car. This is a good example of a battery with a low reserve capacity RC.

A battery with a high RC will allow you to run multiple electronic equipment before the charge ends. Lead-based batteries perform averagely when it comes to reserve capacity. They are not too bad however the AGM batteries have the highest reserve capacity.

With an AGM battery, you are able to run tech straight out of the 12-volt adapter yet still continue to use the truck.

3. Size Of The Battery

First and foremost, the battery needs to be compatible with your vehicle. If it is too large then it will not fit. So before getting any battery, it is important that you get the size and structure of the OEM right.

But the size of the battery is just one part. It is important that you ensure that the size of the terminals is also right otherwise you may get the proper sized battery yet it still cannot fit because of having terminals that are too thin or too thick.

Furthermore, the battery needs to not be too tall otherwise it may touch the hood of the car. Always ensure that the battery has a clearance of 3 to 4 inches between the terminals and the hood of the car.

Diesel Jump Starter

Why do diesels have two batteries? Many diesel trucks out there have two batteries. Others have one large battery. That said, did you know that you could still jump-start a diesel battery with a regular battery from a gasoline-powered engine?

If the battery in your diesel truck is dead and has lost all power, you could still jump-start it using the steps below.

Before jump starting, ensure that both cars are in neutral and that you have engaged the parking brakes.

Step 1

Begin by turning on the heater in your diesel-powered truck. This will help warm the electrical system and keep it safe from surges in electricity which could damage features such as Bluetooth connectivity.

Step 2

Ensure that you have turned off all the lights in the diesel-powered vehicle. Check the cables on the batteries. You may find that one of the cables on one of the batteries is thicker than the other. Take the jumper cables and attach them to the battery with the thicker cables.

If all the cables are the same size, you can jump-start with any battery. Ensure that the jump start cables are connected in the right order if there is only one large battery in the diesel truck.

Step 3

Find the positive terminal on the diesel truck and connect one of the jumper cables. This will usually have a (+) sign next to it. It may also have a red cover.

Step 4

Connect the same line of the jumper cable to the positive terminal of the source vehicle.

Step 5

Connect the other jumper cable o the negative terminal of the source vehicle.

jump starter

Step 6

Connect the other end of this cable to a metallic part of your vehicle that doesn’t have any paint. If you can’t find a part of your vehicle’s hood that lacks paint, you can use the metal bracket that holds up the hood. Just make sure that the cable is not close to any moving parts or any belts.

Step 7

Turn on the engine on the source vehicle.

Step 8

Now turn on the engine of your diesel truck and let it keep running. You can allow both vehicles to keep running for a minute or two. But if the battery had been dead for a long time, you can let them run for longer.

Step 9

Now turn off the source vehicle engine and leave the engine of the diesel vehicle running.

Step 10

Take out the cable that is connected to the unpainted part of the vehicle.

Step 11

Remove the cable that is disconnected to the positive terminals of both vehicles.

Step 12

Next, disconnect the cable connected to the negative terminals on the source vehicle.

Step 13

Take your diesel-powered truck for a 15-minute drive. This will ensure that the battery gets a full charge.

The battery at this point should be able to start the engine next time you want to drive your car. However, if you find that the battery is dead the next time, then the battery is likely faulty and cannot hold a charge. At this point, it’s probably a good idea to get yourself a new battery.

Two Batteries In Series

battery in series

As we’ve seen either diesel trucks can come in a single large battery or they can come with two batteries. Have you ever wondered how you can connect two batteries in series?

Firstly, connecting two batteries in your car can either provide a 12-volt battery system that offers twice the amps. Or they can offer a 24-volt battery system that can provide twice the cranking speed.

Many trucks, SUVs and even RVs will feature a dual battery system that can either provide 12 volts or 24 volts.

For one, if you haven’t connected the two batteries together then each is able to provide 12 volts and give you two areas of connection.

If you connect both the batteries in parallel you will get 12 volts also. This is when you connect the positive terminal to the positive terminal and the negative terminal to the negative terminal. This will also provide double the amp output.

The Series Connection

If however, you connect both batteries in series, you will be able to get 24 volts. To connect the batteries in series you will need to connect one of the positive terminals of some batteries to the negative terminal of the second battery. While the voltage will double, the amps will remain the same.

Connecting the battery in series can provide enough charge to crank up the car’s engine and get it started.

One thing that you should always keep in mind is that the two batteries which you intend to connect in series need to have the same capacity in amps and the same voltage. Otherwise, you could find yourself damaging the batteries.

The series connection doesn’t always have to occur with two batteries. You can also connect a third battery provided it is of the same capacity and voltage.

All you will need to do is to connect the positive terminal of the first battery or the negative terminal of the second battery to the positive terminal of the third battery.

When you will need to connect the negative terminal of the first battery or the positive terminal of the last battery.

Two Batteries In Parallel

If you want to increase the capacity of your batteries, you may want to connect them in parallel. The voltage will be the same but the capacity will increase.

Remember that before you can connect two batteries in parallel they need to be of the same voltage. And capacity. For instance, you cannot connect a 24V battery to a 12-volt battery.

If your car’s battery has a capacity of 100Ah, and you connected two of these in parallel you would get a capacity of 200Ah.

To connect your car’s battery in parallel, you will need to connect the negative terminal with a negative terminal in the second battery and a positive terminal with a positive terminal in the second battery.

By connecting the battery in parallel, you increase the capacity and thus the duration at which the battery can power other equipment. You may want to connect your dual batteries in parallel if you are driving an RV, or have set up your truck for camping or overlanding.

Frequently Asked Questions

How Long Do Truck Batteries Last

Under normal situations, the car battery can last as long as 3 to 5 years. After this time, you may notice a reduction in efficiency and decide to replace it with a new battery.

How To Unhook Jumper Cables

Start by disconnecting the negative cable to the car that is jumped and then from the source car. Next, remove the positive cable from the source car and then remove it from the car that is jumped.

How Long Does A Diesel Need To Be Plugged In

Having plugged in a diesel, leave it for at least 3 hours. If you live in colder climates and the temperature is below 10 degrees, then you may need to leave it plugged for longer such as 6 hours.

What Does Plugging In A Diesel Do

Plugging in your diesel truck warms up the block heater which then warms up the engine coolant. In return, the cylinders are able to remain warm even in cold weather making it easier to state your vehicle.

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