How To Charge or Jump Start a Diesel Truck with Two Batteries

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Are you a recent diesel truck owner, or have recently bought a newer model of diesel truck? Then you might be surprised to discover newer diesel trucks have two batteries. Take some time to learn how the two-battery setup works, understanding your diesel truck can help you out in an emergency.

Why Do Diesel Trucks Have Two Batteries?

Compared to gas engines, diesel engines have higher compression and more resistance. This means diesel engines require more power to start up. Diesel engines have a normal compression ratio between 15:1 and 20:1, while gas engines are around 10:1.

Although a high compression ratio makes an engine more efficient, it trades off efficiency for a higher resistance load. The higher the resistance load is, the harder it is for the engine to start.

This means that two batteries are needed to get a diesel engine started. The higher compression ratio also means diesel engines are made from more durable parts than gas engines.

Charging a Diesel Truck with Two Batteries

When your batteries die, you can charge them with a battery charger or jump-start them with another vehicle using jumper cables. Once your truck is started you should drive around for 30 minutes so the alternator can fully recharge the batteries.

If you use a battery charger, you’ll need two chargers: one for each battery.

Step 1: Disconnect the negative terminal on one battery, then disconnect the battery’s positive terminal. Repeat the process with the other battery.

Step 2: Connect one of the charger’s positive clamps (red) to the positive terminal of a battery. Then attach the charger’s negative (black) clamp to the negative terminal. Repeat this process on the second battery using your second charger.

Step 3: Adjust the charger settings to low amperage. Although the charging will take longer, you’ll get a more complete charge.

Step 4: After charging for two hours, turn off the battery chargers and try to start your truck. If your truck still won’t charge, reattach the chargers with the above steps and charge for another hour.

Step 5: Reinstall your battery terminals starting with the positive cables first. The negative (black) wire should be the last on.

Jump-Starting a Diesel Truck with Two Batteries

Don’t have time to wait for your batteries to charge? You’ll need to jump-start your car batteries. You’ll need a second vehicle running and jumper cables.

Step 1: Turn on the heaters inside both vehicles. All other electronics, including lights, stereo, and windshield wipers, should be turned off. Turn off the engine in the running vehicle and put on the parking brake. Both vehicles should be in park (for automatic transmissions) or neutral (for manual transmissions).

Step 2: Attach the red clamp of the jumper cables to the positive (has a red cover on the terminal post) of the dead battery.

Step 3: Clamp the other red jumper cable onto the positive terminal (red cover) of the battery on the other vehicle.

Step 4: Attach the black cable starting at the good vehicle’s negative battery terminal (has a black cover). Then ground the second cable by attaching the other end’s black cable to a solid piece of metal on the vehicle with the dead battery.

Step 5: Start the engine of the vehicle with the good battery. Then start the engine on the vehicle that had the dead battery. Let both vehicles run for a few minutes. Then shut off the engine on the good vehicle.

Step 6: When disconnecting your battery, first remove the grounded cable from your truck, then remove the positive cables from both vehicles. Remove the negative battery terminal from the other vehicle. When reconnecting the batteries, start with the positive cables and end with the negative (grounded) cables.

Step 7: Drive your truck for 20-30 minutes to let your alternator charge the batteries.

If you need to change the batteries in your diesel truck, make sure to change both batteries at the same time.

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