How to Charge a Diesel Truck with 2 Batteries

The normal compression in a diesel engine is a tad more than 17.1 and the normal compression in a gasoline powered engine is closer to 9.1. That means that a diesel engine requires a lot more cranking amps to get it fried off than the normal gasoline engine requires, so the diesel truck manufacturers often put two batteries under the hood to handle this load.

Your batteries for diesel truck are most likely 12 volt batteries because that is what is normally used on a vehicle. When a truck has two batteries beneath the hood those batteries can either be connected in parallel formation or in a series.

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Batteries in Parallel

When the batteries are in a parallel the positive terminals of each battery are connected to each other. Then the negative terminals are connected to each other. The battery retains the same voltage, a typical 12 volt but the current rating will be increased. This means that you have an increased amount of energy being supplied at one time but your electrical system is not going to be overloaded or put under a strain.

Your battery cables on your truck are heavy duty when compared to the battery cables found on ordinary vehicles that have only one battery connected to them.

Batteries in a Series

When the batteries under the hood of a truck are connected in a series it is done to increase the voltage of the batteries while still retaining the same current power or amperage rating.

When batteries are connected in a series there can be 2 or more batteries connected together. Be sure that you pay careful attention to the connection placements of each battery if you disconnect any of these batteries. You do not want to take the batteries off and then try to remember what order the connection cables should be connected in.

Changing out Batteries when the Truck Requires 2 Batteries

If you ever decide to change out the batteries on your vehicle you want to replace them with batteries that are identical to each other, and batteries that are identical in voltage and cranking amps to the batteries you just removed from the vehicle. This is a critical point. If you hook up two batteries of different strengths or sizes you can fry the electrical system of your vehicle.

Connecting the Battery Charger to the Batteries

If your batteries under your hood have discharged to the point that they cannot crank the vehicle then it is time to charge them up or jump them off.

You typically need two battery chargers working in unison with each other to charge a double battery system. You will also need an open end wrench to use when disconnecting the battery cables from the battery terminals.


Steps to Follow

  • Open the hood and locate the negative terminal on the first battery. Disconnect the cable from this terminal. Then disconnect the negative terminal cable on the second battery.
  • You will now connect the positive clamp from your battery charger to the positive terminal of the first battery. (Tip: The positive clamp will be red and the negative clamps will be black. Do not cross these two clamps or you can damage your charger, your battery, or even cause a spark that creates a fire under the hood of your vehicle.)
  • Now connect the negative clamp to the negative terminal of the first battery.
  • Repeat the clamp connections on the second battery.
  • Check the battery cable ends for signs of oxidation or corrosion. If they appear to be corroded you need to clean away the corrosion or oxidization prior to connecting the battery chargers to the terminals.
  • Set your battery chargers on the low amperage setting. The low amperage setting will take a longer amount of time to fully charge the batteries but when you use this setting you will be less likely to do any damage to your batteries, and the charge will be more complete than when you try and do a fast charge.
  • Wait a minimum of two hours before you disconnect the battery chargers.
  • Test the charge your batteries have received by cranking the truck. If your truck hesitates or does not immediately turn over and crank you need to wait for an additional one to two hours before you disconnect the battery chargers.
  • Turn the battery charger off or unplug the battery charger from the electrical supply source.
  • Disconnect the positive connection and then the negative connection from the first battery
  • Disconnect the positive connection and then the negative connection from the second battery
  • Connect the negative battery cables to the negative battery terminal on the first battery
  • Connect the positive battery cable to the positive battery terminal on the first battery
  • Repeat the cable connections, negative cable first, on the second battery

Selecting a Battery Charger

Battery chargers have undergone a great many changes in the last twenty years. Today there are battery chargers that can actually help to prevent oxidization of a battery, there are chargers that can sense when the cables have been connected to the wrong battery terminal and save you from causing damage to your battery, and there are chargers that can show you exactly what charging state your battery has reached and that will automatically shut off when your battery reaches a full charge.

Some battery chargers can even tell you if the battery is not functioning properly and cannot take a full charge, and some chargers can even tell you if your alternator is functioning properly.

Final Note

You should never charge just one battery if your truck has a two battery system because you can create electrical supply problems, you can interfere with the proper functioning of your alternator, and you can cause one battery to be placed under an undue amount of stress.

You want both batteries on your vehicle to have the same amount of charge.

Batteries can be expensive to replace so you want to make sure that you treat them properly and charge them properly. Do not take short-cuts with your batteries.

Cold weather can affect the amount of cranking amps that your battery has so make sure that your batteries are fully charged and ready to use when the temperatures are expected to dip below the freezing level.

If you have a recurring problem with your batteries being discharged and being unable to crank your truck you need to test your alternator and determine if it is working properly. If the alternator is working properly check your batteries and make sure they can still take and maintain a full charge.

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