How to Charge Dual Batteries in a Car

Written by stephen benham
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How to Charge Dual Batteries in a Car
There are two methods of charging dual car batteries, depending on how the batteries are wired. (dead battery image by Katrina Miller from Fotolia.com)

Cars can be fitted with dual batteries for a couple of reasons: to increase capacity or to increase output. Dual batteries for increased capacity will be wired in parallel. This means that the output voltage remains the same but the time they last doubles. For example, two 12-volt batteries wired in parallel still produce 12 volts but they last longer. Dual batteries wired in series do not last longer, but the output will be the combined voltage of both batteries. For example, two 6-volt batteries wired in series will produce 12 volts but the time they last will be the same as having one battery. Learn how dual batteries are wired before charging dual batteries in a car.

Skill level:
Easy

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Things you need

  • Wrench
  • Battery charger

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Instructions

  1. 1

    Check how the dual batteries are wired in your car before charging. If your batteries are wired in parallel, the cable connected to the positive (+) terminal of the first battery will connect to the positive terminal of the second battery. The same applies to the negative (-) cable. If wired in series, the cable attached the positive terminal of the first battery will connect to the negative terminal of the second battery.

  2. 2

    Check your dual batteries' output. The volts will be marked clearly on the top of each battery. If you have two 12-volt batteries wired in parallel, the output is 12 volts, but if you have two 6-volt batteries wired in series, the output will double to 12 volts. It is important you learn this before charging.

  3. 3

    Follow the steps in Section 2 if your batteries are wired in parallel or follow the steps in Section 3 if they are wired in series.

  1. 1

    Place a battery charger as near to the dual batteries as possible. Disconnect the positive and negative cables that lead from your duel batteries and connect to your car using a suitable wrench.

  2. 2

    Disconnect the positive and negative cables that connect the two batteries together using a wrench. You need to do this as your charger is likely to produce a charge of 12 volts (or 6 volts). Leaving the batteries connected in parallel and connecting to a charger could damage the charger. It will also take a substantial time to charge them.

  3. 3

    Connect the positive (red) cable from the battery charger to the positive terminal of the first battery and the negative (black) cable from the battery charger to the negative terminal of the first battery.

  4. 4

    Plug the charger into an electrical socket. Check to see that the charger is switched to the correct voltage and turn it on. Leave for several hours (or overnight). Unplug and disconnect the cables once charged.

  5. 5

    Repeat Steps 3 and 4 for the second battery and then reconnect the cables linking the batteries, positive to positive and negative to negative. Reconnect the cables that connect to your car to the battery, ensuring you attach positive to positive and negative to negative.

  1. 1

    Place a battery charger as near to the dual batteries as possible. Set the voltage on the charger to the combined output of the duel batteries. If you have two 6-volt batteries, set your charger to 12 volts.

  2. 2

    Disconnect the positive and negative cables that lead from the dual batteries and connect to your car using a suitable wrench. Leave the cable connecting the dual batteries connected.

  3. 3

    Connect the negative (black) cable from the battery charger to the negative terminal of the battery that does not have a cable attached. Connect the positive (red) cable from the battery charger to the positive terminal of the other battery that does not have a cable attached.

  4. 4

    Plug your battery charger into an electrical socket. Recheck the output voltage from the charger to ensure it matches the combined voltage of the dual batteries. Turn on the charger and leave for several hours (or overnight).

  5. 5

    Turn off the charger once the batteries are charged. Reconnect the two negative and positive cables attached to the car to the battery terminals, ensuring you attach positive to positive and negative to negative.

Tips and warnings

  • If you have two battery chargers, you can charge batteries wired in parallel at the same time. Ensure you disconnect the cables joining the two batteries. Otherwise you will fuse your battery charger.

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