How to Wire Two 12-Volt Marine Batteries in Parallel

Written by doug leenhouts
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How to Wire Two 12-Volt Marine Batteries in Parallel
Wiring 12-volt batteries in parallel keeps voltage constant. (Jupiterimages/ Images)

If your boat batteries discharge too quickly, adding a second battery can help. By having 12-volt batteries connected in parallel, they both can be charged by a single alternator while still providing the 12 volts that most marine accessories require. If two 12-volt batteries are wired in series they will produce 24 volts, which can damage your boat's electronics. Before installing a second battery, make sure that your compartment has enough room to safely accommodate a pair of batteries.

Skill level:
Moderately Easy

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Things you need

  • 2 battery cables, AWG 4 gauge
  • Wire strippers
  • 2 battery terminal clamps
  • 2 3/8-inch ring terminals
  • Petroleum jelly
  • Crescent wrench
  • Battery strap

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  1. 1

    Disconnect the positive and negative battery cables of the existing battery. This will prevent current from running while you install the second battery.

  2. 2

    Lubricate the four battery terminals with petroleum jelly. This will prevent corrosion.

  3. 3

    Strip the insulation from both ends of a battery cable. Slide one end of the cable into the opened end of a battery cable clamp and tighten the two locking bolts and nuts with a crescent wrench. Crimp a 3/8-inch ring terminal onto the other end of the cable. Repeat this process for a second cable.

  4. 4

    Slide the battery cable clamps over the second battery terminals. Tighten them with a crescent wrench.

  5. 5

    Slide the ring terminal of the cable connected to the positive post of the second battery over the positive post of the existing battery.

  6. 6

    Slide the ring terminal of the cable connected to the negative post of the second battery over the negative post of the existing battery.

  7. 7

    Slide the existing positive cable clamp over the first battery's positive post, then connect the negative cable clamp. Tighten them securely with a crescent wrench.

  8. 8

    Tighten battery straps over both batteries to prevent them from tipping over and spilling in rough seas.

Tips and warnings

  • Remove any jewellery before working on your boat's battery system.

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