Boatbuilding Blog

February 21, 2010

Wiring Diagram

Filed under: Building - Electric & Plumbing — tomlarkin @ 1:52 pm

I’m working on the 12 volt DC wiring design. The ACR (Automatic Charging Relay) makes the battery charging automatic. Either charging source (the 110 volt charger or the outboard motor charger) will charge both batteries, but will disconnect them as they’re being discharged. This way, I can safely draw down the house battery without taking the chance of draining the starting battery and not being able to start the engine.  I’m still not sure what size wires to use, or if it’s best to have the starting battery under the pilothouse sole or in the stern next to the motor.

The Blue Sea ACR came with a wiring diagram, but I found it unclear.  Adding the color-coding and additional detail helps me understand the design. The picture was drawn in Visio. The ‘Starting Isolation’ wire to the ACR will disconnect the house battery when the engine is being cranked to protect the house electronics from power surges caused by the motor starting.

I’ve got both batteries already, and am making the holders for them. The house battery is a Dyno 4D-size, with 180 amp-hours. It weighs about 114 pounds. It goes under the Port pilothouse sole.  I had initially intended on using an 8D-size battery, but our power needs should be pretty low, just LED lighting and electronics.  The starting battery is a generic deep-cycle starting battery. Outboards don’t take a lot of power to start.

The other task in progress is the dashboard. The frame is complete, and I added the steering wheel mount this afternoon. It’s heavily built because there’s a lot of stress on that area when you’re hanging onto the wheel in bad weather.

(Click the image for full-size)

Wiring - Blue Sea ACR

Edit 2-21 – Here’s a new drawing, with the parts in a more real-life configuration. All the stuff below the switch is below the pilothouse sole. I think this layout clarifies the relationship between the batteries and the ACR.

Original Wiring - Blue Sea



  1. I ran into the same issue with Blue Sea’s original installation diagram. Yours made a huge difference, much clearer. Thanks.

    Comment by Le, L. — March 23, 2013 @ 3:28 pm

  2. Thanks for sharing your work. This is very helpful. I am looking to add an on board battery charger. In this configuration would you use a single bank charger?

    Comment by Strawn — April 7, 2014 @ 11:12 pm

    • No. A single battery bank leaves you very vulnerable to getting stuck if something drains your batteries. It’s happened to me a few times. Once, my bilge pump stuck on until the battery was flat, and another time I just left some lights on accidentally while at anchor for a few days. Both times I would have had to get a jump to get going again if the starter battery hadn’t been there.

      Comment by tomlarkin — April 8, 2014 @ 11:17 am

      • Thanks Tom. Just so I am clear… For an on-board charger, I should get a 2 bank charger so that I can charge both batter independent of the circuit?

        Comment by Strawn — April 9, 2014 @ 9:56 pm

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