Adding a Starting Battery…

The attachments describe a change from 2 batteries (Group27 + Starter battery) to house bank of 2 grp27 plus one starting battery.  The change was made to our Mark 2 (1990), with minimal cost for materials.

In addition to the G27 and existing starter battery in the battery box under the Starboard Settee, I had an extra deep cycle G27, from a previous boat. Without getting into inverters, isolators, -combiners, etc, I just wanted to add “ampacity” to the house bank, and keep the starter battery topped off for turning over the motor.

Wiring the 2 deep cycle batteries in parallel makes it appear as a single big battery with twice the amps.  The ‘1-both-2-off’ switch on the panel works as before; charger has its own leads, so charging at the dock will charge both banks (all 3 batteries). I choose to locate the starter under the aft quarter berth.  I figured the wire length (voltage drop) would be no worse than the current configuration.  I only had to add the ground cable (#2 welding cable from the electrical supply store; they crimped on the ends) from the starter battery to the engine block, and the platform to hold the battery.

The platform is plywood, coated in epoxy.  It has 3 legs made from bolts from the hardware store.  Once I had the platform level, I mixed up some epoxy and glued the bolt heads to the hull.  I already had the battery box, and I bought the battery strap from WM. Last summer I was working for GE Transportation Systems, so I couldn’t swing a cat without hitting an electrical engineer.  Several engineers sprinkled the plan (diagrams) with holy water, so I had high confidence. After one season, it works.  Now on the hard, I took some pictures (which match, but don’t look like, the diagram). Any questions, you can contact me.