Installing Additional Batteries


Adding a third battery

When we purchase Sea Dragon it was equipped with the original battery configuration, one group 27 house battery and one group 24 starter battery. The single group 27 offered 80 amp hours of reserve power. You combine this with the fact that most battery manufactures recommend that you never discharge a batter more than 60%, we had a house bank of only 48 amps hours.

A quick energy audit showed that we need about 150 amps per day. To that end, we now carry two Trojan group 31, 12 volt batteries as our house bank and one group 24 starter battery. It is my plan to add an additional group 31 house batter, upping our house to 405 amp hours. Moving the start battery is not high priority until we add a refrigerator.

Our Energy Audit is as follows:
Based on a 3 day trip.
Our alternator is rated at 75 amps
We carry 2 135 AH batteries (270 AH)

Using the 60% rule, we have 162 AH in our house bank.

We motor about 2 hrs per day at 1,800 rpm. Based on 75% efficiency we produce about 115 AH per day.

Our usage

So much for theory, now for our experience. To date we have never discharged our batteries to even 50%. However, we have have never stayed at anchor for more than two (2) days at which point we will fire up the iron jennie.

Download the spreadsheet


Sea Dragon had the factory battery box when we purchased her. The box was built into forward section of  the port cockpit locker. It had two (2) group 31 plastic battery boxes strapped into the wood frame. In our case the boxes hide a group 27 house battery and group 24 starter battery.

My plan was to expand the house configuration to hold an additional group 31 battery and move the start battery to the starboard locker.

To do this, I simply extended the current design to accommodate an additional battery. Lowering the new box by about 4 inches and cutting a slot through each side of the new section so that the emergency tiller slides under the new battery and through the new battery box.

The piece of wood you see in the picture is holding the sides parallel while the epoxy cures.

Another view of the new box after tabbing into the boat.
Battery box completed with a two (2) 31 house batteries and one (1) group 24 starter battery. Hopefully this season I will have time to move the starter battery to the starboard locker and add an additional group 31 house battery.

This picture was taken before cleaning an repainting the locker.

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Copyright 2006 Garner Bennett. All Rights Reserved