O’Day 40 - Battery setup- what are you using?

Aug 7, 2015
Oday 34 previous, O’Day 40 current Annapolis
Looking to expand upon the minimal battery compartment on my O’Day 40 & am a little stymied. Current batt box beneath rear berth only holds 2 GP27 batteries and my start battery is housed in the stbd lazerette. 200 AH w/ only 100 AH usable is pretty minimal. Would like to know if you have a better solution?
I had wanted to avoid a lithium upgrade which seems to include a 200AH drop-in batt, new alternator & regulator, etc, etc- a $2-$3k upgrade but I’m not seeing a viable workaround- your comments/ solutions appreciated. Also looked at installing Trojan T125 but too tall to fit existing battery box.
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May 17, 2004
Beneteau Oceanis 37 LE Havre de Grace
I’d say the place to start would be with an energy budget based on your intended use and loads. Once you do that you’ll know whether 100 aH is woefully inadequate, just a little short, or maybe ok. Your options will probably depend on how much more you need. Lithium is one option, but carbon foam lead acid is another, or 6V golf cart batteries. An additional charging source like solar could be worth considering as well.
Aug 7, 2015
Oday 34 previous, O’Day 40 current Annapolis
Thanks, I probably should look into carbon foam. However given the difference in usable AH between 2 @ 6v GC’s and an equivalent number of Group 27, FLA’s @50% DOD (112v.100), it doesn’t seem like there is much bang for the buck. Agree that solar is worth a look.
Aug 11, 2011
O'day 30 313 Georgetown MD
Battery setup- what are you using?
On my 30, I'm using two Group 24, 850's. I try to not use the motor so recharging is done mostly via a 200 Watt solar set up, using a double battery controller and monitored by a Ethernet connected EPEVER Meter MT-1 Remote with LCD Display.
Apart from starting my Universal M20 diesel engine as needed, my energy usage includes my auto pilot, VHF, chart plotter, depth sounder and stereo during sailing. I have changed every light bulb or lamp to LED, so my night time usage is minimal. So as long as the sun shines the power source is sufficient while alternating the use of each battery every other day, without getting them too low.
I made my own frame to hold the two 100 watt Renogy compact panels, that are mounted in conjunction with the stern railing. I used all 316 stainless steel tubing and parts. As the panels are encased in aluminum, they are mounted on aluminum square tubing which is attached to the SS frame using nylon washers to isolate the two different metals. It is structurally over engineered and built. Most cabling is internal on the frame, running through the aft combing to the controller. Each battery has an isolation switch and is fused. Inclusive of all, I believe it cost me less than $850.
I have plans to expand my capacity of batteries, to be upgraded to Group 27 or 31with the timing of when my current batteries bite the dust, but that may be a while yet. (Famous last words)


Jan 7, 2011
Oday 322 East Chicago, IN
On my O’Day 322, I switched to golf cart batteries this spring. I could only get 2 in the battery compartment as it is, but I am considering removing the STB water tank so I gave room for an A/C unit and a few more batteries. I have been using only the port water tank to make sure I don’t regret the decision later…

Still only getting 205 Ah in the GC batteries, but they seem to handle the long, slow draw OK.

At a minimum, I want add a starting battery just in case…



Mar 5, 2011
O'Day 35 Severn River, Mobjack Bay, Va.
We have two group 31's on our O'Day 35. Is there room in your battery box to at least upgrade the size slightly to larger 31's? They would offer more amp hours for sure.
Aug 7, 2015
Oday 34 previous, O’Day 40 current Annapolis
Thanks to all your contributions- finally decided to go with 2 Lifeline D4. Built a new battery box to be located beneath rear stateroom bed. Will augment w/375 W solar panel mounted on rear davits.
Jun 11, 2004
Oday 31 Redondo Beach
Wow. That's a big step up from your two gp27's.

Besides the solar are you going to need to otherwise adjust your charging system such as alternator and regulator?
Aug 7, 2015
Oday 34 previous, O’Day 40 current Annapolis
Have already upgraded to a Sterling 50 ah charger. For now, I’ve got one more 5 day cruise this year & I’ll see how it goes. Will likely upgrade alternator /be when Maine Sail reopens his site.