Boat Batteries

In my blog entry a few weeks ago I mentioned that I’ve been thinking of swapping to batteries from This triggered several comments on my blog, most from people thinking I would be crazy to make the swap. Normally, I respond to comments on the blog within a day, but I’ve skipped responding to these questions for weeks.


The honest answer is that it is an issue where my knowledge is light, and I wanted to do some studying before commenting. There are also other issues I’ve been wrestling with. One of the biggest issues is that I’m not completely certain I need all of the batteries I have.


A brief recap on what has me thinking about this: Most of Sans Souci is perfect, however there is one area I’d like to “fix.” The lazarette; the portion of my boat behind the engine room, and beneath the lazarette, is more crowded with equipment than I’d like. We’ve run 10,000 miles with it as it is, and could circumnavigate just fine with things as they are, but it isn’t perfect. I frequently need to work on things in the lazarette, and would prefer a bit more space to move around. I am particularly focused on the batteries because they are crammed into a corner, where I can’t reach them at all. Simple tasks, like shooting the terminals with a heat gun, to verify they aren’t warm, are impossible. Tightening the lugs, on the terminals, would be a major project.


My original vision for the boat was to run it most of the time off of the batteries. The battery bank was sized (1530 amp hours at 24v) so that I should be able to run the entire boat off the batteries with a couple of hours a day of charging. I added an auto-start to my generator, and have huge inverter capacity: 14kw! My hope was to basically forget about electricity, and let the generator auto start from time to time as it decided the batteries needed topped off. This original vision has remained unfulfilled. My preference is for warm weather climates, and the second the air conditioning starts, all hope of running off the batteries goes out the window. In fact, it has turned out that we have run the generator 100% of the time when away from the dock. I never did this on our prior boat, and designed this boat to be run off the batteries, but, as they say, “It is what it is.” Large boats tend to be “generator always on” and somewhere between our old Nordhavn 62, and our Nordhavn 68, we crossed this line.


One of the many questions I’m wrestling with is: how large does my house bank need to be if all I’m not using it for anything? Let’s see .. if I draw 0 amp hours per hour… how long will it last? Actually, that’s not a true statement. I do use the battery bank in two scenarios. 1) When swapping power sources. 2) And, as emergency backup.


My boat has three primary sources of electrical power; a 25kw generator, a 20kw generator and shorepower. Whenever I swap between them, something has to keep the electricity running. This duty falls to the inverters. Because the inverters have less capacity than the generators, I shut off any large loads prior to swapping between generators, or to shore power. The drain on the batteries is only for a few seconds.


The bigger drain to the batteries is their use as an “emergency backup.” When the generator quits, or shorepower dies, the inverters have to handle the load until power can be restored. This is usually a fairly short period of time, but if the problem is serious, such as a fuel problem, or a lightning strike, I’ll want all the hours I can get, to get things running again.


There are a lot of issues involved in selecting batteries, only a few of which I understand. As I mentioned, it was recommended to me to swap from AGM batteries to Flooded Cell batteries. Googling “Marine Deep Cycle Batteries” yields lots of comparisons.


One succinct comparison I found says:


Sealed AGM type are usually the best choice in these situations:

In areas with no or poor ventilation:

In areas or applications where maintenance is a problem, or is poorly done
Any place that a flooded battery might spill or break and cause damage

Where the batteries are in a spot where it is difficult or time consuming to check water levels

In situations where the batteries must be mounted on their sides to fit

Flooded batteries are probably your best choice if none of the above apply:

Flooded batteries are fine for most applications. They are nearly always cheaper, and there is better availability of types and sizes. They are the best choice where:

You have adequate ventilation for the batteries.
You do not have to worry about tipping, spillage, or breakage.
The batteries are in an area where maintenance can be performed without too much hassle.
You need a very large battery bank – AGM batteries have a pretty sparse range of choices for large capacity sizes, such as over 250 amp-hours.


From this list, I would be better served by the AGM batteries, which are what I now have. The primary benefit we were seeking from the battery swap was to make better use of space, by custom designing the batteries. The negatives on flooded batteries if the above comparison is accurate kill them for me. I am a fan of things that are simpler to maintain.


Based on this, I did a little googling to compare different types of AGM batteries. To help myself do comparisons, I focused on the footprint size for similarly sized battery banks to what I have now. My preference would be to shift to a battery which is taller (I have room to even go a couple of feet tall).


Following is a chart, showing only AGM deep cycle batteries, comparing them on the amount of square footage similarly sized battery banks would consume.


 Base Area



 Total amp

Amp Hours

 Per battery

Sq Ft







amp hours

At 24v

 Sq Ft

12 Batteries * 24v

AGM Lifeline












Energy 1 NSb210FT
























Note: 4 volt cells, at 1350 amp hours each – 6 to get to 24 volts














11 5/16

13 1/4

21 3/4









AGM Lifeline

11 5/16


12 2/3











The column I most focused on above is the “Total Sq Ft” column. My current batteries occupy 18.84 sq. feet. Some of the options use half the square footage.


I’m not sure I understand the issues, and am not sure that I’m even pursuing this the right way. Most of all, I’m not sure I’d be willing to throw away perfectly good batteries to make a swap. I’m doing the research partially because it is giving me an education on the topic, and partially because it might help me achieve my goal.


And, on a different topic…


Here are links to all three parts of the “Kosmos Rescue Story”. Must reading:






And lastly….


We visited yesterday with Eric, Annie and their son Bear Bloomquist, owners of a Nordhavn 64 that arrived here in Cabo yesterday. I’m bad at ages, but would guess Bear at 7 years old. They said he has been traveling with them literally all his life. The N64 is their third Nordhavn, as they’ve gone from an N40 to a N47 to an N64. Very cool people!


-Ken W

7 Responses

  1. Makes me wonder where the Nordhavn engineering and design people were at when they installed batteries in such a difficult to access point. I know Nordhavn is a great boat, but as I follow your web site, it appears there are a number of areas that are lacking. Battery access should have been at the top of the list. If your equipment additions,at your request, compromised access, then Nordhavn should have been astute enough to recognize this and either relocate the batteries or your excessive equipment.

  2. The main danger of flooded wet cells is the explosive gas given off during charging. If the ventilation is not adequate and designed with this type of battery in mind, then you could blow the stern off. Acid mist and topping up are added problems.

    Another solution is the 2 volt long life gel battery, a 1500 amp/hr from Victron Energy measures 8.5 inch by 11 inch by 34 inches high. 12 cells would take up 7.8 sq feet. I believe there is more than one manufacturer and the advantage of single cells is that only the faulty cell has to be replaced. Of coarse how easy it is to replace a cell depends on the batteries location and its 240 pound weight.

  3. That’s an excellent chart! I did not know that Rolls could furnish modular batteries in 4V cells. I found one phenomenon with one AGM 8D. It was segregated as the start battery and its capacity kept going down even though it was not being used. The two house batteries (same size and age Lifeline batteries) maintained their full capacity. Once I made the start battery the house bank, it regained its capacity quickly. Then I joined them all together into one bank. This is a liveaboard situation where we are connected to shore power full-time. Arild and I think that this may be an AGM characteristic – they need to be used.

    One AGM characteristic not mentioned is their ability to absorb as much charging amperage as you can produce. That’s the good news. The bad news is that if you are amperage constrained (shore power for example) you must control the in-rush of current so that you don’t pop breakers. This may or may not be relevant to big yachts.

    Lastly, you may recall the photo on the Energy 1 website where their batteries were placed horizontally in a stainless steel rack. That’s another way to save space and yet have access.

    On the issue of possible replacement; a large vessel engaging in a leisurely cruise around the world should carry spares. Based upon my sample of one, their then is the issue of maintenance of those spares. The spares could be used close to the electronics to provide their 12 or 24V power. The spares need to be manageable by one man. AGMs fit this scenario.

  4. I would seriously consider having ROOM for wet cell batteries. I had an AGM, 6 months old, go bad in northern British Columbia and there were no AGM’s within easy reach, but there were wet cellS. I could imagine in world wide travel wet cells will be readily available vs. AGM/Gel types. It is generally not acceptable, other than emergency situations, to mix wet and AGM because of the battery charger. Your battery chargers should have a switch on them to wet vs. AGM, so be careful of that. Chuck

  5. Ken, Is you group going to visit any of the southern Alaska ports such as Valdez, Seward, etc.. The route you show on the map looks like you will bypass much of Alaska if you go from Sitka over to the Aletuians. If you do make it to Valdez or other ports nearby, dinner is still on me.

    Jim E.

  6. Hi you two wonderfull wanderrers. I hope this Thanksgiving fiends you and yours in wonderfull helth and sperit.
    I have been folowing your travles and tribulations since I found the Nordhavn web site. My other fav. ones to follow WAS Always Friday and it’s wonderfull crew.
    I have a Sister that lives just out side of Seatle. She is a school teacher and her husband Ray is a School Addmin. I have suggested to her to try to contact you about your travels and maybe have you and your wife up to there schools for a small slide show and talk about your travels. I hope that this can be worked out, but if not, oow well.
    Maybe this would give some insentives and education to the kids. School isn’t just A, B, C’s and 1, 2, 3’s. As far as inspiering go’s, just count the names on your BIG LIST AND small list.
    Good luck on the prep. and planning your new adventure. I will be following your travels.
    A fantasy traveler
    Ray L
    P.S. If you would like to contact my sister, Marjie, look up her E-Mail address on the Big List (riverann)

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