The Offshore Voyaging Reference Site

Why Most New-To-Us Boat Electrical Systems Must Be Rebuilt

Our new-to-us J/109, like most production boats, came to us with an electrical system, installed in 2004, that is not even close to ABYC compliant. Said system also had some dangerous defects.

And on top of that, the system as installed by the builders, while adequate for the light usage they envisioned, is horribly inefficient, and will be very frustrating, not to speak of environmentally destructive, to live with once away from shore power for more than a few hours—think running the engine for hours, and often, to charge the batteries.

The same applied to our previous boat, a McCurdy and Rhodes 56 that we owned and mostly lived aboard for 30 years, and the one before that, a Fastnet 45.

The Way It Is

But here’s the thing, our experience is the norm, not an aberration, and the older the boat we buy is, the worse the system will be, so there’s no point in wasting time pissing and moaning, rather, we need to fix it.

Two Tips For Buyers

But before we get into that, two tips for those of us buying a secondhand boat, as we did:

  • The boat will almost certainly require significant rewiring and electrical upgrades, so we better budget time and money to do that.
  • The survey will probably not warn us of this impending expense and hassle, so even with a clean survey we need to leave money in the budget for this project.

A Survey Won’t Save Us

On the second point. Yes, I know, most surveyors claim to be ABYC trained and claim they survey to that standard. And maybe some do.

But the fact is that, during the process of buying our J/109, I read through three surveys on the class—two on the boat we bought, and one on another boat—and not one even pointed out that the batteries were not fused, a fundamental code violation and a potential boat-burner.

I also read through several other surveys on other types of boats, and not one had more than one or two trivial mentions of ABYC-compliance violations, and all completely missed problems and omissions that I could see at a glance in photos, like the one at the top of this article.

A North American Problem?

I’m guessing, but do not know, that the situation in Europe, and probably Australia and New Zealand, will be better, at least for boats built comparatively recently, both because in those countries regulations are backed up by the rule of law, rather than just being recommendations as ABYC’s are, and because it seems that surveyors in these countries are, as a rule, more thorough.

But even so, our experience and what we are doing about it will still be of use to those of you in countries other than North America who are buying boats that are over about 10-years old, or boats that have suffered from electrical system modifications from a series of owners equipped with a lot more enthusiasm than knowledge.

So what the heck are we supposed to do about this state of affairs?


What about hiring a boatyard to fix the problems and upgrade the system to ABYC? Generally a bad idea for three reasons:

  • Most boatyard staff are woefully ignorant of basic electrical theory and the applicable standards; yes, including the ones that are ABYC certified—the courses they take are a joke.
  • Even if the yard has a person who does understand ABYC or CE, the likelihood is that they will have absolutely no idea how to make the system more efficient for cruising away from shore power.
  • High cost. More on that in a minute.

How bad are the first two issues? Really, really bad. For example, I know of only one guy here in Nova Scotia and one in Maine who I would trust to rewire my boat to be both compliant and efficient.

Gotta Take Responsibility

Given the deplorable state of training and knowledge among most boatyard “professionals” and even boatbuilders, particularly in North America, even if we can afford to delegate the actual work, we owners have to get at least a basic understanding so that we can check it’s being done right.

The other problem is that even if you can find someone competent (look for independent contractors, not boatyards), you will probably have to wait ages before they can help you. The good people are invariably snowed under.

Don’t believe it’s that bad? Well, read on for a couple of examples:

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More Articles From Online Book: Electrical Systems For Cruising Boats:

  1. Why Most New-To-Us Boat Electrical Systems Must Be Rebuilt
  2. One Simple Law That Makes Electrical Systems Easy to Understand
  3. How Batteries Charge (Multiple Charging Sources Too)
  4. 5 Safety Tips For Working on Boat DC Electrical Systems
  5. 7 Checks To Stop Our DC Electrical System From Burning Our Boat
  6. Cruising Boat Electrical System Design, Part 1—Loads and Conservation
  7. Cruising Boat Electrical System Design, Part 2—Thinking About Systems
  8. Cruising Boat Electrical System Design, Part 3—Specifying Optimal Battery Bank Size
  9. Balancing Battery Bank and Solar Array Size
  10. The Danger of Voltage Drops From High Current (Amp) Loads
  11. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  12. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  13. Battery Bank Separation and Cross-Charging Best Practices
  14. Choosing & Installing Battery Switches
  15. Cross-Bank Battery Charging—Splitters and Relays
  16. Cross-Bank Battery Charging—DC/DC Chargers
  17. 10 Tips To Install An Alternator
  18. Stupid Alternator Regulators Get Smarter…Finally
  19. WakeSpeed WS500—Best Alternator Regulator for Lead Acid¹ and Lithium Batteries
  20. Smart Chargers Are Not That Smart
  21. Replacing Diesel-Generated Electricity With Renewables, Part 1—Loads and Options
  22. Replacing Diesel-Generated Electricity With Renewables, Part 2—Case Studies
  23. Efficient Generator-Based Electrical Systems For Yachts
  24. Battery Bank Size and Generator Run Time, A Case Study
  25. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  26. Eight Steps to Get Ready For Lithium Batteries
  27. Why Lithium Battery Load Dumps Matter
  28. 8 Tips To Prevent Lithium Battery Black Outs
  29. Building a Seamanlike Lithium Battery System
  30. Lithium Batteries Buyer’s Guide—BMS Requirements
  31. Lithium Batteries Buyer’s Guide—Balancing and Monitoring
  32. Lithium Batteries Buyer’s Guide—Current (Amps) Requirements and Optimal Voltage
  33. Lithium Battery Buyer’s Guide—Fusing
  34. Lithium Buyer’s Guide—Budget: High End System
  35. Lithium Buyer’s Guide—Budget: Economy Options
  36. 10 Reasons Why Hybrid Lithium Lead-Acid Systems are a Bad Idea
  37. 11 Steps To Better Lead Acid Battery Life
  38. How Hard Can We Charge Our Lead-Acid Batteries?
  39. How Lead Acid Batteries Get Wrecked and What To Do About It
  40. Equalizing Batteries, The Reality
  41. Renewable Power
  42. Wind Generators
  43. Solar Power
  44. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  45. Battery Monitors, Part 1—Which Type Is Right For You?
  46. Battery Monitors, Part 2—Recommended Unit
  47. Battery Monitors, Part 3—Calibration and Use
  48. Battery Containment—Part 1
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Adam Mansbridge

I’ll take the easy fault: no fuses at the battery. Fun sparks if the battery to switch line disconnects at the switch end especially on a metal boat

Adam Mansbridge

Oh, and yes please do a lithium chapter. I am particularly interested in whether we can trust the battery management system built in to many lithium packs to charge when supplied 14-odd volts from the alternator

Charles Hendricks

I talked with our insurer about lithium and they gave me some requirements that I took to a reputable installer in Ft Lauderdale. They came back with a full Victron quote for 900ah that fully met charging, discharging, monitoring, and alarms. $35,000 was the quote.

I got a second quote, $30,000 but it didn’t include integrating the alternators (catamaran).

When it comes to work that we have done on our boat, we always approach it in the order of safety, reliability, redundancy, and cost.

At this point I just don’t see how the cost/benefit works out for lithium for /us/.

Matt Marsh

Lithium’s not a chapter. It’s a textbook.

“Drop-in replacement” lithium batteries are, almost universally, meant for streetlights and traffic signals and the like. Cruising boat service absolutely requires a properly integrated system, otherwise you will end up with expensive failures (if not fires).

At the very least, anyone considering such a system needs to thoroughly read and understand ABYC A-4, E-10, E-11, E-30, and TE-13, and to understand exactly how the battery chemistry, cell balancing, BMS, charging & load buss architecture, charge/discharge characteristics, temperature management, fire suppression, etc. need to work. You also need a good relationship with your insurer, and for them to sign off on the design and the installation. The underwriters are starting to get wise to the risks of DIY lithium.

I’m thinking of doing another article or two on this, not from a “here’s how to build one” standpoint, but rather “here’s all the stuff you need to become an expert in before you can think about building one”.

Ernest E Vogelsinger

Hi Matt, well said. And as probably most of us don’t have time and experience enough to become that expert, one needs to find one. And I am drop dead sure there are more self-assigned Lithium experts in the wild than good people having real know-how.

Kit Laughlin

Agree 100%.

Dick Stevenson

Hi Matt,
Nigel Calder has an interesting take on lithium and insurance companies in an endpaper essay in the Feb/March “Professional Boatbuilder”.
My best, Dick Stevenson

Dick Stevenson

Hi John,
I get a paper copy. I could not find it on line, but that may reflect more my lack of skills in the computer realm.
My best, Dick

Petri Flander

Hi John, yes, PB current and back issues are online, at least for subscribers. Subsription is free for “marine professionals”, I’m 100% sure that you qualify.

Alissa Winter

The biggest mistake we made with our rewire was to not add extra “fat” to the output calculations of our flexible solar panels. After a couple of years their output was down just enough to break the nice plan we had made, and mean we needed to start running the engine again more often than I’d liked.

+1 vote for a lithium chapter (although it is not in my immediate future).

Gust Stringos

Yes to the lithium discussion!

Mike Evans

John will you put fuses on the batteries themselves?

Matt Marsh

If it’s not fused on the battery itself, it should be fused within 7 inches of the positive terminal. I’d be amazed, and dismayed, if a surveyor missed that.

Blue Sea’s on-terminal fuses are nice and compact, as long as you have some vertical clearance, but spares can be hard to find. (While taking delivery of Maverick V, we got stuck at the dock because someone had installed a 150 A fuse, the largest that the chandlery had in stock — but the starter drew 200-250 A.)

My own preference is for a 6-inch 0AWG or heavier pigtail to a marine MEGA-AMG or Class-T fuse block, for which spares in any rating up to 400 A are readily available. Add double-layer heat shrink insulation over any exposed metal on the terminals, plus a rubber cap to protect the lugs from shorting if someone drops a tool.

Also, am I badly mistaken, or did someone wire the battery switch on John’s boat to the negative terminals? (edit – or is that the shunt, in which case why are they on opposite ends?) And what’s with the gigantic spacers on the lugs?

Petri Flander

I’m with Matt on MEGA/AMG fuses. Previously, I was favoring ANL fuses “because they are readily available”. What changed my mind was two things.

First, an odyssey to actually find one on a foreign city. What I got was either for M8 bolt not M10, or it wasn’t ignition protected (fuse was on battery compartment).

Second thing was finding this document on Bluesea whitepapers, that say:
“Blowing amperage for 40A ANL fuse is actually about 100 amps”…

The only thing wrong with MEGA/AMG fuses is that they too have two bolt sizes, M6 and M8, and they can be tinned or plain copper. So, having a set of suitable spares is pretty much mandatory with them too.
Also, they are cheaper, and more compact than ANL’s, so it is quite easy to make a neat custom main bus with them where the other end is on common copper bar and output is on bolt with cable lug – provided that one can do a hot-dip-tinning for plain copper bar.

Will Kirkness

Good info Petri. I’m trying to sort out what to do on my own boat, as it too, did not come with circuit protection and was also missed by the surveyor. I’m using the blue sea circuit wizard to determine the protection. It asks for the CCA of the battery. I have no idea what that is for my house bank and can’t find it on the Trojan website (anyone know how to calculate this?). So I tried this exercise for the engine starting circuit. After entering the data for the wire size and getting 2/0, I’ve entered the load of the starter, which I estimate to be around 250A and the CCA of the battery which is 1375. It gives me a range of fuses to select from and it describes MEGA fuses as having inadequate AIC, even though it has fuses available at the same rating. I’ve tried different configurations and MEGA is always not recommended for the same reason. ANL and Class T are always on the list.
Am I missing something here regarding AIC and each fuse’s application? Or is that what the Blue Sea paper is trying to point out?
Your comment regarding international availability has me thinking. So maybe Class T is more widely available abroad…? Or I just carry more spares than originally planned for.

Petri Flander

Hello Will, I remember that two T-105’s in series give about 650A CCA. What comes to MEGA/AMG AIC (Amperage Interrupt Capacity), I think that they got derated from 5000A to 2000A on Bluesea somewhere between 2010-2020. Though it reads on AMI/MIDI product page that it is 2000A on 32V and 5000A on 16V. It should apply also to MEGA/AMG.
Anyhow, T-fuse is the universally recommended main battery fuse because 20 000A AIC is always sufficient, and MEGA/AMG ‘s ans AMI/MIDI are good to fuse the main bus cabling, ie. input cables from alternator, mains charger, solar, wind etc. and outputs to main panel, inverter etc.
And yes, BUSS automatic breakers (40-150A) are more “offshore’y” for main bus, but sometimes not ok due to cost, size or amperage issues.

Alan Bradley

Another vote for what it takes to switch to lithium.

Brian Johnson

yes to more on lithium reality

Steve D

Cables not secured every 18″

If either of these batteries are not used for starting then they lack OCP within 7″ (can’t tell for sure but that includes the small wire) of the positive terminals.

The straps and strap eyes, secured with all too common mild steel and now rusty screws, will almost certainly not pass the ABYC battery security test. ABYC standards do allow for 1″ of movement, a rule I’ve been fighting to change for years. Those wimpy strap kits have been a travesty to the marine industry for decades. light duty, hard to take apart and the screws, oh the mild steel screws, which are undersized in any event.

Why are the negative cables connected to opposite sides of the shunt?

Negative cables are the wrong color.

Dieter Kowalewski

Hi Steve, and Matt,

it seems that the two batteries are not paralleled. If the above battery is the starter one, then the negative Side is wired correctly

Ben Logsdon

With a new to me 80s cruising boat with original DC system, I’d love to see the lithium option. If I’m going lithium, I’m going all in with a bigger alternator an external regulator.

“Drop in” lithium batteries are readily available. How would one utilize one of these? Or are they all snake oil?

Also curious about what the comment on what makes a shore power charger dumb.

Jeffrey Werner

Are you saying the concept of “drop in” is BS or are you saying “drop in” batteries, meaning internal integrated BMS, cannot be safely or effectively used? Curious as Rod C states on his site that there are 2 “drop in” designs he would use, one of which I selected. KiloVault. Well engineered BMS, high charge and discharge rates, UL 1973 certified (a Nigel must-have and almost impossible to find on LiFePO elsewhere and certainly not drop ins.) bluetooth monitoring…understanding I had to re-engineer everything: alternator (including protection), regulator, inverter, charging strategy.

Will Kirkness

Cool custom lug on the left terminal of the shunt.
How much stacking is allowed on terminals?

Will Kirkness

I see the batteries are under the settee, so I assume below the waterline. Is there any containment required for this type of battery in this location? Our wet cell batteries are above the waterline and also contained so I’m not sure if requirements differ here. Also, it’s hard to tell in the photo, but what is there to know about battery (+ and -) cables being of equal length in this photo?
Pretty tough game to point out errors after Matt and Steve D had a go…

Will Kirkness

Regarding position relative to waterline, I honestly had no idea, I guess I was just curious. As I thought more I realized it doesn’t make sense. Thanks for the feedback

Gregory Thomasson

4 terminals per stud is allowable.

William Willcox

”Boat Owner’s Mechanical and Electrical Handbook”, latest edition, Nigel Calder. Required Reading. Best $50 I ever spent on boating equipment.

Brad Manter

One No vote on lithium batteries. Don’t care about right now. I’m more interested in what you plan to issue on your system. I need to improve and upgrade exactly as you outlined. I have a J Boats-type stock system. Many thanks. Brad.

Dick Stevenson

Hi John and all,
Your discoveries on your J are really disturbing, especially as I consider Js as one of the medium to higher end boats constructed in the US.
Manufacturing and design standards are not going to change by themselves.
I would like to suggest starting a list of questions (might make a good article) that can (and should) be asked whenever someone is considering a boat.
I would start with whether the boat was fully built to ABYC standards. Then ask (not in order of importance) questions such as:
1.    Are all stainless-steel fasteners in aluminum coated with TefGel/Duralac or equivalent?
2.    Are there high-water alarms?
3.    Is there provision for a reasonably sized anchor?
4.    Is there a place to sleep off-shore: a straight settee?
5.    Are there smoke detectors?
I am showing a senior moment, because I know there are numerous other simple questions that would tell the boat seller the kinds of things that the designer/manufacturer should be including or give consideration to. This could be asked at boat shows etc. and that might get across to the designers and manufacturers (and surveyors) that they need to up their game.
Random thoughts, My best, Dick Stevenson, s/v Alchemy

Dick Stevenson

Hi John,
My point was more along the lines of coming up with a list of questions to ask at boat shows and the like: if enough people start asking: “Are all stainless-steel fasteners in the aluminum mast coated with an anti-seize?”, the manufacturers will get the message. There are a whole list of simple things like this, inexpensively done at the factory, but more complicated later, which should be standard on boats, but are not.
The manufacturers are not going to change in these areas, it seems apparent. Perhaps the buyers asking pointed and pertinent questions might move the bar.
My best, Dick 

Dick Stevenson

Hi John,
I am sorry I was not clear.
I think the boating industry does not work well in taking care of boat owners and does not take good care in a multitude of ways (talk about killing the golden goose). My intention, probably naïve, was to generate some questions to prod manufacturers via boat sellers to do the right thing in putting together their boats (Ask about TefGel when dis-similar metals are in contact for ex.). It was not my intention to generate a list of questions to help people decide what boat to buy or what needs to be done to bring a boat into offshore readiness. I am aware you have been and are amply covering this challenge.
In one area, this boat-owner-concern being expressed has seemed to work: some boatyards, when they hear owners say that they do not wish, when over-wintering outside with hard-stands, to be placed next to a boat with their mast up and jib left on and furled, have changed policy to preclude sails being left on.
This is one example: it would be nice if others would follow.
My best, Dick

John Michaels

Since Lithium is here to stay, an article on how to embrace it properly, would be a welcome addition.

Jim Schulz

Thanks for this John, we have a new-to-us Ericson with an electrical system that looks to have been modified by a couple of smart and well meaning PO’s but has me scratching my head on where to go next after handling the few items pointed out in our survey. I’m excited to follow this series! Also I agree with and appreciate your intent to focus on lead-acid before spending too much time delving into lithium. Since lead-acid is probably what most of us have right now it seems a better starting point to understand and design a safe and functional lead acid system, then perhaps explore lithium at some point down the road.

Jim Schulz

That would be a great way forward if possible, since it sounds from more recent posts that the interest in lithium is big. If it doesn’t get too complicated it will also increase potential learning by comparing and contrasting systems. Thx!

Marc Dacey

It has been our desire for simplicity (and internal ballast) that has kept us in the lead-acid battery camp. We know the shortcomings and pitfalls, but we have a boat with the internal space and access to pretty well do what we want. If we had a sporty racer-cruiser, I would have a different set of goals.

Just the restrictions in shipping lithium to wherever we might happen to be on the boat, never mind the complex management, charging and wiring consideration, are pretty much a deal-breaker for us in a way that lithium batteries in an EV car (which are quite close to “drop in” in terms of replacement after 8-10 years,as I understand) would not be.

Dick Stevenson

Hi Marc and John,
This is also an issue for batteries for electric outboards. A friend had bought a spare battery for his Torqeedo, but could not get it shipped to Iceland where his boat was. I ended up bringing it with me when I sailed to Iceland. With-in country there was no issue sending by mail.
My best, Dick Stevenson, s/v Alchemy

Marc Dacey

It may be for all I know an issue for certain lithium computer batteries. They are not all created equal, a point Rod Collins has (at some effort, as he reports) just made in his first post-stroke article and which has some bearing on the present discussion, which is “lithium-rich”.

Daniel McCarty

Glad you posted the link to Rod’s article about drop in batteries. I read the article a week or so ago, and to say it is shocking, is an understatement.

Dave Warnock

I’d appreciate a guide to the changes to the base design. We are starting from scratch.

Dan Perrott

I look forward to seeing not just the design but the implementation of the design.
I’ve seen lots of photos not what look like great setups, but normally located in an area with great access and lots of space (like on your old boat). But unfortunately often not available on smaller boats.

I have also noticed in more than one manufacturers product installation instructions phrases along the lines of:
This should be connected directly to the battery terminals.
One even stated as a warning that no fuse, switch etc should be in the cable. In this instance to stop a wind turbine going out of control if power is lost.

Matt Marsh

A device that specifies “Connect this directly to the batteries and do not use a fuse or circuit breaker” should, in general, be returned to the store for refund, and also reported to the relevant product safety authority in your country. Explicitly telling the installer to violate electrical safety codes is a good sign that the product itself is likely in violation of those same codes.

A wind turbine that spins up out of control if it doesn’t have a battery to charge could not possibly meet CE (or UL, or ETL, or any other) standards.

Alastair Currie

John, EU and UK surveyors are not any better than USA surveyors in a general context. There’s no standard testing or qualifications for yacht surveyors. We get the same trust and reliability issues with surveyors on this side of the pond. You can’t even trust word of mouth as I have seen the same surveyor praised and criticised on forums. There are no legal requirements on leisure yachts for electrical installations, post new sale, and you will see a lot worse over here in the UK, as we have a long history of unskilled DIY. Yes, on construction, yachts must legally be compliant with EU or UK legislation to obtain the CE mark in the EU based on the Recreational Craft Directive (RCD), or the new system for the UK, post Brexit (I can’t remember the name, but similar to RCD). In the UK (or EU) if you buy a second hand boat, and if the electrical installation has been changed, it is likely it will not comply with any standard. For example, my new to me boat, second hand, had woefully undersized mains power battery charger wires to the batteries, one of those dangerous rectangular pin shore power sockets, complete with burnt out pins and melted terminals, all fitted by the last person who owned the boat. I agree that the lithium discussion should be ignored, at least for a few years, as most of us probably need to focus on the non lithium wiring systems that are currently ubiquitous.

Dick Stevenson

Hi Alistair and John,
In my time in the EU and UK, I had 2 surveys done: regular insurance up-dates. I did my best to find the most well thought of surveyor. Both survey reports were quite disappointing.
My best, Dick Stevenson, s/v Alchemy

Eric Klem

Hi John

I am curious to see what you come up with, you certainly couldn’t find a better group of people to work on it.

One thing that would be interesting to me would be how you would plan to set up charging for an electric outboard as I believe you said you were planning to go that way. Given that the capacity of those batteries represents a significant portion of the capacity of many house banks, just plugging in and charging without any thought or electronic nannies could be problematic.

Another thought is the actual energy budget you plan to work to from generation, storage and usage standpoints. I realize you already have some sizing calculators on this, I was thinking of using your implementation as an example. One of my pet peeves is the people who boast that their solar has them at full charge every single day by 10am, that means the boat is saddled with way too much solar and likely too big of a battery bank or their statement isn’t true. I know some are true, there is a local IP350 I know of with >1200W of hard panels, those boats were not great sailors to start with but it must be hopeless now. I think that I may have been overly conservative in installing 4X GC2’s on our boat and now lug around the extra weight. I have been tracking and I think if I had installed 2X GC2’s we would have only needed to run the engine just for charging purposes about once every 3 years and with the 4 batteries, we have never had to do it. I would think that the ideal system for me if you are at all weight and windage conscious would require you to use the engine just for an hour or two of charging something like 1-3 times a year to cope with the stretches of low generation and high usage.


Kevin Williams

Hello John.

I look forward to further installments on this issue as someone who is migrating more and more from the dock to the hook. One upgrade I am preparing for is to replace my manual windlass with 12V electric. With a 35 foot boat, my thinking is to power it from the existing (or improved!) battery bank. Any inclusion of proper planning for this circuitry in the system would be appreciated.

Eric Klem

Hi John,

That all makes sense. And I totally agree on not ruining the sailing qualities of your boat, it would be extremely easy to do. That being said, I also agree with your plan for a small solar panel for the mooring, it makes the batteries last a ton longer and doesn’t leave you with a quandary of what to do at the end of a nice day of sailing when your batteries are a little discharged. I had selfishly been hoping that you would cave and try to figure out how to discreetly fit a moderate amount of solar without resorting to performance degrading solutions like arches and biminis as one way to make up for a small battery bank is increased solar. I can’t think of many boats I have seen that do this well and I also wonder about longevity of deck mounting panels so we haven’t ditched our single 140W rigid panel yet but I would like to.

Some very rough numbers on solar panels versus burning diesel are interesting. The actual numbers are very specific to how specific solar cells are manufactured but the average CO2 equivalent per kW is about 2500kg. With diesel being about 10kg CO2/gallon, you would need to burn about 1/4 gallon of diesel for every watt of solar installed until the diesel had higher GHG emissions (there are some other nasty emissions I am ignoring). Taking a wild guess that your usage would require a 200W solar panel and that panel would last 15 years in a marine environment, that would mean it would need to displace a bit over 3 gallons of diesel per year to break even. So for someone not cruising full time, while solar is probably a little better, it is not a huge difference and there are many other things you can do to lower your footprint much more such as avoiding a single plane flight.

Sounds like you reached a similar conclusion on charging an electric outboard as me. We almost never spend the night on a dock and don’t have a shore power system so for us, it would be charged at home or only when the engine is running. As we discussed, I think this will work from an energy standpoint but I haven’t given thought on how to make it idiot-proof. I also have been thinking recently that I maybe should put a bit of effort into improving our rowing instead, I have already installed a foot stretcher but the oar blade shape is not optimal and the oarlocks are slightly low.


Alastair Currie

An area that I would like clarified in the upcoming articles, based on your design, is grounding (earthing) and why you choose to ground what you do and why you do it a certain way. An example of my confusion; in Calder’s tome, he shows lots of grounding symbols at the end of circuits but doesn’t explain what that feature is in the real world, similarly on your circuit the engine is grounded – why the keel, why the engine? What is the battery negative’s role in grounding compared to a common negative bus – is that also grounded at the same place as the engine block? I am hoping that your next series of articles will explain that. No need to explain here, just hoping that I can join all the dots and maybe have that epiphany moment when it all comes clear. Thanks, Alastair

Svein Hellesø

Another yes-vote for the lithium option.

Regarding the battery installation in the picture (and schematics) , the main overall requirements in the Norwegian regulations for leisure craft (which closely follow the European regulations), includes the following points:
-proper ventilation to avoid accumulation of explosive gases from the batteries
-proper fixing system
-protected against water intrusion

There are more detailed technical requirements in the regulations for maritime electrical installations.

Ventilation from underneath a settee in the saloon might be difficult to asses.

I can also add that for the start battery connection to the starter motor there is no requirement for a fuse or overload protection.

Svein Hellesø

I think that the requirement for proper ventilation is not fulfilled in many installations. In may local harbor I have heard a few stories of overcharged and gassing batteries (when hooked up to shore power). It is a low probability event that might have large consequences, and it is difficult to properly evaluate the risks. The potential for and extent of gassing increases when the charging current increases.

The “perfect” solution, a dedicated sealed battery compartment with separate ventilation to free air might be easy to implement in a new construction, but might be difficult to add to an existing boat.

In my current boat the batteries are beneath the bed in the aft cabin, with “ventilation” through a narrow cable slit into the engine compartment.

Svein Hellesø

I agree that a for a lead-acid battery ventilation is probably not a big worry.
However, for the lithium option the situation might be different.

I my research on how to properly install lithium batteries I have read “Best Practice/Guide to use of lithium batteries on leisure boats” (an early version of what should become the regulatory requirement for use of lithium batteries on leisure boats). The link is on this page: , but it is all in Norweagian, I am afraid 🙂

There is a firm requirement for venting gases from lithium batteries, due to thermal run-away, to free air.

The gases from a lithium battery in thermal run-away is both flammable and poisonous, so such precautions might be appropriate. I guess it would also be easier for insurance companies to accept lithium batteries if they are installed in accordance with regulatory requirements.

Petter Mather Simonsen

Another vote for a discussion on Li-batteries, John. Reason being that I guess is that almost everybody that has to change batteries, considers Li as an option.

Currently I have 4 AGM’s rated at a total 240Ah/24v on the 7th seasons. Soon they will most likely be exchanged for a lead/carbon mix – real “drop-in’s”.
Have looked into the building of a bank of individual Li cells with separate BMS and concluded that to much of a costly charging system will to be adjusted/changed to accommodate Li.
The lead/carbon bank will last another 7-10 years, and then the battery world may offer something worth changing to.

Maybe the upcoming article on Li will prove my conclusions wrong. Look forward to seeing your views when the time and opportunity arises.

Petter Mather Simonsen

There are alternatives to firefly. I am considering these:
Apparantly allows for drawing down lower than normal AGM without damage cycle life and capacity. In addition they accept a larger charge current than AGM’s.

Victron makes a similar battery

Any views or comments on the above is more than welcome.

Edward Scharf

The thing about lithium that so many people don’t get is it’s a system with the charging system. All of it needs to work together. I think you have pointed that out in your articles but either people don’t get it or there are just new people to educate.

Dick Stevenson

Hi John and all,
Having just had my battery charger turned off and inadvertently not turned back, on I am curious about whether a low voltage alarm might make sense. I caught it but I could have drained the batteries.
Programable settings might be nice.
I am wondering what others think and whether there is any recommendation. I would think the alarm could be rather simple, but, then again, it would have to deal with voltage drops related to short but potent loads.
My best, Dick Stevenson, s/v Alchemy

Dick Stevenson

Hi John,
I have an old but accurate battery monitor which has served well, but has no alarms. I check it a number of times a day, but tend not to do so when plugged into shore power and the battery charger is keeping the batteries topped up (I look forward to your thoughts on battery chargers and battery care at some point).
A quick story and warning possibly to some readers. I believe it is common to have integrated battery charger/inverter units transfer seamlessly from shore power to inverter if shore power is lost. This might be helpful in some scenarios to not have AC disrupted.
In cold areas, I warm the boat with a space heater when on shore power. If power is disrupted from shore my old, but still working well, Trace inverter charger would take over and keep the space heaters going. If not noticed, one can quickly deplete one’s battery bank to the point of damage.
Some research a few years back indicated that new inverter chargers operated the same and were not program-able to undo this seamless transfer, so when it came time, I bought 2 separate units even though the real estate for two was hard to find.
My recent incident was merely operator error: I turned off the battery charger when needing to do some work on a running engine and I do not like 2 charging systems active at the same time. Then I forgot to turn the charger back on. Caught the error a day later when battery level was 12.2v but only because I noticed the circuit breaker switch for the charger was off.
So, I thought of a low voltage alarm. I look forward to your report on the Balmar unit.
Thanks, Dick Stevenson, s/v Alchemy

Eric Klem

Hi Dick,

When I went through this thought exercise, I came out with largely the same conclusion as John that alarming relative to the SOC was best with lead acid. Ideally, you would have an alert that you were getting towards the need to charge point and then a proper alarm when you had reached it. Of course this assumes you are keeping your meter properly calibrated and that you get to full charge occasionally.

For our boat, we concluded that our other risk is when we are ashore for a day or so and the fridge is running. We have it on a low voltage cutoff that shuts it off at 12.1V at the fridge. Yes, we do run the risk of spoiling some food but at least we don’t also have to replace the batteries. I have definitely heard of a lot of weekend sailors who leave the fridge on with shore power and come back to damaged batteries due to a shore power failure. Ours battery switch is off if we are at work so this doesn’t apply to us although it does mean the fridge has to pull down whenever we get onboard. One thing to watch out for with low voltage cutoffs is what voltage they auto-reset at. If it is a big load relative to the battery capacity and this includes batteries that no longer have a big capacity, there will be a lot of voltage sag and just shutting off the load may bring the voltage up enough to cause it to reset, restarting the load and pulling the voltage back down to where it shuts off again, etc. I have witnessed this short cycling on a few boats. Of course, if you put the reset voltage too high, it means that if you do something like run the windlass with the engine off, it will turn off your stuff and leave it off without resetting, also not good. If you are running a cutoff, I think you have to put all your big loads on it and really think it through. I am not totally convinced they are a great idea but alarms are useless if you are not there to hear them so no perfect solution. One good rule of thumb is that if you can’t clearly write out a set of rules for an alarm or control to follow, it isn’t going to work and I don’t know of a simple set of truly fail-safe rules here.


Thomas Harries

Hi John. Are the followup articles with the proposed design done? Am I looking in the wrong place? Thanks.

Thomas Harries

Thanks John. Do you by any chance have the schematic done? I’m stuck on the hard for a few more weeks waiting for a rudder repair and thought I’d take the opportunity to work on my electrical system.

Mark Young

A very interesting article. I am sure many people are interested in this topic.

I have been mulling (cogitating?) over this exact issue. I have recently bought a new to me 80’s Alu cruising boat. Forgetting that I had this subscription (I joined up when I thought I had closed on a cruising boat – only to have the sale fall through) I was asking about this exact issue on forums. Most poo-poo’d the idea saying it was an unnecessary expense.

Yet here it is on your site – for exactly the reasons as I was thinking, you have outlined the same rationale. My boat, built in the 80’s is bound to have had PO’s add circuits over the years. The electrical panel has pieces of tape with names of the circuit scrawled on the tape and stuck over the original circuit breaker names. The most recent owner who I purchased the boat from has self installed a N2K network – and not that well done. My fear about the electrical system is it going wrong in some remote part of the world and not be able to figure out the wiring (since it was most likely done DIY style).

I want to make extensive changes to the galley (pull it out and reconfigure it to suit my needs). I want to take out the nav station since I dont need it in my use case, and so give more room to the galley. This gives me a chance to take the electrical panel out and rewire the boat. I want to do this with new tinned wire, as it seems that Garcia did not use tinned wire in the 80,s (dont know about now).

Ok so that is all well and good and its refreshing to get a take on this from a seasoned sailor.

But how do we redesign the electrical system? Who do we go to figure out all the items needed? Your article on battery selector switch is good to know and well worth doing. Having an alternator isolator is another good tip well worth knowing. Not getting this work done by the boatyard is a great tip – I thoroughly concur with getting outside specialists to come in and help on this. But how to incorporate all these ideas into a wiring schematic if one is not an electrical engineer ?

Is there ANYWHERE we can go to get a consultation to figure all this out?

Your thoughts are highly appreciated.

Jim Schulz

I’m starting a systematic read of your online book John and look forward to it in preparation for an electrical system refit on our 1989 Ericson 38. In the meantime I’d like to plug Nigel Calder’s online electrical systems courses at I just finished the 101 course and found it much easier to digest than his book. I don’t have any experience in marine electrical systems and was overwhelmed by the book, but this course broke things down into easy to understand chunks that built on each other very intuitively. I found it well worth the expense.

D Teesdale

Regarding the Lithium route, I keep reading and learning all that I can while still likely to remain with the Lead Acid / AGM for now. Any new equipment I buy is with the option of lithium, just in case. Meanwhile, maybe something new will break through like Lithium Sulfur or the Solid State Batteries. 🙂