The Offshore Voyaging Reference Site

Battery Containment—Part 1

Many boats we buy, either new-to-us or brand new, will be fitted with DC electrical systems that are not close to cruising ready, and even further from offshore ready.

So, over the next few weeks, I’m going to add what I have learned while fixing that on three boats—twice on one of them as I came up with a better system after living aboard for 10 years—to our Online Book Electrical Systems for Cruising Boats.

As I already promised—sorry for the delay, went down the lithium rabbit hole—this series will be a deep dive into how to do this right, with lots of diagrams and photos, as well as the full system I designed for, and recently installed on, our new-to-us J/109.

Let’s start with battery containment, since most production boats I see have woefully inadequate battery compartments that need to be fixed, at least before we take the boat offshore.

A Real Problem

If you doubt that, read, as I have, accounts of knockdowns and rollovers at sea. A common thread is that the batteries came adrift, often doing huge damage in the process.

That was bad enough back in the day when a typical cruising boat had a couple of Group 31 or similar-sized lead acid batteries, but these days, with huge battery banks, the need for proper battery containment has never been higher.

Specifying The Solution

The first step when designing a new battery containment area, is to consult the relevant standard. Yes, I know, getting access to these standards costs money, but, seriously, do we want to go through all the grief and expense of this project and end up with a battery area that’s not compliant? Clearly not.

Those of us in North America should join ABYC as I did, and read their E10-Storage Batteries document. Lots of good stuff there about ventilation, covering contacts, and the like, but let’s zero in on requirements for the structure that will contain our batteries:

10.7.1Battery mounting materials and surfaces potentially in contact with corrosive electrolytes (e.g., lead acid type) shall withstand electrolyte attack.

10.7.3 Fasteners for the attachment of battery boxes or trays shall be isolated from areas intended to collect spilled electrolyte.

10.7.4 Batteries, as installed, shall be restrained to not move more than one inch (25 mm) in any direction when a pulling force of twice the battery weight is applied through the center of gravity of the battery…


We Need Better Than ABYC

So ABYC were doing great on spill containment and the two-gravity requirement, but then they had a complete brain fade. One inch of movement allowed? WTF?

That might be OK inshore in smooth water, but Matt Marsh, AAC tech-guru, once calculated that a passage to and from Bermuda would subject the boat to around a quarter of a million wave cycles. And I’m betting, based on having done those passages over a score of times, that many of those cycles would likely move batteries constrained only to the ABYC spec, at least a bit.

And one thing I know for sure from my years at sea, is that once something heavy is allowed to move back and forth, failure is a matter of when, not if. One of the immutable laws of the sea is:

Movement begets more movement.

Me…I think…unless I heard it someplace else…or maybe Churchill said it, or Mark Twain.

So we need to change 10.7.3 to be:

Batteries, as installed, shall be restrained to not move in any direction when a pulling force of twice the battery weight is applied through the centre of gravity of the battery¹.

AAC Pain-in-the-ass Offshore Standards

¹Before the engineers around here jump down my throat (with justification), I do understand (I think) that to actually achieve zero movement we would have to preload the containment to a bit more than double the weight of the batteries, which is probably not practical, so there will be some movement at least at the upper end of the accelerations we are planning for, but practical experience says we can, and should, reduce that to no more than an eighth of an inch (3mm) or so, and even less is better. (If I have that right, we can all thank Eric Klem and Matt Marsh, and if I have it wrong, it’s my fault and I would appreciate correction from the engineers around here.)

Figuring The Design

Now, at this point, wouldn’t it be cool if I could publish drawings and specifications for containment based on engineering calculations for all boats and situations?

Clearly, that’s not going to happen, because:

  • I’m not an engineer, or even close.
  • There are way too many variables in battery and bank size, as well as space available on different boats, to make a single solution useful.

The Ideal Solution

So in a perfect world we would all hire a professional engineer to custom design our battery boxes. And that’s definitely the best alternative, particularly when dealing with big banks.

The Practical Solution

But this brings up a general problem that we boat owners are faced with on a regular basis:

Many of the upgrades we need to do to our boats to make them offshore ready require quite complex design, but actually finding an engineer willing to mess with these small projects is difficult, and said design work will often exceed the cost of the project.

What to do? No perfect (or even close) answer, but here’s what I do:

  1. Learn everything I can about basic engineering.
  2. Recognize that, notwithstanding #1, I’m still pretty ignorant about how the forces at work offshore will try to break whatever it is I’m building.
  3. Listen with rapt attention when engineers comment here at AAC.
  4. Draw things out before I build them and really think about the forces at work.
  5. Overbuild the living crap out of things that really matter, like battery containment—Matt Marsh calls safety margins “margins of ignorance”, and I’m plenty ignorant and so need plenty of margin.
  6. Watch new things I build for any signs of impending failure (typically flexing) and improve as required. I can’t tell you how many gear faulures I have avoided over the years by being really vigilant about this—things seldom fail without warning.

A Team Effort

So how can I translate that into an article about battery containment that’s useful to you members? Not easily. In fact, I have been fussing about the best way forward for months before coming up with an AAC team effort:

  1. I will share two of the containment solutions I came up with and the challenges that shaped those solutions.
  2. I’m hoping you members will share your own solutions.
  3. I’m double-hoping that the engineers that give so generously of their time in the comments will identify weaknesses in our solutions and suggest improvements.

Let’s do it.

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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—Part 1, BMS Requirements
  31. Lithium Batteries Buyer’s Guide—Part 2, Balancing and Monitoring
  32. Lithium Batteries Buyer’s Guide—Part 3, Current (Amps) Requirements and Optimal Voltage
  33. Lithium Battery Buyer’s Guide—Part 4, Fusing
  34. 11 Steps To Better Lead Acid Battery Life
  35. How Hard Can We Charge Our Lead-Acid Batteries?
  36. How Lead Acid Batteries Get Wrecked and What To Do About It
  37. Equalizing Batteries, The Reality
  38. Renewable Power
  39. Wind Generators
  40. Solar Power
  41. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  42. Battery Monitors, Part 1—Which Type Is Right For You?
  43. Battery Monitors, Part 2—Recommended Unit
  44. Battery Monitors, Part 3—Calibration and Use
  45. Battery Containment—Part 1
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James Evans

Nice job, John. But for 20 bucks might something like the photo have been easier?

Arne Mogstad

From what I can see in the picture, that is a few orders of magnitude worse than John’s design. For one, it looks to only be attached with four screws/bolts through the small plastic tabs in the corners.

Eric Klem

Hi All,

Honestly, I wouldn’t be surprised if that technically meets the ABYC requirement. From an engineering perspective, even at 2G, all of these loads are actually pretty small. The question with this is what kind of plastic is it made of and have they gotten the details right. For example, the mounting tabs do look a little thicker than everything else and they are gusseted so there is hope. The question is whether they tie into the rest structure well enough and whether the material is one that holds up over time (anyone remember the UV degradation of early ABS seatbelts?). I don’t know if they are threading directly into plastic or have inserts but that is something to watch. It certainly does not look overbuilt to me and when you don’t leave a lot of margin like that, you need to get the design right and have process controls on the manufacture so that you don’t have a situation like a purchasing guy quoting the base with a different molding vendor who changes feeds and vents and doesn’t do mold-flow analysis or strength testing.

Would I use it? Maybe on a coastal cruiser if I could actually see it and feel good about it. No way on an offshore boat. And I would definitely be real careful that nothing else could get loose and hit it. If something like an improperly stored emergency tiller were nearby in the same locker, that could easily spell trouble.


Matt Marsh

A light-duty plastic tray and crossbar like that is not appropriate for an offshore boat. It’s an improvement over the “just set it in the bilge and maybe put a buckle strap over it” approach used in most <20′ runabouts, but would have a hard time passing muster in a car or in a severe-service boat.

In well-built cars, you see substantial stamped-steel trays under the battery (either welded to the vehicle structure, or bolted to it with a few 3/8″ cap screws) and removable stamped-steel crossbars above it, with the crossbar tie rods (typically 1/4″ or larger) pre-tensioned so that the battery movement is indeed approximately zero.

A lashing or strap will probably work well enough, but is unlikely to restrain a battery as securely and rigidly as a good metal or composite (not injection-moulded plastic) crossbar and tie rods.

If the battery is moving, at all, then its wires and its case are probably chafing on something. There’s a grid of ribs moulded into the bottom of the battery case to absorb that chafe. But I still see a worrying number of batteries where those ribs are chafed away and the case bottom is now at risk of being breached. (Usually, these mechanically poor setups are seen in conjunction with electrically poor setups, so the batteries die of sulphation or plate damage and get replaced before the motion has enough time to wear through the case.)

Arne Mogstad

Hi. I have little knowledge on the area, but from my time working on bigger ships offshore, it was always a big issue if any construction was done that impacted the constructions ability to flex. On your design, once you’re gluing your very strong and rigid battery frame to the hull, will that not induce new stress-areas in the hull? I don’t mean to pick on your design, quite the opposite! I need to do a major electrical job myself in not too long, and find these articles very interesting.

Arne 🙂

Eric Klem

Hi Arne and John,

Provided that a little caution is exercised, I suspect John is right that you will not cause major issues by bonding to a hull skin of most cruising boats. Ideally of course you would run the numbers but that is hard even for an engineer as the layup is often not known. A few thoughts on what caution means to me:

  • Stay away from highly stressed areas like chainplates and keel attachment.
  • Stay away from existing stress concentrations. Some are obvious like a thru hull and others may be a little less obvious like the end of a stringer. You should be pretty safe if you are in a big area with no major structures ending.
  • Try to limit stress concentrations from the structure you are adding. The most obvious one is to taper out the ends of the structure. While an engineer can give you a more optimal curve, most people just use a low angle chamfer at the end and that does a decent job.
  • Use your best judgement to avoid making something stiffer than what it is attaching to. This is not a place to try out carbon fiber, a low modulus glass is just fine. Also, avoid structural pieces that are large in the perpendicular direction to the hull skin as stiffness is proportional to the square of the height here.

One other way to do this that may make sense for people is to glass in a few landing pads on the hull and then bolt a box that you make separately to that. 3 or 4 pads is really all you need, just make sure that they mate nicely to the box and you don’t need to shim or something. It is certainly possible to make this stiff enough to cause a stress concentration on the hull but it is not super likely as you have to know how to transfer that stiffness through and I suspect most home-brew solutions won’t do that effectively.


Matt Marsh

I’m in general agreement with Eric’s response above. The key is to stay away from existing stress concentrations and try to avoid creating new ones.

If you lightly tab or bond something stiff to a hull skin, it’ll peel or crack as the hull flexes.

If you want it rigid and don’t want it to peel or crack, then what you’re adding needs to be effectively another structural member. The tabbing / bonding needs to be large enough and stiff enough to withstand that. And, critically, it should be tapered / feathered out to avoid creating a new stress concentration where the reinforced area ends.

The alternative is to soft-mount it, and expect some motion or flex to be absorbed at the joints.

Dave Warnock

We have an old book by Ian Nicholson in which he suggests fitting tapered plywood flat onto the hull and then tabbing the bulkhead to (and beyond this). This to avoid creating hard spots on the hull.
We have used this when adding new watertight bulkheads and a new chain locker in our forecabin. Hard to do unless part of a large refit due to the impact of the surrounding area.

Arne Mogstad

Thanx to all three of you, John, Eric and Matt! Very good and helpful answers! Even though I have an aluminum boat, I suppose a lot of the same principles would apply. I have a battery box that is welded in aluminum and fit my current batteries so well, that I only need a vertical restraint, but I won’t be able to do that for my next batteries, so I need to make something new.

Dave Warnock

We changed our minds a few times but have ended up building a watertight box on top of the encapsulated keel. We have the 8 LiFePo4 batteries in two layers (4 x 120Ah on the bottom, 4 x 300Ah on top).
The 18mm ply outer structure is glassed to the bilge sides with dividers around every battery (no room to move at all, but there are some ventilation holes within the compartment). Each battery in the lower section has an 18mm ply lid that will be secured by multiple 5mm bolts. The top is two pieces of 18mm ply (aft section is slightly higher due to narrower hull at that point). The lids are going to be bolted onto a 3mm x 50mm neoprene seal. The forward ply forms the cabin sole, the aft is behind the companionway (wet locker has a separate floor just above).
The 4 cables go through watertight glands in the aft end into the electric motor space, each bank has an ANL fuse in the motor compartment (cable length 150mm which I think complies with ABYC).
We have put in new bilge drains on both sides around the battery box leading to the sump.

Dave Warnock

Maybe but the logic of your article is still helpful 🙂
My bug bear is people not protecting their battery bank from water (especially when they put it in the bilge). Obviously ventilation needs depend on battery type but I wish more care was taken to ensure that the first water sloshing about the bilge doesn’t short out the electrical supply to the electric bilge pump etc.

Matt Marsh

Lithium battery cells have hermetically sealed cases around the reactive parts. Water can’t penetrate unless the cell is already badly damaged.

If it’s fresh water, not much would happen. If it’s salt water, you’d get a current flowing through the water between the terminals, which would discharge the battery and cause a mess of corrosion. In other words, pretty much the same as happens with sealed lead-acid cells.

Metallic lithium touching water is Very Spectacularly Bad. But there should be no metallic lithium in such a battery unless it is severely discharged beyond the point of permanent damage, and even then, it’s inside a hermetically sealed case.

In general, you should keep batteries out of the water. I’d only install them in the bilge if the boat had a sufficient sump and pumps below that to ensure that the batteries remain dry at all times.

Dave Warnock

Thanks, helpful.
I’m happy with our choice to put the batteries in a watertight box and ensure that the cables come through watertight glands with the fuses as high as the 150mm max cable length allows.
We do have a big sump and the rest of the bilge will drain around the battery box. This should ensure that our pumps etc can run reliably for as long as possible should we get a severe leak (of fresh or salt water).

William Willcox

I commend, for your readers consideration, the battery box design documented in chapter 3 of “The Boat Owner’s Mechanical and Electrical Manual”, 4th Ed., by Nigel Calder.

In use for about 30 years on my boat, with good success.

Bill Willcox

Alex Borodin

Hi Bill,
while I do not feel qualified to criticize Nigel Calder, I did notice that his design leaves the battery free to move in the vertical direction. Neither the battery, nor the box’s lid are fixed against shifting in the event of a capsize.
This doesn’t seem ABYC compliant to me.

William Willcox


You are absolutely correct. For my installation I epoxied two 2x4s with horizontal slots to the floors under the box and then strapped the box and batteries through the slots and over the tops of the batteries. Have been in some interesting seas on the way to Mexico outside the Baja peninsula with no issues. Nigel was focused on containing and ventilating multiple battery banks. I suspect he avoided the mounting details since all boats are different.

Eric Klem

Hi John,

This looks like a good solution for your situation. Like you, I have noticed a lot of disaster reports mentioning the danger of stuff flying around. Batteries happen to be heavy items but at least they are an easy to deal with shape, doing all the other large items like spare anchors can be quite time consuming.

I agree with your take on the best way to secure batteries depending on the form factor. In design, I generally like to bolt stuff to a flat surface whenever possible but sometimes the aspect ratio calls for more support. I don’t know the answer on battery acid resistance of dyneema but UHWMPE has pretty good acid resistance so I am optimistic that it is good. RC recommends cam straps like this in one of his articles and I assume he has checked acid resistance of those. I have seen them set up that way several times and never seen a strap that look weakened.

My start with custom battery boxes was doing them for the homebuilt electric cars I used to do. The biggest single box housed 20 Trojan T145’s at 72 lbs each for a total of just shy of 1500 lbs and I think that the rules required a safety factor of 4X and a maximum movement of something like 0.050″ if I remember right. 2 things quickly emerged as the hardest parts of the design which were securing the box to the vehicle as nothing it was right next to could take much point loading and the second thing was securing the batteries in the box. Building the actual box was quite easy, it was a bunch of welded angle iron with fiberglass panels. To limit battery movement to meet the rules, we constrained the batteries both on top and bottom. On bottom, there were cleats that fit the batteries tightly similar to what you did. On top, there were hold down bars going all the way across the box which bore both on the top and the sides of the batteries and were reinforced on top by steel crossmembers.

I have built a few battery boxes and have generally just built smaller and lighter duty versions of the ones I did with electric cars. The best have probably been boxes made from something like Coosa that are then glassed up. In the bottom, I have glassed in angle stock just like you used to constrain the battery. Since I have made the boxes go all the way up the sides of the battery, I have always also constrained the batterys on top. For this I make a frame that pushes down on the perimeter of all batteries at once and then put little pieces in between to keep the batteries constrained in the horizontal plane. The trick with something like this is getting it to install at the right height. I have done 2 things here. The most straight forward is to just run a few pieces of threaded rod vertically in the box and thread on some nuts pulling this top assembly down in to the box. The other thing I have done (and what is on our current boat) is to install captive nuts in the frame and then slot the sides of the box so that it can be bolted from the outside at the correct height. The reason I did this was that not all corners at the top are accessible but I can do all sides. Sorry, I don’t have any pictures. I have also done something similar to what RC suggests for a small skiff and it was fine for that, more battery movement inside the plastic box than I would want for a cruising boat though. One box I built on a commercial boat was lucky enough to have some structural members not that far above the 16 GC2’s we were trying to constrain so the box was not a self contained unit, the top was braced up and the bottom mounted down.


P D Squire

Thanks for the RC links Eric.

I am now pleased that the man who built my yacht in his shed 35 years ago mounted the flooded-cell battery with cells running port to starboard. He must’ve known something (or got lucky)

After my ratchet strap deformed the battery casing I replaced it with three of the cam straps you linked. My one concern is that I don’t feel I was able to get enough tension with the cam system. I suspect my battery would move 1/2″ or more if subjected to 2G.

George L

What did you do to ensure that the box is gas-tight and vented to the outside?


Alastair Currie

In the industry that I work in (offshore oil and gas), there have been hydrogen explosions reported from battery banks. These reports are for smaller batter banks somewhat analogous to some sailing yacht set ups: –

The second link certainly supports your observation that the lid must be vented, otherwise hydrogen gas concentrations will build up.

The risk is easily reduced to a low, or improbable event by adding a vent to the box if the batteries are a type that can vent hydrogen.

P D Squire

Jaguar XK120’s had two 6v batteries mounted below the floor behind the seats. They vented into the cabin. After a few years the fumes caused the headlining material to deteriorate.

P D Squire

I wonder if battery manufacturers specify the strength of their casings, especially in respect of securing them in place. One benefit I perceived with the standard hold-down tray was that it spread the clamping pretension over a fairly long section of each side’s top edge. By contrast, a spectra strop would apply the same load over a much smaller area. Is the battery’s case manufactured to tolerate that much long-term, point-load along it’s top edge? If there is a manufacturer’s casing strength specification it might help us design matching securing systems.

I thought of this when I noticed that tensioning my boat battery’s securing ratchet strap had put a bulge into the battery side. (I released the tension somewhat and installed a couple more straps to spread the load across. If it ever comes out or moves, or worse; if the battery case fails, I’ll let you know.)

P D Squire

This seems to raise a question for the A40 engineer(s.) Do they design the battery box(es) to perfectly secure a specific battery chosen and recommended because it was found to have the best casing structure (among other advantages,) or is a generic box built that will secure any battery(ies) of the size specified.

One of the A40’s attractions is that so much is specifically specified (no options) and that a great deal of experience is going into the selection process (and that nothing will be present that hasn’t proved itself over 10, preferably 20 years.) Perhaps there is a case for including specific battery(ies) in this specification. Although there will need to be some flexability in case the specific battery isn’t available in some remote locations where owners might need to acquire a replacement.

Perhaps Eric Klem has already provided the answer.

Stein Varjord

Hi P D,
I also like the “conservative” spec system for the A40, but I think battery systems is an area where it should be as open as possible. The reasons are the obvious ones: The right choice for battery type and size will vary widely between owners even in one given moment. On top of that, battery tech develops faster than almost any other tech. Loads of seemingly realistic and dramatically improved solutions are already moving towards the consumer market. In 10 years, we will all definitely want completely different battery types than what is available now. The A40 cannot possibly be ready for all future options, but locking into one specific option might become a problem.

P D Squire

The A40 will come with hard points for owner-mounting future options such as radar, Jordan Series Drogue, etc. I agree the standard boat would come with an engine battery and one or two 4D or G31 house batteries. What we are learning here about securing battery boxes to the boat’s structure suggests that it would be beneficial for the A40 to have suitably engineered points for securing an additional house battery box for owners who want to add extra capacity.

P D Squire

Yes, tricky situation; designing for something very heavy that might or might not be installed. Perhaps a space low down where users could install an extra water tank, diesel tank, OR battery bank. Although I’m not sure what the owner who needed none would do to maintain the ship’s balance and stability. Also, lead-acid is way heavier than water or diesel.

Alternatively, perhaps ship the A40 with somewhat more than reasonable lead-acid house battery capacity. Then users who want lots of electricity can head down the lightweight lithium path. As you point out elsewhere, they will need to add a duplicate charging system and other complexities, but at least the large lead-acid backup will already be in place. And they won’t be looking for somewhere to put perhaps a 1/4 tonne of extra stuff.

It might be an interesting idea to treat lithium as another charging source for a lead-acid system. This would seem to be virtually the same as the equivalent capacity increase, but simplified because the lithium system never has to be effected by the house devices (or them it.)

Stein Varjord

Hi P D,
I think that’s a good point. I’ve also seen batteries showing clear wall distortion. I don’t like any of the ready made battery strap down solutions I’ve seen, but perhaps inspiration could be found in how separate lithium cells should preferably be clamped?

They should be kept in compression. Normally one would put the cells next to each other and have strong plates, often aluminium, on each side. Often also stiffener bars across the plates. Then clamp them together by at least 4 threaded rods. The cell pack is then strapped or bolted down using the clamping plates for attachment, or at least support for the cell walls.

Especially tall batteries seem suitable for this. Like 2v cells or 6v batteries. I don’t know how smart it is to clamp lead acid batteries at all. At least it seems possible to exaggerate. Anyway, maybe worth a thought?