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:
- Learn everything I can about basic engineering.
- 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.
- Listen with rapt attention when engineers comment here at AAC.
- Draw things out before I build them and really think about the forces at work.
- 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.
- 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:
- I will share two of the containment solutions I came up with and the challenges that shaped those solutions.
- I'm hoping you members will share your own solutions.
- 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.