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What if you could double your battery bank capacity without any cost or adding any additional weight to your boat? Wouldn’t that be awesome? Well, that is essentially what we accomplished yesterday.

Have you read my Voltage vs. Amp Hours Consumed post? One of our most popular, as of this morning it had garnered 93 comments. A large part of that discussion related to the fact that, in order for a battery monitor to give accurate readings, it not only needs to be hooked up correctly but it also must have the proper info inputted into it. Specifically, you need to tell the monitor just how big your battery bank is so that it can tell you what percentage is remaining.

After checking the distilled water in our batteries yesterday, and noting that the fluid was down a bit in half of them yet hadn’t moved at all in the other half, I was inspired to ask a question. That being, just how do I know that all of the batteries are actually connected? I mean, although they appeared to all be connected, I didn’t run the wires myself nor had I traced each wire or checked the current flow through them. And so, I went exploring.

My exploration didn’t take long though. In fact, in mere seconds I took note of the two breakers which are hidden way back in the far corners of our center battery compartment, one on each side. Can you guess what I’m going to say? They were both off! Yes, off, and being so, they took half of our battery bank completely out of the circuit. How long have they been this way? A wonderful question for which I have no answer. A week? A month? Since we installed those new batteries 1.5 years ago? I have no idea. All I know is that, as soon as I flipped those breakers on, current started flowing and our battery bank doubled in size.

Before all the “doom and gloomers” start suggesting that the batteries, having been out of the circuit and not charging for a period of time, are either dead or damaged, they are not. The voltage reading on each battery showed it to be at, or above, 50% charge. Once the batteries were connected we fired up our generator and ran it for the better part of the day, bringing the entire 8-battery bank up to full charge.

Did having these four additional batteries affect our morning readings, which was the impetus for the post that I linked above? Definitely. This morning, instead of reading of 12.1-12.2V, the Link monitor showed the bank’s voltage to be 12.4V, a significant improvement!

Moral of this story? If you didn’t do it yourself, or check it, how do you know that it is set up correctly? In this case, there is no tell-tale indicator that those breakers are off. Nothing stops working nor do any warning lights flash. If you have things like that on your boat that you are assuming are working, you might want to give them a look, just in case.


  1. What are the breakers intended for? Also, are they genuine ‘contact breakers’ designed to trip out if overloaded in some way. Or are they simply rather fancy switches? And why two?



    • There are four batteries in that compartment, two go to Bank A and two go to Bank B. There is a switch for each. What their purpose is exactly I don’t know.

      • The usual separation is Engine starting, say bank A, and General on bank B.

        I set mine up that way when I rewired etc, following roughly what was there before. However, I leave it on both most of the time so that it all gets charged.

        Anyway, well done for solving it and having full power!

        So now it is ok for Hair dryers, Microwave, Electric kettle, All computers at once, etc etc! 🙂

        Cheers! Mike

  2. Th moral of the story is sell those batteries and get the weight off your boat. If you have managed without them thus far, you clearly do not need them!

  3. Keeping with the electricity flow, how often do you use your generator? If it is often, do you say that the solar was a waste? Or do you only use it infrequently and the solar has been a big help?

    • Solar is a NECESSITY! The more, the better. We only run the gen to make up for a shortfall. If we had more solar we would probably never have to run the generator.

      • Any opinion on wind gen? I read Plastiki, and they were an all green boat (to the extreme!). They used solar and win gen. They also had a wind generator in the water, which I thought was really cool.

        • A wind gen would be a nice addition but to be honest, before I put one on, I would max out my solar based upon the available real estate.

  4. M&R,

    I’ve been lurking lately and not posting, obviously, but..

    NICE CATCH Michael. Well done.

    Ain’t it amazing what you find when you go ‘sploring?

    Good job.


    • Thanks, Jim! I was pretty happy about the find, although a bit peeved to know that they weren’t hooked up for an undetermined amount of time.

  5. so now the AH vs BV dilemma should make a little more sense 🙂

  6. You have 8 batteries on a 32′ boat? No, really?

  7. Congratulations on finding the problem. Keep those batteries. The boat’s been handling them weight and a good reserve is nice to have. They’ll all last longer if they aren’t working as hard.

  8. The circuit breakers are there to protect all the wiring on the boat, and stop a fire.

    Large capacity CB on the +ve side as near the battery as possible is best practice and seems to be what you have. It means any short circuit anywhere on board but especially between the battery bank and the distribution panel will turn off the power supply rather than starting a fire.

    Surprising how few boats have them, yours is new enough for it to have been required, many older boats haven’t retrofitted them.

  9. Hi, as stated allready I guess the two banks are for domestic 50% and engine start 50% although if there were more batteries I would think that the domestic may have been a greater percentage, it maybe a thought that you move the isolators to a position that is easier accessed so that in long “on the hook periods” you simply use the domestic bank and isolate the engine start bank or even change the bank ratio to 75/25% ie a quarter of your bank is used for engine starting and 75% for domestic? (just a thought) I guess that your engines are easy starters and the remainder of your domestic battery energy could be used to assist when the starting wasn`t so good ? therefore if this were the case then the engine bank, or one battery, could be a traction battery for the starting and the other three could be deep cycle, leisure, batteries. You may also be less likely to need to replace all the batteries at the same time too.cost may also become an influencing factor when pricing up normal (leisure/traction) batteries against golf cart versions? JMHO Rob

  10. Let’s clarify this.
    You have EIGHT 12V batteries (weighting …. what? 40 lbs apiece) on a 32ft catamaran and your outboards are hand started?

    What exactly requires all that capacity?


    On a tangent issue, solar only charges while the sun shines. A wind generator charges while the wind blows (which could very well be at night too, seeing as you’re in the Caribbean).
    With strictly solar, you discharge the battery bank at night. A solar/wind mix could potentially mean you don’t appreciably discharge at any time. Thus less need for such a large & heavy battery bank.

    • The previous owner of our boat doubled that battery bank. I have no intention of changing it. The engines are not pull start, they are electric start. They COULD be pull started in a pinch though (in theory, I have never tested this).

      You are right about solar vs. wind. Both would be best. Real world experience by the multitude of cruisers I have talked to about this subject though point to the conclusion that solar is way more valuable in the tropics. So again, if needed, I would consider a wind generator AFTER I had maxed out the solar I can fit on the boat’s available real estate.

  11. The whole battery thing confuses me so much. A couple of things I read in your post made no sense to me (like doubling your battery supply once you threw the breakers on only gave you .2V more power when I thought it would double it) but I am not gong to ask about them because … well, I’m just not.

    • On a lead acid battery…

      12.6V = full charge
      12.4V = 75% charge
      12.2V = 50% charge (the lowest you really should let them go)

      As you can see, 0.2V is a LOT.

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