Batteries/Inverter (was Music on Board)

L & M Nunas nunas at
Mon Mar 16 19:04:08 EST 1998

Oops!  I meant to post this to the list, not reply to the author.


-----Original Message-----
From: L & M Nunas <nunas at>
To: P G <pgslo at>
Date: March 16, 1998 8:59 PM
Subject: Batteries/Inverter (was Music on Board)

>-----Original Message-----
>From: P G <pgslo at>
>To: nunas at <nunas at>
>Cc: Kogia at <Kogia at>
>Date: March 16, 1998 10:04 AM
>Subject: Re: Music on Board
>>Maurice, how about giving the list some idea of what your experience has
>>been with the inverter and batteries.  This is a question I'm trying to
>>address.  How many batteries to handle a known load.
>My experience has been very good.  But, as I have posted before, our boat
>was much smaller than those people here are writing about.  At just under
>feet LOA, we did not have much room for a lot of batteries and a gen. set.
>It was sort of a one-or-the-other proposition.  We opted for the
>If you are going sans gen. set, then there are some things you need to do.
>These all include creature comforts.
>Aircon--Forget it.  Even the smallest one takes about 6 Amps @ 120 V (i.e.,
>over 60 A at 12 V).  Running full time, one of these babys should take a
>standard 8D battery (200Ah) down to nothing in a little more than 3 hours.
>Never tried this, but by the numbers, that has to be right--right?.  Nope,
>wrong!  You would be lucky to get 2 hours out of it.  It is impractical to
>run these things via an inverter, as the useful life of a charged battery
>decreases with higher current.  In other words, if an 8D will give 200 Ah
>when you draw a constant 20 A from it, it will give much less if you draw
>100 A from it.
>Our aircon was an open hatch with a wind scoop.
>Refer--Ours was a small RV style unit with a built in inverter (i.e., it
>on 120 V AC or 12 V DC).  Most RV dual voltage refers are really 120 V
>with a little built in inverter.  If you have a look at the DE390 in the
>West Marine catalogue, you will see one very much like it.  The current
>of ours was about 4 Amps at 12 V.  Since most of our boating was done in
>Canada (Rideau Lake, Great Lakes, St. Laurence), the wx is fairly cold most
>of the time.  I recun that the refer ran only about a half of the time.
>that's 1/2 of 4 amps every hour, or 2 Ah.  Looking at it another way, it
>would run a standard Group 27 starting battery (more than 100 Ah at a 2A
>rate) flat in about 2 days.  Common sense test--judging from the times I
>something stupid (it happens), that is about right!  I figure I have run
>them flat in about 2.5 days more than once.  Please don't ask how I know
>this so certainly!
>We always brought cold stuff to the boat and that helped a lot.  We also
>kept the beer in a separate cooler with ice and stowed it in the shade.  On
>long cruises, we would put stuff in the fridge when we were underway, but
>not when anchored, unless we had to (meat and so on).
>Cooking 1--Forget electricity for most things.  Each burner on a standard
>stove takes about 10A @ 120V (i.e., over 100 A @ 12V).  So two running at
>the same time would draw a whopping 200A
>Cooking 2--You can run a small microwave oven off an inverter.  That is
>we did.  A small microwave will draw about 9 A @ 120 V (i.e.,  over 90 A @
>120 V).  A standard 8D battery will run down in just about 40 minutes.
>most of the time, we only used the microwave a few minutes at a time to
>things up.  So this was not a problem.  Math wrong?  Nope, don't think so.
>Remember, an 8D will only give you its rated 200 Ah if you draw 20 A from
>Other Stuff--We did not have much other stuff.  My wife liked to use her
>hair dryer once in a while, but only for brief periods.  Of course, we had
>the standard 12 V lights scattered all over the boat, VHF and a few other
>toys.  If you add it all up, it does not come to much, providing you have
>two or three big batteries.
>So, what to do?
>You will have to establish an electricity budget.  To do this, look at each
>thing you operate and see how much it draws in Amps.  If it runs on 120 V,
>you will have to multiply by 10 to get the equivalent current draw from
>inverter (no, this is not "correct" scientific thinking, but for quick
>thinking it will do).  Then figure how many hours a day the thing actually
>runs.  Multiply the two together and voila!  This gives you the approximate
>number of Ah (ampere hours) it will suck from your battery.
>An example.  A standard reading light takes about 1.25 A @ 12 V.  So, if
>read two hours each evening before going to bed, that is 2.5 Ah.  And so
>Add 'em all up and you get your electricity budget for a typical day.
>I've already given some clues as to what you can expect from commonly used
>batteries, so you can figure from there how much battery you will need.
>But, now for the bitter bit.  The more often you take a battery down to its
>discharged state the quicker you will wear it out.  This includes so-called
>deep discharge batteries.
>So, let's say you run the numbers and figure that everythig you have will
>suck an 8D battery (i.e., 200 Ah @ a 20A rate) dry in two days.  So, you
>figure that this would be great for a typical two-day weekend.  Well, used
>like that, an 8D, even a good one will barely make a season if you are a
>busy boater.  For good battery life, figure that you need twice as much
>battery than the actual stated rating.  This means that an 8D in our
>would only give you 1 day, by which time it would be 1/2 discharged. so,
>need two of 'em for a weekend.
>This is great if you are a weekend boater.  You run plan the system to take
>the batteries to 1/2 charge and then plug in the charger all week while you
>work. But what if you are not a weekend boater.  Or, like us, if you are,
>but you live on the boat during your entire vacation.  Now, you gotta get
>the charge back into the batteries without shore power, maybe for a week or
>two (i.e., for all intents and purposed indefinitly).
>Let me know if you want me to go into this half of the problem and I will
>put my thinking cap on.  It is a tougher question, but I've made most of
>mistakes already!
>Oh yes, forget those wimpy batteries you see in all the catalogues.  Forget
>gell cells, forget for sure anything you can buy in a department store.  Do
>yoursaelf a favour and buy "traction baatteries".  If you are serious about
>using batteries as your primary source of power, this is the ONLY answer.
>How's that for a strong opinion?  i can give reasons and recite chapter and
>>What do you mean by "minimal" reefer?
>See above.
>> Why does a bigger boat need a generator?
>More people, more food, more beer, bigger expectations.
>> I'm going to build  a boat and originally I thought I would
>>go all propane -  stove, fridge, hot water, and heating.  However, I have
>>had a change of heart.  I think it might be better to go all electric -
>>it's safer, easier, and you don't need an additional fuel (propane)
>>aboard.  I thought I would have a minimal generator of 4 to 7kw and about
>>4 or 5 solar panels to keep the batteries charged on a daily basis.
>>Under this scenario, I figure I would only have to run the generator once
>>or twice a week.  I realize that air cond. is the real wild card.  A/C
>>does require a large generator.  I am looking at the small portable
>>Cruise Aire model that you place over a hatch to cool a small space.
>>We're willing to live with compromises.
>I had my little 6 gal. hot water hooked up to my main engine (it had
>antifreeze cooling so this was easy).  When on shore power, I used the 120
>V.  I don't know for shure what it drew, but I'd guess about 5 A @ 120 V (I
>could pop the 15 A breaker if I ran it and the two burners on my little
>Origo stove).
>We used a propane BBQ a lot, like about every day.  But, I would not have
>that stuff plumbed into the boat--no way. (That otta get some discussion
>I don't think your 4kW gen set will run your aircon, stove and a few other
>tings at supper time.  Maybe . . .you just have to go to all your applinces
>and run the numbers.  I can give some basic formulas, additional guidance
>and so on if you need it.
>>I sure would appreciate the benefit of your experience.  My friend,
>>Jacques, on the TW list is also interested.  I think a reply to the
>>general list would be a benefit to all.  Otherwise, a return post would
>>be much appreciated.  Thank you.
>Well, that's enough for now.  There are lots of guys with more experience
>than I have lurking around this list.  Let them jump in now.
>Best Regards,
>Boatless in Singapore

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