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Old 18-12-2007, 00:05   #1
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Designing my electrical system

For my new to build boat, I will use a dual voltage system. 12V for the engine (starting & controls), 24V for the house bank (with 12 DC/DC convertors for the nav equipment, and few others that need 12V). My boat is being built in South Africa, and most lights, fans, water makers, etc. are all available in 24V.

I made some basic calculations to dimension batteries, chargers etc.. I made use of the calculation tables that can be found on the web, or in books like the ones by Nigel Calder.

Based on everything in 12V I would use about 260 Ah in a 24h period.
For 24V that would be 130 Ah in a 24h period.

To be on the safe side I would take a battery capacity at 24V that is three times the max consumption in 24hrs, so 3 x 130 = 390, so take 400 Ah.

Does this look right so far?

Now for the 24V alternator, I am planning to take a 100 - 150A marine alternator. Would that be the right size for a battery bank of 400 Ah?

Also could anyone point me into the right direction what I would need in solar panels to effectively keep my batteries charged when I leave the boat in the marina for a while?

Thanks in advance,

Andreas
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Old 19-12-2007, 09:34   #2
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You need too also consider the number of hours or days between running your generator, and the time you wish to charge your battery back,

To determine the Solar panel size you need counter normal self discharge to consider the cloud vs sun and the latitude.

Up on the BC coast the size is much greater than elsewhere.

I have 1000 ah bank and require in a 50 watt panel in through a long cold snowy dark winter
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Old 19-12-2007, 10:24   #3
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Thanks for your reply, jscott!

I assume while cruising to run the generator about 1 - 1.5 hours a day, during which period I would also run my watermaker.

50 W panel to keep a 1000 Ah bank charged is actually not too bad (I assume you have a 12V installation). I would have room for more than that, which would reduce my generator running time. The boat will be mainly in the tropics.
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Old 19-12-2007, 11:20   #4
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dont forget refrigeration

Unless you empty the fridge every time you leave the boat you will need a lot more than 50 watts. I have 225 watts and it is not quite enough in hot weather when the fridge runs a bit more.
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Old 19-12-2007, 11:40   #5
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Andreas,

Any particular reason you have chosen to go with a mixed 24V and 12V system? On a 43' boat, it would not seem necessary, and there are lots of power losses and some complications in conversions from one voltage to another.

Most folks with 40-footers choose to go with all 12V systems which greatly simplifies things.

Just curious.

Bill
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Old 19-12-2007, 18:08   #6
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Quote:
Originally Posted by henryv View Post
Unless you empty the fridge every time you leave the boat you will need a lot more than 50 watts. I have 225 watts and it is not quite enough in hot weather when the fridge runs a bit more.
That's a good point, thanks!
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Old 19-12-2007, 18:32   #7
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Quote:
Originally Posted by btrayfors View Post
Andreas,

Any particular reason you have chosen to go with a mixed 24V and 12V system? On a 43' boat, it would not seem necessary, and there are lots of power losses and some complications in conversions from one voltage to another.

Most folks with 40-footers choose to go with all 12V systems which greatly simplifies things.

Just curious.

Bill
Bill,

Actually the only equipment which will run on 12V is some of the navigation equipment, for which the DC/DC converter will be quite small.
All other equipment (windlass, lighting, refrigeration, watermaker etc.) will be 24V. So over all with 24V there will be smaller losses, and therefore lower relative voltage drops.

Andreas
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