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  #1  
Old 06-06-2008
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Tapping 12V from 24V System

Am ferrying an old Euro boat with 24V windlass and starter/ alternator. I will be adding temporary 12v user circuits for electronics, lights and so forth. The boat is currently stripped without a single piece of wire in the hull.

So-- I could run it at 12V with a 12V alternator and combine the batteries in 24V series by switching when I need to start or use the windlass. Some problems with that though-- no load on the disconnected 12V alternator at engine start comes to mind.

Or-- and here is the question-- can I tap off 12V through leads to one battery while both batteries are hooked in 24V series?

Thought it might work but I like to avoid the larger explosions when possible!

Keith
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Old 06-06-2008
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Using 12VDC off of one battery in a 24VDC system is a really bad idea. IT will cause the batteries to age differently and lead to other issues in the long run.

Your two options are:

1) Set the boat up with a 24 VDC system and use a DC-to-DC converter to provide 12 VDC where necessary.


2) Set the boat up with a 12 VDC system and use a DC-to-DC converter to provide 24 VDC where necessary.

However, given the amperage of a windlass and starter motor, it might be wise to use a third option, which is to have both a 24 VDC and a 12 VDC system on the boat, with separate panels, charging, etc.
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Old 06-06-2008
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I would tend to agree with SD. Using step-down converters is generally acceptable, except I would run a separate 12v system for the high amp stuff. I believe that's how Hallberg-Rassy does it.
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Even on a 12V with different banks your batteries would age differently anyway (house and starter for instance), there is no problem at all to tap for a 24V eventually. DC -DC converter is a bad idea, it will pose unecessary loss, and will be a beast in size if you intend it to start your engine or a windlass from 12V system. Problem will be to have a combined bank for starter rather a separated one (single point fo failure), and you'll need your alternator a 24V, or your switching will be a big mess (big switches and many contacts).
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I have two systems on my boat - 24v. for the house, winches, windlass and 12v. for the electronics. That means I have two battery banks, two battery chargers, and two alternators. Lots to go wrong or break - and something often does.
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Old 06-08-2008
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Funny this should come up: a friend preparing to go south gave me a call this morning and said there was an Air-X wind gen still in its box, never used at a boater's yard sale. I got it for $350, about 40% of what I should've paid for new and frankly cheap enough for me to "experiment" with before we go south.

Only issue is that its a 24 VDC model. So I'll have to wire it in a particular way to charge a bank of either two 12 volts (the current house) or four to eight 6 volt golf cart batts in series/parallel.

I agree with the separate 24 VDC system for the windlass, but that's a lot of work for the single application...but the payoff is power when you want it
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One other point... the wiring in the boat, which is setup for a 24 VDC system, isn't going to be heavy enough for a 12 VDC in most cases. Since the higher the voltage, the lower the amperage, it is generally okay to go from a 12 VDC system to a 24 VDC system in terms of wiring, but not the other way around.
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Thanks for some good ideas.

Another input from another thread here:

Look up the wiring for a series/parallel switch used on big rigs (trucks). 12 volts to run all the trucks accessories except the starter, which is 24 volt and activated by a solenoid that changes two banks paralleled to series (and it doesn't mess up the rest of the system)
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