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post #1 of 4 Old 02-10-2009 Thread Starter
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Question Another 12v power question

Upon excellent advice from my Sailnet peers, I have chosen installing mostly 12v devices -- lighting, etc. -- enabling me to use most of those while away from my 120v shore power. (It made sense to me from the start, but I wanted to hear from some of you more experienced folk anyway.)

The next question is: I have purchased a new 120-12v converter, one that has multiple, automatic settings (full charging, trickle, etc.) So, in port I have unlimited 120v shore power (at least up to 30amps, of course); and I now have a new 12v converter. But, if -- as a liveaboard -- I am using a greater number of 12v devices than the boat's electrical system was designed for -- despite having a brand new converter -- am I harming my batteries? Does the majority of the power used go into and out of the batteries; or does it go from the converter, to the battery terminals, and -- bypassing the batteries, for the most part -- go straight to the devices?

I know it's a strange question. And, as a former Navy electrician, I should know the answer. But I have some-timers disease -- sometimes I forget things -- and I just don't know whether using more lights, more chargers, more fans, etc., is going to wear out my batteries.

Sorry for the long drawn out question.

TonyInNewportOregon
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post #2 of 4 Old 02-13-2009
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It would seem to me if the 12v power is comming from the charger then the batteries will stay charged, plus you are not useing full power (30a) 24hrs a day so the batteries will charge then I don't see a problem
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post #3 of 4 Old 02-13-2009
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Classic simple electrical planning calculation. Amp hours in must be greater than amp hours out. Figure out how many hours or parts of an hour each device will run each day and how many amps each one draws. Multiply to get amp hours. Add em up and it must be less than the number of amp hours the charger puts out in 24 hours. For example, a 15 amp charger puts out 360 amp hours per day. That means you can use devices totaling 15 amps all day without losing your battery charge.

There is one slightly obscure side issue. As the battery approaches full charge it will not accept the full 15 amps. Even so, the charger will continue to bypass the battery and provide the full 15 amps as long as the load is connected. It's pretty unlikely that scenario would develop unless you are using a whole lot of stuff, like that 12 volt tanning booth.

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post #4 of 4 Old 02-17-2009
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Generally, if you're drawing more 12 VDC electricity than the charger can supply, the excess will be drawn from the batteries and draw down the batteries until they are depleted.

For example, say you have a 15 amp battery charger. Say you are using the refrigerator (12 vdc 2.7 amps), and have the stereo cranked (16 amps), and the lights on in the cabin (four at 1 amp each), you're effectively drawing 22.7 amps or so... the charger will supply 15 amps of that, and the batteries would generally make up the remainder... however, if your batteries are discharged, you will eventually kill them by doing this.


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Originally Posted by TonyInNewportOregon View Post
Upon excellent advice from my Sailnet peers, I have chosen installing mostly 12v devices -- lighting, etc. -- enabling me to use most of those while away from my 120v shore power. (It made sense to me from the start, but I wanted to hear from some of you more experienced folk anyway.)

The next question is: I have purchased a new 120-12v converter, one that has multiple, automatic settings (full charging, trickle, etc.) So, in port I have unlimited 120v shore power (at least up to 30amps, of course); and I now have a new 12v converter. But, if -- as a liveaboard -- I am using a greater number of 12v devices than the boat's electrical system was designed for -- despite having a brand new converter -- am I harming my batteries? Does the majority of the power used go into and out of the batteries; or does it go from the converter, to the battery terminals, and -- bypassing the batteries, for the most part -- go straight to the devices?

I know it's a strange question. And, as a former Navy electrician, I should know the answer. But I have some-timers disease -- sometimes I forget things -- and I just don't know whether using more lights, more chargers, more fans, etc., is going to wear out my batteries.

Sorry for the long drawn out question.

TonyInNewportOregon

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