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Old 11-03-2010, 12:23 PM   #1
Join Date: Oct 2010
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Electrical Questions...

I have a few electrical questions...

Does the engine alternator charge the house batteries along with the engine battery? The reason for the question would be if I hook up a inverter to the house batteries, I would want them to be charged as I am driving.

How large of an inverter can typically be plugged into the 12volt cigarette lighter?


2011 Forest River Georgetown 350, 36 ft, 3 slides and bunk beds for our 2 boys.
RVing since 2002.
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Old 11-03-2010, 12:28 PM   #2
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I would think that on a new coach like yours they would be charged while driving. I believe this is usually done through a bi directional solenoid which connects the two battery banks. It's the same one that's energized if you press your "emergency/aux start" button. I think a common way for you to answer your question is to measure voltage of you house batteries at the battery and then start the engine...give it a few minutes and then measure again. If it's higher then I think you're charging.

Good Luck...


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Old 11-03-2010, 12:47 PM   #3
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Yes, your house batteries are charged from your alternator while driving. Typically, an inverter up to about 400 watts can be used from the cigar lighter plug for powering laptop, GPS, maybe a small TV, etc... NO larger loads should be attempted. It would be better to attach the inverter to the batteries with a much larger set of wires (cables) and extend the ac to the required area via a regular 120v extension cord.
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Old 11-03-2010, 01:45 PM   #4
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Thanks, another related question.

The generator would also charge the house batteries, correct?

So, if I wire my TV outlets to run on a inverter all the time, the house batteries will never die as long as the engine is running, I'm plugged into shore, or the generator is running, correct?

Thanks again, I love this forum....
2011 Forest River Georgetown 350, 36 ft, 3 slides and bunk beds for our 2 boys.
RVing since 2002.
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Old 11-03-2010, 02:41 PM   #5
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Hi Craig M,
Your post is correct at a high level. However, I would never say never. Ya gotta put in more than you are taking out.
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Old 11-03-2010, 04:47 PM   #6
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Here's a rule of thumb for inverters: the current required on the input is 10 times the current desired at the output. So, if you're trying to pull a 10A load on the 120V output of the inverter it is going to pull 100A out of your 12V system. The numbers aren't exact, but they're close enough for you to make sure your 12V wiring is adequate for the load.

For example: let's look at a typical 400W inverter. If you use its full 400W capacity, that's 400W/120V = 3.3A available at the output of the inverter. In order to supply 3.3A at the output you need to put 33A in at 12V (actually a bit more due to inefficiencies in the inverter).

Many 12V accessory plugs are provided with a 20A or 30A fuse, so running a 400W inverter at its max output may or may not work without popping the fuse.

Scale that up to a typical 1000W inverter and you'll be surprised: that's 1000W/120V = 8.3A available at the output, and 83A required at the 12V input! I've never seen a 12V accessory plug wired to supply 83A - that's why decent inverters are installed real close to the battery bank, using LARGE 12V wiring. The currents involved are considerable.
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Old 11-04-2010, 12:36 PM   #7
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Question 1: Yes
Question 2: 75 to 100 watts tops.

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