Installing 750W Inverter
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URL: http://www.rpod-owners.com/forum_posts.asp?TID=2693
Printed Date: 25 May 2025 at 1:05am
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Topic: Installing 750W Inverter
Posted By: Seanl
Subject: Installing 750W Inverter
Date Posted: 21 Jun 2012 at 10:48am
My wife uses a CPAP machine that she needs to sleep. It draws quite a bit of power so I needed a fairly high power Inverter. High power inverters need to have either short wire runs or really big wires. In the past I have tapped right off the 12 volt terminal on the converter and ran an extension cord to the CPAP Machine but this year I wanted to install a plug next to the bed so that I would not have a long cord going from one end of the trailer to the other. There was already a plug where I wanted it so my Idea was to install a switch that would let me change from shore power to the inverter when we had to shut down the generator and she could use the same plug regardless of the power situation. This drawing is what I came up with. all of this wiring is under the dinette bench with the electrical panel and I have mounted the inverter there as well. I will post pictures of the install when it is done. The only Thing that is missing from the schematic is there is a 60amp inline fuse in the 12volt line for the Inverter. Excuse the missspelling of schematic in the drawing just noticed it.  ------------- Sean, 2011 Rpod RP-173,2009 Jeep Liberty Rocky Mountain Edition 
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Replies:
Posted By: techntrek
Date Posted: 21 Jun 2012 at 11:09am
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I recommend a DPDT switch, switching L1 and neutral. Better yet, take a look at this $50 automatic transfer switch: http://www.solar-electric.com/pomaxpmautrs.html - http://www.solar-electric.com/pomaxpmautrs.html
Some inverters (especially modified sine wave) won't work right or will be damaged if you don't switch both lines.
Something else to consider - make sure you have enough battery reserve so you don't damage the battery by running it down too low. From CPAP discussions I've seen on a solar forum I visit, usually the machine itself isn't a very big load. The problem is when you turn on the heater to add moisture. That is a very large load, and unless you have 3-4 batteries is probably too large of a load. If she can go without the heater you should be good. If she must have the heater expect to enlarge your battery bank. ------------- Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ http://www.rpod-owners.com/forum_posts.asp?TID=1723 - Pod instruction manual 
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Posted By: techntrek
Date Posted: 21 Jun 2012 at 11:17am
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I didn't re-read much of this thread, but here's a 4-page discussion you can look at.
http://www.wind-sun.com/ForumVB/showthread.php?14881-CPAP-Backup - http://www.wind-sun.com/ForumVB/showthread.php?14881-CPAP-Backup
------------- Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ http://www.rpod-owners.com/forum_posts.asp?TID=1723 - Pod instruction manual
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Posted By: Seanl
Date Posted: 21 Jun 2012 at 12:27pm
Yes I was considering a DPDT switch for that very reason and one of the reasons I put this up here was for peer review. I have two batteries now. I have used one in the past and found that it discharged the battery to much. The inverter is supposed to shut before it damages the battery but I have never tested it. My wife says she cannot use it without the heater as her nose gets too dry. The other thing I am thinking is that the Inverter is chassis grounded and so is the neutral on the AC, I think. So I might have two ground paths which may not be good. so I might need the double pole switch to force the return to use the return path through the inverter.
------------- Sean, 2011 Rpod RP-173,2009 Jeep Liberty Rocky Mountain Edition
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Posted By: techntrek
Date Posted: 21 Jun 2012 at 2:38pm
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Have you tested it with something like a Kill-A-Watt so you know what amps it draws over time? I imagine the heater cycles on and off so you'll need the average per hour.
You'll want to parallel those 2 batteries and then always use them and charge them as one.
The problem with most inverters, especially the nonprogrammable ones, is they are set to cut off at 10.5 volts. At that point you have damaged the battery. See the colorful chart 2/3 of the way down on this page: http://www.windsun.com/Batteries/Battery_FAQ.htm - http://www.windsun.com/Batteries/Battery_FAQ.htm
Most of the time you want to keep it above 50% SOC, occasionally you can go down to 30%, but that is 11.75 volts, far above 10.5.
Let me know the amp-hour draw of the unit at 120 volts and I can work with you to figure out if your 2 batteries are even enough. There are losses and inefficiencies in the inverter, plus that 30-50% SOC you don't want to go below... which is why I was saying you may need more than that. ------------- Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ http://www.rpod-owners.com/forum_posts.asp?TID=1723 - Pod instruction manual
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Posted By: Seanl
Date Posted: 21 Jun 2012 at 5:53pm
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According to the manufacturers website My wife's CPAP machine needs 46 Amp hours for 8 hours with with a 50% safety margin on an inverter. ------------- Sean, 2011 Rpod RP-173,2009 Jeep Liberty Rocky Mountain Edition
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Posted By: cane2
Date Posted: 21 Jun 2012 at 6:41pm
| Does your wife have a newer machine, if so they run on 12 volt check your manual. You can get a patch cord made at battery plus or shack. You will not be able to use the heater but you should not need it in the summer. |
Posted By: Cmtpod
Date Posted: 21 Jun 2012 at 7:12pm
| Let me just throw this out there, but I had a similar issue but found that it was way cheaper and easier to buy a unit with a built-in battery. Cpap.com was the site I've used. Got one with built-in heater and battery for like $400. You can charge it pretty quick with the generator if you are off grid, or just charge it via 12V and then run at night on battery. |
Posted By: techntrek
Date Posted: 21 Jun 2012 at 10:12pm
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Seanl, those numbers don't add up. Dividing that up would mean 5.75 amp-hr at 120 volts, or over 60 amps-hr from a 12 amp battery. That would kill a battery in less than an hour. From what I've read people can run these for 1-3 nights from one battery. That's why I asked if you did an actual measurement on it, that's the only way to know for sure.
------------- Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ http://www.rpod-owners.com/forum_posts.asp?TID=1723 - Pod instruction manual
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Posted By: Seanl
Date Posted: 22 Jun 2012 at 12:30am
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Ok re read the specs and it says "Battery size for 8 hours use (amp-hours) (includes 50% safety margin) 46 amp hours." I don't have any way to measure the current at this time but the spec sheet says the power supply uses 2.5 amps at 110 volts. So that's 275 watts. I still think 2 batteries should carry it. The only reason I have a 750 watt inverter is that was all I could find in Watkins Glen New York when my 400 watt inverter gave up the ghost a few years ago. ------------- Sean, 2011 Rpod RP-173,2009 Jeep Liberty Rocky Mountain Edition
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Posted By: techntrek
Date Posted: 22 Jun 2012 at 7:06pm
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Ok, much better. I'll assume they mean 23 amps over 8 hours, or about 2.8 amp-hr at 12 volts, 35 watts. Although now that seems a bit low if they include the heater in that figure. I wouldn't count on the power supply reading, that is just the max it can put out. Maybe it needs to be that big to handle the heater when its running, but the heater cycles on and off quickly so over 8 hours it doesn't use much (the difference between peak amps and amps used over time). That would make the 8-hour figure make a little more sense, but it still feels a bit low.
------------- Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ http://www.rpod-owners.com/forum_posts.asp?TID=1723 - Pod instruction manual
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Posted By: Seanl
Date Posted: 19 Sep 2012 at 11:08pm
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I promised to post pictures of this install and never got back to it. I mounted the inverter under the dinette seat behind the electrical panel. I got a 60 amp toggle switch and and 60 amp inline fuse. I wired it off the DC Panel and instead of installing a cutoff switch to switch between shore power and the inverter I installed a dedicated plug for the inverter. Here are the pictures of the install. The duel battery setup worked just fine for out two week vacation this summer. The inverter installed on the partition between electrical and water pump area and pass through storage.  I picked up the 12 volt power on a spare lug in the DC Panel. It is Red Wire on the left on the bottom left of the picture below.  I put in an inline fuse  and ran it to a 60 amp toggle switch mounted next to the electrical panel.  And I installed a 115 volt plug under the dinette.  ------------- Sean, 2011 Rpod RP-173,2009 Jeep Liberty Rocky Mountain Edition
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Posted By: techntrek
Date Posted: 20 Sep 2012 at 12:00pm
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Nice, clean install. ------------- Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ http://www.rpod-owners.com/forum_posts.asp?TID=1723 - Pod instruction manual
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