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  1. #11
    Long Hauler howson's Avatar
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    Quote Originally Posted by Cate&Rob View Post
    Hi Howard,

    I went through a similar “dilemma” several years ago, when trying to figure out how to power the 120 VAC (only) fridge on our boat. How many batteries? How big an inverter? What was the actual duty cycle? How far was I drawing the batteries down? How much battery charging was I getting from different sources?

    The key to all this is investing in a quality amp-hour meter. I went with Victron, many years ago. A previous version of the current BMV700 https://www.victronenergy.com/battery-monitors/bmv-700. Not “inexpensive” . . . but money that I have never regretted spending. This meter will let you see exactly what is going on and you can adjust accordingly.

    Paul (Poppy’s 5th Wheel) installed one of these . . . I wonder where he got that idea . . . and I think he will agree that this was money well spent.

    Rob
    I'm sure that's a quality piece of equipment, but I don't see how it will help since I don't have the batteries (or inverter) to monitor. ??? A similar device that tracks AC consumption would work, though, to determine my actual requirement.

    Let me expound on that last statement. If I could measure and track internal camper temperature along with the AC electrical draw by the refrigerator over time (given the refrigerator stays <=40 degrees), I'd have a known basis for the electrical load that could translate into a battery and inverter requirement. Since my camper is in the back yard with power connected, I could easily accomplish this testing by leaving the air conditioners off and closing up the camper to simulate the towing environment. There'd always be a cold reserve of adult beverages handy, too.

    Doh! I just remembered I have a Kill-A-Watt. It can handle up to 15A, so it should help me determine my battery (and inverter) requirement for the refrigerator. Time to crank up a spreadsheet...

    Another thought just entered my cranium. I have upfitter switches and the associated wiring built into my truck. The wiring for switches 5 & 6 are fused to 40 amps each (12V obviously). Assuming I can parallel their outputs, isn't that 80 amps of 12V DC or 960 watts? I'm probably doing something wrong, but can an inverter ideally make 120V/8A from that 960 watts? That should be more than enough to power the refrigerator (I think) even with the inevitable loss during inversion.
    2017 Ford F-350 DRW 6.7L Platinum
    2019 315RLTS (purchased 16 Jul 18 from Campers Inn RV in Byron, GA)

  2. #12
    Long Hauler DaveMatthewsBand's Avatar
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    My ears are burning.

    [emoji3590]


    Sent from my iPhone using Tapatalk
    Resistance is Not Futile, It's Voltage Divided by Current.


  3. #13
    Long Hauler howson's Avatar
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    Quote Originally Posted by DaveMatthewsBand View Post
    My ears are burning.
    LOL. They shouldn't be! You need all your equipment to power your 200 watt amplifier when you crank up "Kit Kat Jam".

    To recap, I want to power my Dometic 12 cu ft refrigerator with AC via an inverter while the trailer is being towed. My assumption is the truck can provide the power required. Here's what I know (in no special order):

    - The Dometic draws just over 404 watts (3.39 amps @ 120V). This was measured by a Kill-A-Watt. The EMS showed 3A on one leg and 1A on the other, so the Kill-A-Watt value looks accurate.

    - The refrigerator is protected by a 120V/5A and an 12V/8A fuse. (I'm not sure why a refrigerator that runs off of AC and propane needs an 8A fuse on a 12V circuit?) That equals 600 watts of AC (120x5) and 96 watts (12x8) of DC. For planning purposes as long as the inverter setup can meet these thresholds it will work.

    - This thread about a 12V requirement while on propane started by @CaliforniaJeff is interesting.

    - My truck is equipped with an extra heavy duty alternator capable of generating 220 amps @ 12V. (I visually verified by reading the part number off of the alternator and looking it up online.) Since it is a diesel it has two batteries, too.

    - The truck is equipped with upfitter switches. Two of them are fused at 40 amps.

    - I need (require) a pure-sine inverter.

    Still in Research:
    - I'm unsure if I can parallel the two 40A upfitters together to provide 80 amps to an inverter. (All I'd require is 50 amps, since the maximum expected by the refrigerator is 5A of 120V, or 600 watts.)

    - Does the refrigerator also use 12V when plugged into AC?

    - How many amps, if any, does the truck supply through the 7-pin connector to the battery? Is it enough to power the 12V aspects of the refrigerator (if required)?

    - Inverters are best installed close to their 12V source, protected from weather, and must be ventilated. These requirements seem to conflict since the 12V source will be the truck's battery and alternator (via the upfitter switch if possible) yet the best protected area is the front storage on the trailer. More thought is needed on this...

    Comments, suggestions and criticisms welcome. (Exception: No propane comments, please!) I'm in no hurry on this project--I'll keep at it until I work through all the possibilities.
    2017 Ford F-350 DRW 6.7L Platinum
    2019 315RLTS (purchased 16 Jul 18 from Campers Inn RV in Byron, GA)

  4. #14
    King Pin
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    Howard- It sounds like you're on the right track. Of course, the duty cycle of the refrigerator comes into play. When the reefer is not running, the 12VDC from the truck will be recharging the batteries. And If the refrigerator is running, the batteries will provide the needed current to the inverter.

    One other thing you could consider is adding a few solar panels on the roof to provide even more power to the battery bank. I run my three 100 watt panels in series through an MPPT controller. On a sunny day, I am usually producing about 15-20 amps of power into the battery bank depending on the angle of the sun.

    Jim

  5. #15
    Rolling Along carnolddsm's Avatar
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    To answer the question does the refrigerator use 12V when plugged into a 120VAC power source: yes

    For both Dometic and Norcold 12VDC runs the electronics and controls. The 120VAC runs the heating element.

    BTW - same principle applies to the water heater, controls are 12VDC, only the heating element is 120.
    Colan and Marilyn Arnold
    Des Moines, IA - kind of, on the road full time.
    Currently in Durango, Colorado
    Momentum 350M originally, now a 397TH

  6. #16
    Long Hauler howson's Avatar
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    Quote Originally Posted by carnolddsm View Post
    To answer the question does the refrigerator use 12V when plugged into a 120VAC power source: yes

    For both Dometic and Norcold 12VDC runs the electronics and controls. The 120VAC runs the heating element.

    BTW - same principle applies to the water heater, controls are 12VDC, only the heating element is 120.
    Thanks, Colan. I'll factor in 3A of 12VDC, too. (The maximum, based on the fusing, the circuit expects to draw.)
    Last edited by howson; 12-24-2018 at 06:22 AM. Reason: Corrected fuse rating in post.
    2017 Ford F-350 DRW 6.7L Platinum
    2019 315RLTS (purchased 16 Jul 18 from Campers Inn RV in Byron, GA)

  7. #17
    Seasoned Camper Bob/OlallaWA's Avatar
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    Darn, I won't ask why no propane, but will ask if you have access to large cooler and dry Ice. You make my head hurt, and our clouds and trees make solar a dream, not something really happening....
    2019 GMC 3500 HD:)
    2016 Reflection 313RLTS:cool:

  8. #18
    Site Sponsor Cate&Rob's Avatar
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    Hi Howard,

    It would seem that you have far more truck alternator capacity than you have wire gauge capacity to the trailer batteries. The trailer battery charging circuit is usually sized and fused for 40 amps. If you could upsize this wiring and fusing to supply 80 amps to the trailer batteries when towing, you would be able to run the fridge and keep the trailer batteries fully charged when towing.

    Rob
    Cate & Rob
    2015 Reflection 303RLS

  9. #19
    Seasoned Camper ncitro's Avatar
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    Howard I hope I can lend some real world numbers for you. I apologize in advance for what turned out to be a pretty long post. I have a similar truck, and am similarly looking to increase the charging from the truck to take advantage of the plentiful power there. Unfortunately I have my rig wired so that the fridge cannot run off of my inverter so I cannot give you draw numbers from that, I run mine on propane. However hopefully the following charging information will be useful to you:

    I have the battery monitor mentioned above, and can tell you from watching it while towing that once the truck ramps at speed I get a whomping 10 amps of charging from the truck. That's not to say I see +10 amps on the monitor, depending what I have on while towing I usually still see a loss, just 10 amps less of a loss than if I did not have the truck connected. I frequently run with my crock pot plugged in while towing so dinner will be ready when we arrive on travel days, so with my four golf cart batteries and the little charging I get from the truck I have enough capacity for this task. I will say on a recent trip to Florida we spent a night in a cracker barrel parking lot and intentionally did not conserve much power. We got the batteries down to about 60% remaining, and after a full day of towing (probably 6-7 hours) we were only up to about 75% charge.

    I too have considered either tapping into the 40 amp upfitter feeds, or even running a new set of larger wires straight to the truck battery (through a solenoid of some kind of course) to take better advantage of the plentiful supply of alternator power there. I have read some posts over the years that the problem with the low amount of charging from the truck is a combination of small wire size in the truck as well as the variable output of todays alternators. The first issue is somewhat easily solved with a new larger pair of wires from the truck, but the second has me somewhat stumped. The posts I read claimed that modern alternators are controlled by the computer in the truck, and they vary the output of the alternator based on the voltage of the trucks batteries. Therefore when the trailer batteries are depleted but the truck batteries are full the alternator is throttled back and not outputting enough voltage to charge the batteries at any appreciable rate.

    What I want to do to test this (but have not had enough time) is to discharge my batteries and then connect nice big 2AWG jumper cables from the truck to the trailer. If I see a appreciable amount of charge going into the batteries I know that the bit about the alternator is hogwash. If I still see a small amount of charge I know I am looking at a more complicated solution - a DC to DC charger. My understanding is this device will take the lower voltage supplied by the truck and step it up to a higher voltage to get the batteries into bulk charging (by drawing more current, which I should have plenty of available). Unfortunately they are fairly expensive, and since I am considering solar anyway I stop myself figuring I might as well just put that money towards panels and let them do the charging. If however you are interested in the DC to DC route this is the unit I had recommended to me:

    https://www.amazon.com/CTEK-40-186-D...+to+dc+charger

    Sorry again for the length, but hopefully at least some of this is helpful to you.
    2018 Reflection 28BH
    2019 F350 Platinum 6.7L LB 4x4

  10. #20
    Long Hauler DaveMatthewsBand's Avatar
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    Hey keep in mind if your frig pulls say 3.1 amps on alternating current it is going to pull 31 amps running off an inverter.
    (AC Amps x 10 = DC Amps) Our frig pulls 30 amps while we’re traveling (with the LP off) but the 1155 watts of solar pulls in 75 amps so it’s not a problem replenishing what the frig is using. I can’t tell you how long it would take to run our batteries down because that would vary greatly depending on ambient temp. It’s about 40F outside right now so the fridge is barely running so it wouldn’t use much power at all overnight then tomorrow the solar will charge them back up. You could probably do the same with at least 800 watts of solar and a minimum 200ah of batteries? Just my guess.


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    Resistance is Not Futile, It's Voltage Divided by Current.


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