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Renewing the debate on the best Fridge/Freezer for 6 seater MX

Discussion in 'Model X' started by Whisky, Jun 18, 2018.

  1. Cowby

    Cowby S E X C R

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    Ha! Talk about perfect timing! I’ve followed your thread from the inception and finally bit the bullet on getting the 40W model. I was wondering the exact same thing regarding extended down time and what happens to the internal temps when not powered.

    Question, how long did the the temp stay “cool” enough before warranting a battery backup? I know that’s relative but maybe under 45-50 degrees before I’d start to worry?
     
  2. ecrsail

    ecrsail Member

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    I am glad my initial review was helpful. The X is a great road trip vehicle, and I love the flexibility of carrying my favorite breakfast and lunch supplies and fresh fruit to encourage healthier eating habits while on the road.

    I recall the Dometic temp getting above 60 F after my vehicle sat parked at a trailhead all day while I went for a hike late last spring. This was in southern California. For shorter stops on cooler days the rise was much less significant, but ambient air temperature plays a role here as well.
     
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  3. TaoJones

    TaoJones Beyond Driven

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    May be worth investigating whether the Dometic has an insulating jacket accessory.

    One of the reasons I went the route that I did was to ensure as stable a temperature range as possible - especially while parked in inhospitable environments.

    Between the jacket and the 35% window tint, all has been well, and well under 50F - unless I stuff warmer leftovers into the fridge, in which case it takes some time to chill those down.
     
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  4. scottf200

    scottf200 Active Member

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  5. ToddS

    ToddS Member

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    I am about the venture down this road.

    Instead of the extra battery for cooling while away from your car, has anyone tested just leaving it in camper mode to keep the interior cool? I know it is relative to how hot it is outside, tinting and shades, and how cool you keep your car. I was just wondering if anyone had any thoughts about this (assume the Tesla battery is not an issue with superchargers close by).

    Cheers!
     
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  6. hly

    hly Member

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    Thanks for the idea of an external 12V battery to keep the fridge running when the car is off. I wonder if instead of a lead acid type a lithium-ion 12V battery is not cleaner and lighter. Time to do some research I guess.
     
  7. aesculus

    aesculus Still Trying to Figure This All Out

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    I am thinking about the Dometic 28 with the insulated cover for my MX 6 seater. I have another application that only the 28 would work for and can swap it into the MX when we travel. Hoping the narrower profile of the 28 plus the cover would have less impact on the seats.
     
  8. ecrsail

    ecrsail Member

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    Not exactly sure what is meant by "cleaner" in this context, but the 12V 18Ah sealed lead acid battery like this one is in fact sealed and non-spillable. This type of battery is commonly available and used in wheelchairs, uninterruptible power supplies, etc, so it is relatively inexpensive as well.

    However, since you asked, you might be interested in this 10Ah LiFePO4 battery which runs about 3-4X the cost of an 18 Ah lead acid battery. It has less capacity, but is lighter and supports more charge cycles. However, for this application I (somewhat begrudgingly) concluded the traditional sealed lead-acid was the more appropriate choice. If you do try this battery out, please post an update!
     
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  9. hly

    hly Member

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    Thanks! In your use case how much electricity on average does the Dometic consume per 24h?
     
  10. scottf200

    scottf200 Active Member

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    @hly , You can see SSonnetag calculate 200 Wh.

    I just put my new Dometic 35 in my 75F X. It hit 40F in an hour of on/off testing as I went in and out of the vehicle trying to see how long the 12V outlet in the back would stay on (a couple minutes). It seems crazy efficient as others have mentioned above. Obviously empty but if you put cold food or drinks in there it would be similar. Seems it maintains the temp well.

    Specs show 84 Watts when running (cooling I presume) on 12V.

    [​IMG]
     
  11. hly

    hly Member

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    Thanks. So by SSonnetag calculation the dometic uses about 200Wh per 12h or 400Wh per day. At 12V that's 33Ah. The 18Ah lead-acid battery would last about 13h and the 10Ah Li-Ion battery about 7h.
     
    • Disagree x 1
  12. ecrsail

    ecrsail Member

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    Actually, that calculation assumes continuous compressor operation which does not occur in practice. There is a duty cycle to consider as in normal operation (after the unit has reached the set temp) the compressor runs intermittently.

    Fortunately, Dometic has published average energy consumption over time and rates the CFX-35W at 62 kWh/annum and the CFX-40W at 64 kWh/annum (source: Dometic website). Also consider the fact that you would like to avoid depleting your battery in normal usage. Fortunately if you have battery protection feature turned on, the Dometic will protect your battery and shut itself off before the battery voltage becomes an issue -- but this means we have a bit less than the rated battery capacity actually available.

    Anyway, so these were some of key factors as to why I chose an 18Ah battery -- I wanted to be sure I could leave my X parked for at least 24 hours without worrying about CFX-40W keeping things refrigerated. Another reason was that I simply did not like seeing the unit power off and then back on whenever I left the vehicle for more than a few minutes.

    It's been working great, and I'm really happy with my system.
     
    • Informative x 1
  13. hly

    hly Member

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    Thanks for the great infos!
    I thought SSonnetag calculation already included the duty cycle. If running continuously I am sure it is more than 400Wh per day so maybe his calculation was done in a hotter weather?
    Anyway based on Dometic info it is about 170Wh per day or about 14 to 15 Ah, you are right that a 18Ah lead-acid battery is better for a full day operation. I found a 20Ah Li-ion battery but it is too expensive at $280.
     
  14. Saghost

    Saghost Well-Known Member

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    Conventional wisdom is you shouldn't plan on more than 50% DoD for lead acid if you want longevity; I'm not sure how often the car would wake-up and charge in those 24 hours but if it's all one period that'd be too much for the 18Ah pack as a regular duty cycle.
     
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  15. ecrsail

    ecrsail Member

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    Yes, and the Dometic's battery protection feature is anticipating a lead acid battery (the typical configuration found in all of those ICE RVs).
     
  16. Legolad

    Legolad Member

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    #56 Legolad, Jun 27, 2018
    Last edited: Jun 27, 2018
    Yep. There is a world of difference. The Dometic is a fine cooler and gets far colder than the Peltier, but I think its energy use in DC is actually higher than the Koolatron (7 amps for Dometic and 4 for Koolatron). The .7Ah is for 120V usage. I am electrically ignorant so I might be misunderstanding all these numbers, but it looks like it is similar to the Koolatron on DC but better on 120V.

    Regardless, It's just a matter of what you prefer and how you use it.

    Before we even bought our Model X we bought a Koolatron Peltier unit for road trips. It's cheap, sturdy, and we only use it to keep road trip snacks cool. It's large enough to hold juices and snacks and sandwich fixings. It works on AC and DC. It can even be set to Cool or Warm.

    As for the temps, Peltier units are always relative to ambient temperatures. The Koolatron will always be ~30 degrees different on the inside. So, if you keep your car at 70 degrees, the Koolatron will be 40 degrees or 100 degrees.

    40 degrees is the highest temp you'd want in your fridge at home and it's good for keeping fresh foods from spoiling too quickly. Milk and juices and sandwich meats won't spoil quickly at 40 degrees. Sodas and beer will be refreshing, but not ice-cold. Once we get to our destination, we use the Koolatron in our hotel room to keep our supplies fresh for the trip home. For us, that's all that matters on a road trip.

    As for the cost of the power, we use our car power (free with the Model X) and our hotel power (also "free"). About once or twice a year we use it at home for a few hours for a party. Not even worth looking up the cost for that.

    The Dometic is a superior unit in terms of durability and temps, but at $732, I'd call it overkill for our usage.
     
  17. Saghost

    Saghost Well-Known Member

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    Nope.

    You're confusing the maximum draw with the average draw or draw needed to achieve and hold a temperature, and somehow assuming that thermoelectric coolers will always be at the best temperature they can achieve.

    The thing is, Peltier devices are very, very inefficient. They have the advantage of being solid state, which makes them both quite reliable and fairly cheap to make, but they eat a whole lot of energy for every bit they move - usually ten times as much or a little more.

    Compressor based heat pumps are expensive to make, require careful seals for the working fluid and moving parts, but they also routinely move three times as much energy as they consume.

    There's no way that the Peltier device can come close to the same performance for energy consumption when it has to spend 30x as much energy to move heat out of the box.
     
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  18. ecrsail

    ecrsail Member

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    For those who are interested, Wikipedia has a decent description of thermoelectric cooling (Peltier effect) and discusses the relative efficiencies with compressor based refrigeration (Carnot cycle).

    Thermoelectric cooling - Wikipedia
     
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  19. Saghost

    Saghost Well-Known Member

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    Interesting. It looks like they've made some progress - the 94% energy loss I had in my head from articles a few years back seems to be no longer the case - but certainly a Peltier is no substitute for a proper heat pump (unless you're in an environment where you can't easily use a traditional heat pump, of course.)
     
  20. Legolad

    Legolad Member

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    ok
     

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