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Off-grid gurus: How do I get 2.5kW & 5kWh from batteries into my RV?

Discussion in 'Tesla Energy' started by deonb, Jul 1, 2016.

  1. deonb

    deonb Active Member

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    Relatively simple scenario:

    I would like to be able to run the A/C in my RV for about 2 hours when I'm at a rest stop, and then recharge it overnight at my destination. The A/C can run on a TT30 (110V / 30 amp), which works out to about 2.64 kW at 80%.


    So for 2 hours that draws about 5Kwh. According to GM I should be able to make that work for oh, $725, right? Right??

    Apparently not... Instead, it looks to be about a $10'000 setup.

    For starters, here's a 5kWh battery for $7500:
    MasterVolt 66025000 MLI Ultra 24/5000

    And then, you can't actually use a 3kW inverter, because that seems to give you 1.5kW per leg, so instead you need a 6kW inverter. Which is another couple of grand.


    Is this really that hard of a project? I though it's something I can throw together for a couple of thousand, but apparently not. I'm even fine with AGM instead of Lithium, but it seems you run into issues when you draw down AGM that fast


    Aside from literally just ordering a Powerwall, and then unmounting it and throwing it in the back of my truck, is there an easier option?


    PS: I realize I can run a generator. I would prefer not to.
     
  2. BrokerDon

    BrokerDon Member

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    • Informative x 1
  3. deonb

    deonb Active Member

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  4. PV-EV

    PV-EV Member

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    By the time you add in the inverter and gear the Powerwall will be pretty expensive too. Xantrex makes a modified sine wave inverter DR3624 that is 24VDC in and 120volt out, rated 3600 watts continuous. I don't think your AC will care about the modified sine wave but other electronics in your RV might.
     
  5. miimura

    miimura Active Member

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    With a quick look at Wholesale Solar I found these batteries and inverter.

    4ea Crown 390Ah 6V AGM $475ea $1900
    1ea Magnum MSH4024M 120VAC 24VDC 4,000W Mobile Hybrid Inverter $2,240.

    That's 9.36kWh total and 4kW output at 120VAC for just over $4,000. The inverter has AC input and will act as a battery charger as well. Of course you need some thick wire and terminals to hook up the batteries, but it is a single serial string, which makes things simpler. The batteries will weigh just over 500 lbs so they will also require some beefy mounting for an RV application. You could go smaller on the batteries, but it's better, especially with AGM, to restrict the depth of discharge.
     
  6. deonb

    deonb Active Member

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    That's interesting... Is a 6V AGM battery going to be able to handle a 110A continues load?
     
  7. PV-EV

    PV-EV Member

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    The spec sheet says 255 minutes at 75amps. It might be worth your time and money to put an amp meter on your planned load and see exactly how much It draws. Going by the size of the trailer plug is not going to be very accurate.
     
  8. nwdiver

    nwdiver Active Member

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    If you don't plan on using the batteries more than a couple times a month, if you have enough space and don't mind flooded batteries you can get 8x 6v golf cart batteries from Costco for <$1k and get a 48v Magnum inverter. That's what I use for back up power for my home.
     
  9. deonb

    deonb Active Member

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    Now we're talking!

    That Magnum inverter is a bit high-end. Do you think I can get away with something like this?
    6000 Watt Power Inverter Charger - 48 Volt to 120 Volt

    I can get that one for close to $1000
     
  10. Saghost

    Saghost Active Member

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    I don't know if it's the best answer, but since we're talking a bunch of money already and no one's mentioned it: Does it make sense to consider changing the A/C to a 12V or 24V unit instead? That'd eliminate the need for a big inverter with soft starting...
     
  11. deonb

    deonb Active Member

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    That's actually a great suggestion, but I have a few other small devices that I may occasionally want to run or charge e.g. laptop, camera, flashlight, GPS trackers, signal boosters, 2-way radio etc. They use barely any power, but they all have proprietary 110V jacks & bricks which would have to be changed. And any visitor you have will come with more of those.

    I would love to just run a 12V or 24V system throughout - even in my house, but lack of standardization have always hampered progress in that area. At least we have standardized 5V now.
     
  12. Saghost

    Saghost Active Member

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    I wasn't thinking you could do away with 120V A entirely - but all those ancillary bits you're talking about can easily be handled by a ~$200 ~1000W pure sine inverter, rather than the expensive high power stuff you need for the A/C. Just a thought. :)
     
  13. miimura

    miimura Active Member

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    That one has a transfer switch for the AC output while the Magnum I linked is a hybrid which means that it will charge the batteries when the AC input is connected and the output will be uninterrupted. I don't know if uninterrupted power is important to you.
     
  14. miimura

    miimura Active Member

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    If you're worried about the amperage, you can buy 8 pieces of the 220Ah 6V AGM batteries and build a 2P4S pack that will be rated for 150A and have 10.56kWh capacity at 20hr rating or 9.12kWh at 5hr rating. The cost is $173/kWh vs. $200/kWh but you will need more cable and connectors.
     

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