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Total number of 225 Amp/hr batteries need to...

Discussion in 'Off Grid Living' started by Blackjack1, Jun 10, 2015.

  1. Blackjack1

    Blackjack1 Neophyte Monkey

    Ok, so I've done the math and need a confirmation on my battery string.

    Here is the project: Running a small window A/C unit 6 hours in the evening.

    Window AC unit specs: Draws 6 amps at 120VAC = 720 Watts
    720 Watts * 6 hours = 4320 watt/hrs required

    Battery string: 24 volt system - 2 strings of 4 - 6 volts 225 Amp/hr deep cycle batteries (in series and in parallel) - gives me 24 volts and 450 amps. (in series doubles the volts, in parallel doubles the amps)

    Drawing batteries to only 50%: 24 volts * (450*.5) amps = 5400 watt/hrs

    Will this allow me to run this small A/C unit for 6 hours?
    Thanks in advance for your thoughts,
  2. ghrit

    ghrit Ambulatory anachronism Administrator Founding Member

    Looks OK without checking the arithmetic, but there is nothing left over for sunless days or other loads. Bear in mind that the unit might be cycling on lower demand, and the startup current draw (for really cheap single phase motors, it could be as much as 10X of running amps) might give you fits without some extra capacity. Also, you haven't accounted for losses in conversion from 24 volt DC to 120VAC.
    Last edited: Jun 10, 2015
  3. Blackjack1

    Blackjack1 Neophyte Monkey


    Thanks for the reply - the math is what I most need verification of...
    If the math is correct, wouldn't there be 1080 watts (5400 - 4320) extra for conversion from 24v DC to 120VAC?

    Granted a 3rd string of 4 - 6 volt 225 Amp/hr batteries would be better... but I'm most concerned with having the basic calculations correct for this specific scenario.

    Please let me know,
  4. BTPost

    BTPost Old Fart Snow Monkey Moderator

    That is the Battery Bank I use every Day.... 3 Strings of 4 ea. L16HDs feeding a Trace 4024....
  5. Blackjack1

    Blackjack1 Neophyte Monkey

    How many and what watt panels do you have keeping this string charged?
  6. BTPost

    BTPost Old Fart Snow Monkey Moderator

    I do NOT do Solar, as I live in the Rain Forest of Southeastern Alaska, and Solar is useless for over 6 months a year. My System is charged off the Twin 20Kw Diesel Gensets, in my Winter Powerhouse. My Energy costs are not mine, but part of the Cost of Doing Business, as I am the local Fuel Distributer, and I operate the Gensets, for 10 Hours a day. (7Am to noon, & 5Pm to 10Pm) That allows my Multiple Battery Banks to recharge, and keeps all my Freezers cold.
    Gopherman likes this.
  7. Gopherman

    Gopherman Sometimes I Wish I Could Go Back to Sleep Site Supporter++

    Take a look at the New Batteries Tesla is selling, 7k or 10k $3,500. I have 7 x 250 watt panels. I also am going to run separate electrical, autonomous of the panels.
    Problem Solved!
    I am going to get 1 of the 10k Units for my house this summer for a start. They also have a Huge one capable of running the entire house available, It's expensive but they are going to finance for $15 a month.
    Tesla to announce home and utility-scale batteries that offer energy independence | Computerworld
    Tesla Powerwall
    Last edited: Jun 10, 2015
  8. Airtime

    Airtime Monkey+++ Site Supporter

    So, a hellava salesman sold an honorary Eskimo in Alaska not just one, but it would seem multiple freezers! Wow.
    kellory and ghrit like this.
  9. Tully Mars

    Tully Mars Metal weldin' monkey Site Supporter+

  10. techsar

    techsar Monkey++

    Not sure if I would call that "problem solved." finding inverters for 350 - 400 vdc to household AC isn't going to be cheap...but I'm sure Tesla will sell those as well ;)
    Gopherman likes this.
  11. BTPost

    BTPost Old Fart Snow Monkey Moderator

    Well it is +56F right NOW, so how else would you expect to keep things Frozen..... and in October after the last Barge leaves, all my Winter Meat and about half my other Grub is Frozen, and it is still +40F most of the time... Alaska is a VERY BIG Place.... and the Wx is very different between Regions.... No Igloos around here for 1000 Miles.... or maybe even 1500 Miles.... and nobody lives in Igloos any more, anyway....

    All it takes is a VERY BIG Transformer, and a Stack of Hercky NMos FETs, on a Hugh Heatsink, and that shouldn't be much of an issue... Heck I bet OutBack Power Systems could whip one up, if there was a market for them.... Switching Dc/DC Power Conversion is a mature Technology, these days....
  12. Gopherman

    Gopherman Sometimes I Wish I Could Go Back to Sleep Site Supporter++

    One article I read had the entire package for a pretty affordable price broken down into $15 a month payments. Thats doable for just about anybody. Vheck around their site I'm sure it's there.
  13. BTPost

    BTPost Old Fart Snow Monkey Moderator

    I was a really FUBAR Serial thing, that was around BEFORE Ethernet... Mostly it was like a Remote Control Panel substitute that they never quite got ready for PrimeTime, before they Left.... One of the first things they did when they were not constrained by the FUBAR New Owners was design the Mate System, and Protocol, to incorporate into all the New Products they had in the Pipeline. Very successfully I might add.... and that was very early on before Ethernet really got out of 10Base2 Cabling and Hardware...
  14. BTPost

    BTPost Old Fart Snow Monkey Moderator

    The Mate Protocol runs on Cat5 Wire and 10BaseT/ RJ45 connectors but it isn't Ethernet, just Looks like it.... Not sure what the difference is, Hardware wise, but the Protocol is an InHouse Design...
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