How many KW ?

Discussion in 'Tech/Mech General' started by Robert Clark, Jan 16, 2016.

  1. Robert Clark

    Robert Clark Funster Life Member

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    A question for the techies please

    At 220v output (via a inverter) how many KW are there in 4 x 80ah gel batteries?

    Next - how many KW per day can 2 x 100w solar panels put back into these 4 batteries (assuming MMPT controller)

    Thanks !

    Robert
     
  2. Boringfrog

    Boringfrog Funster

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    Summer or Winter? UK or abroad?
     
  3. Boringfrog

    Boringfrog Funster

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  4. Robert Clark

    Robert Clark Funster Life Member

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    Summer in Europe - thank you
     
  5. John Laidler

    John Laidler Funster

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    :) I'm thinking...

    12 * 4 * 80 = 3,840W for one hour. Assuming the inverter is 100% efficient then you will get 3.84KW for one hour - except batteries draining at that rate won't give anything like 80 Ah, more likely half I think. So you can probably run a 2KW appliance for about an hour, probably less I fear.

    The solar panel question is harder as it depends how much power they might actually produce, because it won't be 100W per panel. For argument let's say they only average half that over the day. So 100W for 10 hours is the same as 1KW for one hour. But if your load was 2KW then they are not going to replenish the batteries, even assuming charging is 100% efficient, which it isn't.

    Of course you might get more out of your panels, 150W total over 12 hours gets pretty close to replenishing the 2KW drain over an hour but will they produce 75% over 12 hours? Sounds optimistic.
     
    Last edited: Jan 16, 2016
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  6. Robert Clark

    Robert Clark Funster Life Member

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    Sorry - it's just that I know how many watts each appliance consumes, so thought I could more easily see how long the batteries might last
     
  7. Boringfrog

    Boringfrog Funster

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    My Brain Hurts.........:)
     
  8. hilldweller

    hilldweller Funster Life Member

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    kw is kw no matter what voltage. S0 12V 200A is 2.4kw. 240v 10A is also 2.4kw.

    Now if you use 1kw for 1 hour you have used 1kwh - now see the link to batteries.

    You have 4 x 80ah but can use only half so 160Ah available.
    Since these are 12V you have 12 * 160 = 1920kwh.

    Easy ? No ! Because if you use loads of amps the batteries will only deliver 50% or less of that 1920kwh after which they need to recover to get a bit more out.
     
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  9. tonka

    tonka Funster Life Member

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    Do it another way, and very rough... Full batteries 4x 8oamp = 320 amps. BUT you never want to go down to 50% or you start ruining the batteries so you may have 320÷2 = 160 amps .
    160 amps x 12v =1920 watts (1.9kw).

    Remember if you draw through an inverter there will be more loss so add on 10-20% to the appliance figures..

    As for how much the solar put in, harder to work out as depends on how good the panels are, the regulator and what the weather is doing....

    (Backing up Mr Hilldwellers maths)..
     
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  10. John Laidler

    John Laidler Funster

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    Ignore what I wrote initially, I've had another look at it, see above - #5. :)
     
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  11. Robert Clark

    Robert Clark Funster Life Member

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    Thank you guys - that makes sense
    I expected the batteries to last longer than they are doing, but I guess we're using too much juice
     
  12. John Laidler

    John Laidler Funster

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    But it is a useful example of what folk can expect from an inverter if they want to run high wattage appliances - for which the answer is not for very long!
     
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  13. Lenny HB

    Lenny HB Funster

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    You have G80 Gel batteries which you can safely discharge to 80% DOD giving you 256 A/H of useable power. Allowing for the inverter being about 85% efficient it would give you 2.6 kw.

    Too many variables on the solar, on a good day in southern Europe you could get as much as 1 kw back into the batteries, however a small amount of cloud could reduce that figure drastically.
     
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  14. maxi77

    maxi77 Funster

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    Certainly on their website I could find no claim from Exide that 80% was a safe discharge level, even if they did I am pretyy sure regular discharging to that level would dramatically reduce the number of cycles the battery could achieve from the 1000 they seem to claim. I would stick to the 50% cut off and if more capacity is needed look at fitting more battery
     
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  15. Lenny HB

    Lenny HB Funster

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    From Exide leaflet on Gel batteries

    upload_2016-1-16_23-36-19.png

    More Exide info here you can see a Gel battery gives 500 cycles @ 80% DOD compared to a Wet Cell giving only 50 cycles & 250 @50% DOD.
    [​IMG]
     
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  16. peterc10

    peterc10 Funster

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    Robert, just go out and have another meal.
     
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  17. Robert Clark

    Robert Clark Funster Life Member

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    Thanks for that Lenny
     
  18. maxi77

    maxi77 Funster

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    But equally that shows that restricting discharge to 50% will give double that life, your regular discharge to 80% is not free. Another thought is that the voltage of the battery under load when discharged below 50% may activate the low voltage trip on the invertor
     
  19. funflair

    funflair Funster Life Member

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    But if you get 1000 cycles at 40% discharge and 500 cycles at 80% discharge what is the difference as essentially you had the same power, agree about the low voltage trip on the inverter.

    Martin
     
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  20. Lenny HB

    Lenny HB Funster

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    I was pointing out that taking a Gel to 80% DOD is still going to give a life cycle of many years, in practice you are hopefully not going to discharge to 80% every time particularly when you have 200 watts of solar to keep the battery topped up.
    When you look at the chart which compares good quality Exide batteries, at 80% DOD the Gel has a life cycle of 10 times that of a wet cell and at 50% DOD the cycle is still nearly 3 times that of a wet cell.
    In normally use discharging to 80% DOD occasionally is not going to have much effect on the life of a Gel.

    If nothing else I've convinced myself that spending twice as much on a pair of Gel's to replace the crap AGM Banners was well worth it.:)
     
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