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Thread: How to calculate the capacity of storage battery?

  1. #1
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    Question How to calculate the capacity of storage battery?

    In stand alone solar system, how do you calculate the capacity of storage battery? Is that any formula?

    Please make an example. Thanks so much for any answers.

    Helen

  2. #2
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    Default

    The battery manufacturer makes that calculation when they design the battery, and then they put a label on the battery with the specs.

    example 12V 110Ah @ 20 hour rate


    In reality, you can only use half the power stored, otherwise, you severely shorten the life of the battery. There is a good battery FAQ here:
    http://www.windsun.com/Batteries/Battery_FAQ.htm That will answer a lot of questions I hope.

    Mike
    Since the dawn of time it has been mankind's dream to blot out the sun.
    Montgomery Burns

    "Always listen to experts. They'll tell you what can't be done and why. Then do it."

    spreadsheet based voltage drop calculator:
    http://www.solar-guppy.com/download/...calculator.zip
    http://www.solarpaneltalk.com/showth...oss-calculator

    http://www.mike-burgess.org/PVinfo_2.html

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,
    battery lugs http://tinyurl.com/LMR-BigLug
    Setting up batteries http://tinyurl.com/LMR-NiFe

    gear :
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||

    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||

    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

  3. #3
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    Default

    hi, Mick90250,

    Thanks for your answers, but I think you did not catch what I said. You know if I want to make an off-grid system, I need to figure out which capacity of battery I should use.

    Ok, let me make an example,
    Load :2* 11W lamp; 10 hours working time per day.
    system voltage:12V
    Rainy days: 3 days

    Then how will you calculate the capacity of the battery?

  4. #4
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    Quote Originally Posted by solarf.helen View Post
    hi, Mick90250,

    I want to make an off-grid system, I need to figure out which capacity of battery I should use.

    Ok, let me make an example,
    Load :2* 11W lamp; 10 hours working time per day.
    system voltage:12V
    Rainy days: 3 days

    Then how will you calculate the capacity of the battery?
    Much better question. I knew you were not going to manufacture a battery.

    Batteries have a shorter life, the deeper you discharge them. example:
    1000 cycles 10% discharge
    800 cycles 20% discharge
    300 cycles 50% discharge
    80 cycles 90% discharge

    Generally, discharge to no less than 50%, and size 1 day to about 10%

    You are asking for 3 days. I 'll assume 12V CFL, so no inverter losses

    Your load is 22W for 10 hrs = 220W hour x 3 days = 660w hour consumed in 3 days

    So a battery bank of 1320wh would be the minimum size.

    Working all in watts, allows conversion back to any voltage. Volt x Amp = watts

    12V = 110A
    24V = 55A

    So a standard group 24 size battery would be just under the spec, but a group 27 would
    http://www.mysears.com/DieHard-Marin...ze-27M-reviews
    just make it. at 105A.

    Then you have to recharge it with something.
    Since the dawn of time it has been mankind's dream to blot out the sun.
    Montgomery Burns

    "Always listen to experts. They'll tell you what can't be done and why. Then do it."

    spreadsheet based voltage drop calculator:
    http://www.solar-guppy.com/download/...calculator.zip
    http://www.solarpaneltalk.com/showth...oss-calculator

    http://www.mike-burgess.org/PVinfo_2.html

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,
    battery lugs http://tinyurl.com/LMR-BigLug
    Setting up batteries http://tinyurl.com/LMR-NiFe

    gear :
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||

    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||

    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

  5. #5
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    Default Cycles vs Life

    Quote Originally Posted by Mike90250 View Post
    Batteries have a shorter life, the deeper you discharge them. example:
    1000 cycles 10% discharge
    800 cycles 20% discharge
    300 cycles 50% discharge
    80 cycles 90% discharge
    Generally, discharge to no less than 50%, and size 1 day to about 10%
    .....
    Then you have to recharge it with something.
    Last edited by MarineLiner; 07-17-2009 at 11:35 PM.

  6. #6
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    Quote Originally Posted by MarineLiner View Post
    so it differs with "cycle" in the AC (Alternative Current, Hertz/cycle).
    and Depth of Discharge (DOD) is what i just know.
    Right, battery cycles, are:
    a) Discharge to some value
    b) Recharge back to full
    = one cycle

    Not the same thing as AC powerline frequency (cycles)
    Since the dawn of time it has been mankind's dream to blot out the sun.
    Montgomery Burns

    "Always listen to experts. They'll tell you what can't be done and why. Then do it."

    spreadsheet based voltage drop calculator:
    http://www.solar-guppy.com/download/...calculator.zip
    http://www.solarpaneltalk.com/showth...oss-calculator

    http://www.mike-burgess.org/PVinfo_2.html

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,
    battery lugs http://tinyurl.com/LMR-BigLug
    Setting up batteries http://tinyurl.com/LMR-NiFe

    gear :
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||

    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||

    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

  7. #7
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    Default

    Quote Originally Posted by Mike90250 View Post
    Right, battery cycles, are:
    a) Discharge to some value
    b) Recharge back to full
    = one cycle

    Not the same thing as AC powerline frequency (cycles)
    So with that said that means that the average battery coming in with a 50% discharge would last less than a year?

  8. #8
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    Default

    It's a giodeline to start. Same batteries, same site, but more batteries, sized to only 20% dishcarge, should last over 5 years. (Deep cycle batteries, not Kmart deep/starter batteries)
    Since the dawn of time it has been mankind's dream to blot out the sun.
    Montgomery Burns

    "Always listen to experts. They'll tell you what can't be done and why. Then do it."

    spreadsheet based voltage drop calculator:
    http://www.solar-guppy.com/download/...calculator.zip
    http://www.solarpaneltalk.com/showth...oss-calculator

    http://www.mike-burgess.org/PVinfo_2.html

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,
    battery lugs http://tinyurl.com/LMR-BigLug
    Setting up batteries http://tinyurl.com/LMR-NiFe

    gear :
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||

    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||

    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

  9. #9
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    Join Date
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    Default need help.. confusing :(

    which is ri8.. case1 or case2?
    CASE I:
    our fan needs 220 v and 0.2 A for an hour = 50 watts.
    a 12volt 7AH battery @20 hour rate -> 7/20 = 0.35 A.
    The battery can sustain a 0.35A load for 20 hours.
    But we can use only the 50% capacity of battery so tat it doesnt get damaged.
    So 0.35A load for 10 hours.
    If we consider the losses in invertor, the time will be around 3 hours.
    CASE II:
    power consumed by the fan is 50watts.
    lets consider that we want the fan to run for about 3 hours in battery.
    power required = 50 watts * 3 hours = 150watt hours
    So, the battery capacity should be 150 watt hours.
    what we need : Two 12volt 7AH in parallel. (in parallel v is constant and I= 7+7, so P=12*14=168 watt hours)
    Loses in invertor will occur. We can not drain the full power from the battery.

  10. #10
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    welcome. Lets' un-confuse this a bit.

    power consumed by the fan is 50watts.
    power required = 50 watts * 3 hours = 150watt hours
    Good so far. If you use only 50% of battery, then you need a 300 watt hour battery.
    At 12V, that would be a 25amp hour battery.

    Now to recharge that battery, to account for all the system losses, you need to have a PV panel with a wattage nameplate that, when multiplied by your SUN hours
    use PV watts to find out your hours http://www.nrel.gov/rredc/pvwatts/about.html
    Ths solar SUN hours are quite different than when the sun is up , most areas get 6 or fewer hours.

    Anyway, you need to harvest 300 watt hours, divide by ____ sun hours = ______ watts of PV

    If you use an inverter , add 30% to the needed 300W harvest, you will need to harvest about 400 wh

    Hope this helps
    Last edited by Mike90250; 04-22-2012 at 10:26 AM.

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