Battery Storage

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rporrata
rporrata Registered Users Posts: 1 ✭
in General Solar Power Topics #1
I have a Schneider Electric 6848 Inverter and two Schneider Elec. 60/150 Controllers.  For power storage I have 8 12v 250 AGM 250 GEL batteries.  Questions:  How much power storage in watts these batteries provide?  How much of this storage I may use before they need recharging?  It is my understanding that I cannot use more than a certain percentage of power storage.  

Thanks for assistance.
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  • mvas
    mvas Registered Users Posts: 395 ✭✭✭
    edited September 2019 #2
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    rporrata said:
     It is my understanding that I cannot use more than a certain percentage of power storage.  
    That is correct ... you cannot use more than 100% of power storage.
    Try to leave at least 20% charge remaining = 80% Depth of Discharge max.
    An average of 50% DOD is typical, but not mandatory.
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  • mvas
    mvas Registered Users Posts: 395 ✭✭✭
    edited September 2019 #3
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    rporrata said:
      For power storage I have 8 12v 250 AGM 250 GEL batteries.  Questions:  How much power storage in watts these batteries provide?  
    Please reply with ...
    Battery Manufacturer =
    Battery Model # =

     Do you know how to compute Watts or Watt-Hours ?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #4
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    Welcome to the forum RP,

    In general, the "optimal" usage (full time off grid, using system every day) is to use ~25% of state of charge, for 2 days to 50% state of charge (for longer battery life):
    • 2 strings of 4x 12 volt @ 250 AH batteries = 48 volts @ 500 AH of bank capacity
    • 48 volts * 500 AH * 0.85 AC inverter eff * 1/2 days storage * 0.50 max typical discharge = 5,100 Watt*Hours per day average
    The typical maximum discharge (one heavy usage day, perhaps Weekends, with a couple days to recharge--Take to 50% state of charge:
    • 48 volts * 500 AH * 0.85 AC inverter eff * 1/1 day storage * 0.50 max typical discharge = 10,200 Watt*Hours per day heavy load
    And the not to exceed (full bank to 20% state of charge). To exceed this even once, is is possible that you will ruin one or more cells/batteries (take a cell to 0% SoC then reverse charge). You need to immediately recharge the battery bank:
    • 48 volts * 500 AH * 0.85 AC inverter eff * 1/1 day storage * 0.80 max discharge = 16,320 Watt*Hours not to exceed 1 day discharge
    Then there is the question of what kind of GEL batteries you have... The typical flat plate batteries (at least in the US), tend to have a C/20 or 5% maximum charge rage.

    For many European/Tubular type GEL (and other sealed lead acid batteries), they may support 10% or even 20% rate of charge.

    For a full time off grid system, you really want a 10% rate of charge minimum for daily solar usage. For a "weekend" or emergency backup system, you can possibly live with 5% rate of charge (and more backup genset usage).... There are generally not enough hours of sun in a day to recharge a deeply cycled Lead Acid battery bank every day.

    So--The other question is--The size/angle of tilt of your array, and where is it located (free of shade between, at least, 9am to 3pm--Including NO Power Line or other shadows on panels). I guess you are in/around San Juan Puerto Rico?

    The above assumes that you charge during the day and use battery power mostly at night/during bad weather.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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