Relation between measured Isc/Voc and power produced by MPPT controller

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  • eugenesan
    eugenesan Registered Users Posts: 11 ✭✭
    edited August 2017 #32
    > @mvas said:
    > @eugenesan ,
    >
    > How do you know that your AGM Battery Bank is at 20% DOD when under load?
    > Do you have an Shunt and an Amp-Hour Meter?
    > How do you know when your AGM Battery Bank is recharged to 100% SOC?

    I estimate the bank charge status by observing the voltage before the sun is out and the load is none/minimal. I usually expect voltage to be above 12.8V. I've calculated that value by reverse extrapolating manufacturer's specs that state top 50% of charge are in 12.4-13.0 range.
    Also my charge controller provides daily statistics which I can use once the batteries hit float stage (voltage drops from 14.8V to 13.8V the values I've configured the controller with).

    I know above is not exact science but my load is pretty much stable and the approach seems to work.
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  • eugenesan
    eugenesan Registered Users Posts: 11 ✭✭
    #33
    > @mcgivor said:
    > @eugenesan
    >
    > Rewinding to your original questions, your batteries manufacturer states minimum charge current of 8 amps, you have ~11 amps, but you have 2 batteries in parallel, so assuming internal resistance of each are equal, the current flow would be equal or 5.5 amps through each battery, below the minimum requirement. To add to this the minimum guidelines are  based on grid charging,  where unlimited hours are available, not just a small window of opportunity per day, so you need much more charging  capacity to account for this. You really don't want to be on the fine line  of battery failure where you currently are, sure lowering loads to almost  nothing may help, but what's the point of having a system that can't support the loads you require.

    I agree.
    Despite being able to keep up with my reduced load the system is not optimal especially for the batteries' health.
    Atm, I am planning to improve (hopefully) performance of the existing panels by reinstalling them properly (better cooling and tilt) and adding additional panel.
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    edited August 2017 #34
    @eugenesan said 

    I estimate the bank charge status by observing the voltage before the sun is out and the load is none/minimal. I usually expect voltage to be above 12.8V. I've calculated that value by reverse extrapolating manufacturer's specs that state top 50% of charge are in 12.4-13.0 range.
    Also my charge controller provides daily statistics which I can use once the batteries hit float stage (voltage drops from 14.8V to 13.8V the values I've configured the controller with).

    In order to estimate SOC using voltage, the battery must be disconnected from all loads and charging for about 4 hours before measurements are taken.

    http://batteryuniversity.com/learn/article/how_to_measure_state_of_charge
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #35
    Here is the link to copper pipe resistance:

    http://forum.solar-electric.com/discussion/comment/108378#Comment_108378

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    #36
    > disconnected from all loads and charging for about 4 hours before measurements are taken.

    I think you mean "not charging".

    I am available for custom hardware/firmware development

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  • myocardia
    myocardia Solar Expert Posts: 118 ✭✭✭
    #37
    jonr said:
    > disconnected from all loads and charging for about 4 hours before measurements are taken.

    I think you mean "not charging".
    Well of course that's what he meant. That's why he said it. The phrase "disconnected from all charging" can mean nothing other than not charging.
    DoD= depth of discharge= amount removed from that battery   SoC= state of charge= amount remaining in that battery
    So, 0% DoD= 100% SoC, 25% DoD= 75% SoC, 50% DoD= 50% SoC, 75% DoD= 25% SoC, 100% DoD= 0% SoC
    A/C= air conditioning AC= alternating current (what comes from the outlets in your home) DC= direct current (what batteries & solar panels use)
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  • Horsefly
    Horsefly Registered Users Posts: 476 ✭✭✭✭
    #38
    English grammar can trip people up easily. I think you could read what was typed either way.
    1. The batteries need to be disconnected from all loads, and they need to be charging for 4 hours...
    2. The batteries need to be disconnected from both loads and charging for 4 hours....
    So I don't think it's fair to say "that's what he said" because the way he said it was not clear. 

    Grammar police mode off.   :)
    Off-grid cabin: 6 x Canadian Solar CSK-280M PV panels, Schneider XW-MPPT60-150 Charge Controller, Schneider CSW4024 Inverter/Charger, Schneider SCP, 8S (25.6V), 230Ah Eve LiFePO4 battery in a custom insulated and heated case.
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #39
    Could be a little ambiguous perhaps, completely disconnected from all loads with no charging input for at least 4 hours, may have been a better choice of words.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    #40
    IMO, one disconnects from a charger (vs charging).  But OK, enough grammar.

    I am available for custom hardware/firmware development

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  • eugenesan
    eugenesan Registered Users Posts: 11 ✭✭
    edited August 2017 #41
    I wonder what is the discrepancy between "connected" (without charging) and "disconnected" measurements.
    Is it significant? Is it linear and can be used as a fixed coefficient?
    I ask because charge controller seems to be able to figure the charge state. My MT50 shows a battery icon gradually filled. I see it dropping the charge from time to time (probably to read the battery voltage) but the break is usually takes only seconds not 4 hours.
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  • mcgivor
    mcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #42
    The indication of SOC on the controller is, for all sence and purpose, useless, the only way it could possibly know is if it knew how much capacity went out, versus what went in, even then accuracy would be questionable, because there are other losses, such as heat. The CC is merely reading the voltage and giving a value based on that, the only accurate method is reading SG, however with AGM that's not possible, so the previously outlined 4 hour method is about as good as it gets.
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #43
    For flooded cell lead acid batteries, the standard procedure is to rest the battery (no charging or discharging current) for 3-4+ hours, then measure the resting voltage to estimate the state of charge of the battery bank.

    Lead acid batteries can get a "surface charge" and read higher voltage, etc.

    The resting time is to let everything come to equilibrium before measuring voltage.

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