Charge controllers: Rated by input or output amps???

MontanaclanMontanaclan Registered Users Posts: 9 ✭✭
in Solar Beginners Corner #1
I'm finding conflicting information on this... I'm looking at using an MPPT controller to use some cheap used 24v panels and feeding a 12v battery bank on an RV.   If controllers are rated by input amperage, a 20 amp controller should be fine.   If rated by output amperage, I'll need a 40.    There must be a definitive answer available from this group... help!   Many thanks.
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  • mcgivormcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #2
    Montanaclan said:
    I'm finding conflicting information on this... I'm looking at using an MPPT controller to use some cheap used 24v panels and feeding a 12v battery bank on an RV.   If controllers are rated by input amperage, a 20 amp controller should be fine.   If rated by output amperage, I'll need a 40.    There must be a definitive answer available from this group... help!   Many thanks.
    Rated by maximum output............best not to run at maximum though.
    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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  • MontanaclanMontanaclan Registered Users Posts: 9 ✭✭
    #3
    @mcgivor
    Ok... so if I'm running 2 * 275w panels in parallel that spec at ~9 amps each, not accounting for de-rating (in order to be conservative on controller needs).... I'd have about 31v at 18 amps going in, and would have about 46 amps at 12v coming out... so I should go with a 60 amp MPPT controller?

    Thanks again - the help is really appreciated.
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  • mcgivormcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    #4
    The rated output of the panels will unlikely ever be seen unless in perfect conditions, freezing temperatures in full sunlight, the normal output will be in the 75-80% range so ~35A, having a controller with more potential capacity would allow for some expansion as well as putting less stress on its components.
    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: 33,313 admin
    #5
    The rough math:
    • 2 * 275 Watt panels * 0.77 panel+controller deraring * 1/14.5 volts nominal charging = 29.2 Amps on cool/clear day/noonish
    That would be about the best I would expect to see spring/fall clear days.
    • 2 * 275 Watt panels * 1.0 cold panels * 1/14.5 volts = 37.9 Amps winter peak
    Given that you are in/around Montana (?), then in very cold/sub freezing weather, you could see more.

    It is one of those things... You probably get 20% more power in deep winter, but your actual harvest time is pretty short in winter.

    And your maximum current is during cold weather--So that is "better" for running near full power...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • 706jim706jim Solar Expert Posts: 508 ✭✭✭✭
    #6
    So 40 amps should do it.
    Island cottage solar system with 2500 watts of panels, 1kw facing southeast 1.3kw facing southwest 170watt ancient Arco's facing south. All panels in parallel for a 24 volt system. Trace DR1524 MSW inverter, Outback Flexmax 80 MPPT charge controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 30th year.
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  • MontanaclanMontanaclan Registered Users Posts: 9 ✭✭
    #7
    That’s great. And even better to learn about the cold panel performance. On the on hand we have a lot of cloudy winter days, on the other, I’d need more power in the cold season due to furnace cycling.  ... all this to power a small rv, with my cpap as the single biggest draw.  Thanks!
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  • mike95490mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #8
    Yes.

    PWM controllers rated INput amps
    MPPT  rated by OUTput amps   Enjoy
    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

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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