MPPT amperage output on 12 volt system

rebelbah
rebelbah Registered Users Posts: 4
in Solar Beginners Corner #1
Good Day All, newbie here
im trying to better understand the MPPT charge controllers better. i have a 12 volt system, a 40A MPPT charge controller, i have 2 12volt(max 16.* volt output) 50watt 3.**amp solar panels (for now)
i know if i connect the 2 panels in parallel i can get 16volts max 6.* amps
i know if i connect the 2 panels in series(in theory) i can get 32volts max 3.* amps but

1) if i connect the 2 panels in series getting a max of 32volts what amps should i get with a MPPT charge controller?
2) if i were later to add panels(maybe 24v 200+ panels) would it be better to have higher or lower voltage in groups?

Thank you
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #2
    Re: MPPT amperage output on 12 volt system

    Would be interesting to know what MPPT controller you have - - - some are more efficient than others thus less loss.
    An easy way to look at it is to multiply the voltage of the panels under load, by the amps flowing. That will give you the power in watts. Regardless of voltage as long as it's within the range the controller can handle, that wattage enters the MPPT controller, which then, minus losses, passes that wattage on to the batteries, with the amperage depending on the battery voltage at the moment.
    To simplify for easy understanding, lets assume the panels are putting out 24 volts and 5 amps. lets also assume the controller is 100 % efficient and that the battery voltage is at 12 volts. What you'll have coming into the controller is 5A X 24V = 120 watts.
    The controller now down-converts the voltage to 12 volts to match the battery voltage, but since there's 120 watts available, the controller will up the charging amperage to 10 amps, up from 5 amps. 12V X 10A = 120 Watts. Of course in real life there are losses, and both panel and battery voltages will vary. Still the principal remains the same. One big advantage of going with a higher voltage is less line loss between the panels and controller, especially if it's a long run. It also can allow the use of smaller diameter wire, depending on the situation.
    It's more complicated if you want to go into detail, but this is the basic principal. Hope it helps.
    OH! And welcome to the forum :)
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  • NorthGuy
    NorthGuy Solar Expert Posts: 1,913 ✭✭
    #3
    Re: MPPT amperage output on 12 volt system

    It is better to connect the panels in series. This way, the voltage between your panels and controller will be higher (meaning less loss), and you will need only one breaker and one MC4 cable.

    Your MPPT controller probably can handle this, but it's better to check. Multiply Voc (should be on the back of the panel) by the number of panels(2), then multiply by 1.2 (1.3 if you live in a cold place). What you get must be less than the max voltage your MPPT controller can handle. If it isn't, parallel connection is your only choice.

    When you decide to buy more panels, you need to figure out how you are going to combine them with existing panels before purchasing.
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  • rebelbah
    rebelbah Registered Users Posts: 4
    #4
    Re: MPPT amperage output on 12 volt system

    the charge controller is a tracer 4210rn with meter,
    so its better to use higher voltage in and get higher amps our at your battery level if im understanding correctly
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,626 admin
    #5
    Re: MPPT amperage output on 12 volt system

    In one sense, it does not matter... As long as

    Maximum Controller input voltage>Vmp-array>Minimum controller input voltage

    then it does not matter.

    There are some things to understand... The MPPT Controller is usually most efficient when Vmp-array~2x Vbatt-charging. Going higher or lower can cost you a percent or so of lost power (something like 94% to 96.5% efficiency range).

    Another is that Vmp-array is sensitive to array temperature... You need Vmp-array-hot>minimum controller input requirement (typically a volt or so above maximum battery charging voltage).

    And Vmp-array-cold needs to be less than the controller maximum input limit.

    High Vmp-array means that Imp-array is going to be less (power=voltage*current). So 2x Vmp means 1/2 Imp for the array. Heating is the square of the voltage, so 1/2 the current is 1/4 the power loss in the wiring. Also, voltage drop is a percentage of operating voltage, so the same drop with 2x the voltage means 1/2 the percentage drop (or allows you to run the wiring 2x farther with the same percentage drop).

    As a first approximation, assuming all of the voltage requirements are met, then Power IN = Power Out. A second approximation is around:

    Power In * 0.95 = Power Out to battery bank
    Vmp*Imp * 0.95 = Vbatt*Ibatt

    And from a practical point of view, the typical maximum output for a good clear/not hot day:

    Panel Rating * 0.81 panel derating * 0.95 controller losses = Panel rating * 0.77 derating = Typical day to day maximum Power

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • rebelbah
    rebelbah Registered Users Posts: 4
    #6
    Re: MPPT amperage output on 12 volt system

    im in the Bahamas so its normally hot here

    MPPT charge controller specs
    System Voltage: 12/24 VDC
    Rated Charge Current: 40A
    Rated Discharge Current: 20A
    Max. PV Input Voltage**: 100VDC
    Max. PV Input Power: 12V system 500W / 24V system 1000W
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #7
    Re: MPPT amperage output on 12 volt system

    it isn't so much the ambient temp that is of concern, but the actual pv temp. with higher ambient temps it could allow the pvs to go much higher in temperature thus lowering the vmp noting that mounting with poor air circulation can cause the same effect at a lower ambient air temp. with a vmp of 16v i would put the pvs in series assuming that cc works about as well as the ones we are familiar with.
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  • rebelbah
    rebelbah Registered Users Posts: 4
    #8
    Re: MPPT amperage output on 12 volt system

    my panels are mountes at anout 25degree angle and mounted on metal rails plenty of air circulation
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #9
    Re: MPPT amperage output on 12 volt system

    well that's good that you won't be making it even worse with bad circulation, but those pvs will still get plenty hot and that could lower the voltage below the threshold that would allow the cc to operate properly at if the pvs are paralleled. in series that worry is eliminated and the higher voltage will overcome the inline resistances better from wires, fuses, etc.
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