Vmp vs V into MPPT

Morningstar states 12, 36 & 48 volts are possible with the Tristar MPPT controller. My RV roof mount system is (6) 120 watt 12v panels. I am wiring as 24v or 36v. First choice is to come into a TriStar MPPT controller at 36v; 2 groups of 3 panels wired in series then parallel at a combiner thus 36v to the controller. BUT the Vmp is 51 Vmp at the controller. Is this a problem? If so I can go 3 groups of 2 panels resulting in 34 Vmp at the controller. The problem is the stated panel 12v vs the 17 Vmp. I maybe over thinking this!

rocmoc n AZ/Mexico


  • BB.BB. Super Moderators, Administrators Posts: 31,614 admin
    Re: Vmp vs V into MPPT

    The nice thing about MPPT charge controllers is that they can take high voltage/low current from the solar array and efficiently "down convert" it to low voltage/high current needed to charge the battery bank.

    So, for a 12 volt battery bank, Vmp-array needs to be from ~17.5 volts to ~100 VDC maximum (standard conditions--Always double check charge controller for its actual limits). Note, most vendors have a software tool to help design the array:

    String Calculator » Morningstar Corporation

    You need higher Vmp-array for 24 volt (~Vmp-array=35 volts minimum) and 48 volt (~70 volts minimum) battery banks.

    And Vmp varies with temperature--The hotter it gets, the lower Vmp gets. The colder it gets, the higher Vmp and Voc become--You need to configure your array based on panel Vmp, battery voltage, and high/low temperatures for your region.

    It is complicated the first time or two--But becomes easier as you get practice and better understand what is happening.

    Note that as Vmp rises, usually the MPPT charge controllers become slightly less efficient (run hotter)--So, unless you have reasons to run higher voltage (like a long wire run from array to charge controller), the "optimum" Vmp-array is typically around 2x the battery charging voltage (i.e., for a 12 volt battery around Vmp-array~30 volts). However, you still have the maximum input voltage to the controller of ~140-150 volts for many high end MPPT controllers--Which usually works out to something like ~100 Vmp-STD-array VDC for cold weather climates.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rocmocrocmoc Registered Users Posts: 4
    Re: Vmp vs V into MPPT

    I used the String Calculator with the Morningstar TriStar MPPT 60; my first choice of 3 panels in series with 2 groups in parallel is Optimal Configuration. Their words not mine but that is great. I was looking for just that type of tool but had not found it. Bookmarked for future use.

    rocmoc n AZ/Mexico
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Vmp vs V into MPPT
    rocmoc wrote: »
    Morningstar states 12, 36 & 48 volts are possible with the Tristar MPPT controller.

    It should be made clear that these Voltage choices are nominal system output Voltage, not input Voltage from the array.

    The one caution is that the higher the array Voltage is in respect to the system Voltage the lower the efficiency of the controller.
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