Mixing Voltages

Northern_BC Registered Users Posts: 33 ✭✭
My solar array is made of multiple strings 3 solar panels in series (133 VOC). I have room on the end of my frame to put up two more panels. This series would only be 88 VOC. The strings are run to a combiner box and then to the charge controller. Will mixing the voltages like this reduce the entire output of the system?

XW6048 inverter, EnerSys 1520 Ah battery bank, 1500 watt tracker & 7500 watt stationary solar, 10kw Baldor NG generator


  • softdown
    softdown Solar Expert Posts: 3,825 ✭✭✭✭
    I have read that one gets the ~ voltage level of the lowest string. That was also my very recent experience and not a pleasant one. 

    Somehow I lost the serial connection of two panels. So I got ~40 volts from one panel and ~ 80 from the other panels in serial output. Total output for the whole array was about 47 volts. A disaster with a 48 volt system.

    ***Considering that my clamp meter showed no output from the panels that had lost their serial connection - and that the clamp meter was used first - it is all very inexplicable. How did two panels lose their serial connection after many years? Yet hooked themselves back up again - but wrong this time?  (This last sentence is for the "solar detectives".)
    First Bank:16 180 watt Grape Solar with  FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries
  • Horsefly
    Horsefly Registered Users Posts: 472 ✭✭✭✭
    If you put strings in parallel that have different voltages, the voltage of the combined strings will be whatever the lowest string voltage is.

    If you panels have a voltage of V and a current of I, and you have three strings of three panels, the voltage of the combined 3 strings will be 3V, and the total current produced will be 3I. The power of these parallel strings of 3 panels will then be 3V * 3I, or 9VI. Now if you take another two panels, put them in series, and put this new string in parallel the others, your voltage will drop to 2V, but your current will go up to 4I. Your total power will therefore be 2V * 4I, or 8VI. That means the total power produced by your array will actually go *DOWN* from 9VI to 8VI because you added this 4th string.

    This is without even considering the damage that could be caused to the panels in such a configuration.
    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.
  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    If you really need/want the extra power from 2 extra panels, IMHO the way to do it would be to wire them separately via a PWM controller to the bank.  ~60Vmp is a bit low for mppt, but should be mostly okay for pwm. 

    As noted by others above, putting them in the mix on the existing controller will like be counterproductive.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
    Agree with Estragon..add another small string and a cheaper controller, or if you have headroom on the controller, add one more panel (three new in total) for another string.  Try to find room for the extra somewhere?  Tony
  • Northern_BC
    Northern_BC Registered Users Posts: 33 ✭✭
    I was looking at mounting panels to the side of my frame, facing west, to get the late evening summer sun. So, my other panels would really not be doing much at that time. I can easily fit two panels. Three would take some extra framing and thought to mount.
    XW6048 inverter, EnerSys 1520 Ah battery bank, 1500 watt tracker & 7500 watt stationary solar, 10kw Baldor NG generator
  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,805 ✭✭✭✭✭✭
    I usually get 2KW + on a tracker for my clients. What kind is yours?
    "we go where power lines don't" Sierra Nevada mountain area
    E-mail offgridsolar@sti.net

  • Northern_BC
    Northern_BC Registered Users Posts: 33 ✭✭
    I have a 1.5kw tracker. It is great, but gets partially block from the sun for about the last 2 hrs during a few months of the summer.

    This discussion has got me thinking. I have my 1.5kw tracker combined with 1.5kw of fixed solar panels which then run to one charge controller. In the mornings and late afternoons when my tracker is perpendicular to the sun, is it actually going to be downgraded to the output of the fixed system which is getting the sun from a side angle? I have my other 3kw of fixed solar panels to a 2nd charge controller and notice that the output reading on both charge controllers are similar at these times of day. When really the tracker side is getting much better sun. I probably need a separate charge controller just for the tracker?
    XW6048 inverter, EnerSys 1520 Ah battery bank, 1500 watt tracker & 7500 watt stationary solar, 10kw Baldor NG generator
  • BB.
    BB. Super Moderators, Administrators Posts: 33,501 admin
    The MPPT charge controller is trying to maximize the Vmp-array voltage... Hot panels, Vmp falls, cold panels Vmp rises, higher current flow slightly lower Vmp-panel/array, etc.

    When you put two different sets (parallel strings of panels), you will have different Vmp-string voltages. There is no one Vmp-array voltage that is the optimum of all strings--Some Vmp-strings will be lower peak, others will have a higher voltage peak. Which "local peak" voltage will the MPPT controller "follow", hard to say.

    For optimum harvest, you want all of the panels to one MPPT controller to be under the same conditions (same angle to sun, same mounting in free air/over roof/etc, same cell temperatures, etc.).

    Will you see a big difference--I cannot guess (I have never had the fun of doing the experiment).

    You could do your own experiments. Connect one string/set of panels at a time to your shared MPPT controller (a few minutes on A, a few minutes on B, back to A, then back to B). Average the wattage and compare A+B separate wattage on controller vs AB single array on one controller.

    You will need your battery bank to be discharged and/or a heavy load (like an electric heater) to make sure the MPPT controller is doing maximum harvest (i.e., A=1,000 Watts, B=1,000 Watts, but A+B = 1,500 Watts because battery bank/loads do not accept more current--Not a valid comparison).

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