Adding Panels to current system

SidMSidM Registered Users Posts: 4
I’m fairly new to solar but have studied forums, manuals, etc. to try to get up to speed.

I have Twelve 75 watt panels (Vmax 21.8, Voc 30.6) that I have been using for a few weeks in 4 parallel strings of 3 panels in series, feeding a 24 volt battery system using an Outback FM60 controller with 4 golf cart batteries. I just acquired 3 Trina 240w panels (Vmax 30.4, Voc 37.2) that I would like to add.

Would the best for me be to reconfigure the 75 watt panels to 3 parallel strings of 4 panels in series (Vmax= 21.8 each, Voc 30.6) giving me 87.2 Vmax) and use the three new 240 watt panels in one string (Vmax 30.4, Voc 37.2) giving me 91.2 Vmax? This would be to try to get close to matching output voltages, or is the Voc the voltages to match?

Comments

  • nielniel Solar Expert Posts: 10,311 ✭✭✭✭
    Re: Adding Panels to current system

    for the one fm60 i believe adding all 3 new trinas may be pushing things to the edge and beyond. it is borderline on the voc nearing the max voltage to the cc and it is over on the power as 24v x 60a = 1440w and you would have 1620w with all of those pvs.

    it would work out with 2 trinas added in series and paralleled to the present system if one likes within the 10% voltage margins.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Adding Panels to current system
    niel wrote: »
    for the one fm60 i believe adding all 3 new trinas may be pushing things to the edge and beyond. it is borderline on the voc nearing the max voltage to the cc and it is over on the power as 24v x 60a = 1440w and you would have 1620w with all of those pvs.

    Welcome to the forum SidM,

    I wouldn't be as concerned about putting 1620 watts on an FM60 as I would be concerned about exceeding the FM60 Vmax.

    Another issue is the amount of current that your batteries can handle. Your batteries are about 220 ampHours capacity... you shouldn't be pumping 60+ amps into them. Fortunately the FM60 can be set to limit its output current during bulk.

    The Imp of your 75 watt panels is about 3.44 amps and the Imp of the Trinas is about 7.8 amps. If you put four 75 watt panels in series/parallel (Imp= 6.88 ) and put one Trina in series with that, you will have a string of five panels. This will limit the current through the trina to 6.88 amps, effectively reducing the trina to a 209 watt panel. Each string of five panels would be 509 watts, and you could make three strings of these.

    You might consider adding another controller for the new panels. If you do that, you need to find a way for the controllers to not overcurrent the batteries.... that may be difficult to achieve unless you have an Outback hub and FlexnetDC installed. A better option is probably to trade in the FM60 for a Midnite Classic200 and use the string configuration that you suggested. The classic can take the higher voltage and current of that configuration.

    Your situation is very common... it's one of the reasons why it is so often repeated on this forum that it is difficult to expand systems. Sometimes the best way to expand a system is to add another system, and split your loads between the two systems.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • SidMSidM Registered Users Posts: 4
    Re: Adding Panels to current system
    vtmaps wrote: »
    Welcome to the forum SidM,

    I wouldn't be as concerned about putting 1620 watts on an FM60 as I would be concerned about exceeding the FM60 Vmax.

    Another issue is the amount of current that your batteries can handle. Your batteries are about 220 ampHours capacity... you shouldn't be pumping 60+ amps into them. Fortunately the FM60 can be set to limit its output current during bulk.

    The Imp of your 75 watt panels is about 3.44 amps and the Imp of the Trinas is about 7.8 amps. If you put four 75 watt panels in series/parallel (Imp= 6.88 ) and put one Trina in series with that, you will have a string of five panels. This will limit the current through the trina to 6.88 amps, effectively reducing the trina to a 209 watt panel. Each string of five panels would be 509 watts, and you could make three strings of these.

    You might consider adding another controller for the new panels. If you do that, you need to find a way for the controllers to not overcurrent the batteries.... that may be difficult to achieve unless you have an Outback hub and FlexnetDC installed. A better option is probably to trade in the FM60 for a Midnite Classic200 and use the string configuration that you suggested. The classic can take the higher voltage and current of that configuration.

    Your situation is very common... it's one of the reasons why it is so often repeated on this forum that it is difficult to expand systems. Sometimes the best way to expand a system is to add another system, and split your loads between the two systems.

    --vtMaps

    Thanks for the welcome and information from you and Niel. I knew the max wattage would be high, but I didn't know I would be exceeding the Vmax that I thought was 150 volts for the FM60. If I understand it correctly, 3 Trina's at 37.2 volts should be 91.2 with the Open max of 37.2 times 3 at 111.6.

    My desire is to have enough output to run my office during the daylight hours (approx 800 watts/hr) and still have a charged battery bank, but I see that considerable planning is required. I plan to double the battery bank, but not sure that will solve the high current issue. I had a feeling that the FM60 was not going to be adequate for system growth.

    Much to consider here before I hook both strings up.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Adding Panels to current system
    SidM wrote: »
    but I didn't know I would be exceeding the Vmax that I thought was 150 volts for the FM60. If I understand it correctly, 3 Trina's at 37.2 volts should be 91.2 with the Open max of 37.2 times 3 at 111.6.

    Vmax for the controller is 150 volts. The "max" means maximum voltage.

    The panels have a Vmp (voltage at maximum power), and a Voc (voltage with open circuit). The Voc of the string of panels must not exceed the Vmax of the controller.

    Four of your 75 watt panels will have a Voc of 122. When they get cold, the Voc is elevated. You can probably get away with it, but it's too near the edge for my comfort.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • SidMSidM Registered Users Posts: 4
    Re: Adding Panels to current system

    Thanks, I thought the concern was for the 3 Trinas. So I won't do the 4 series 75 watt ones. Again, maybe two separate strings would be the best and most straight forward approach. I'm going to put the three 240 watt ones on line now and leave the other ones off until I can work it out.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Adding Panels to current system
    SidM wrote: »
    I'm going to put the three 240 watt ones on line now and leave the other ones off until I can work it out.

    You need to put them in series, not parallel. If you put them in parallel the Vmp is not high enough for 24 volt battery. If you put them in series, the Vmp will be over 90 volts. That will work, but is stressful on your controller, especially when it is hot out and the controller is running at high power.

    Your controller is more efficient down converting 60 volts to a 24 volt battery than down converting 90 volts. That means it runs cooler with a 60 volt input than a 90 volt input. With panels such as your trinas, it is recommended to have strings of two panels in series. Can you aquire another trina?

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • SidMSidM Registered Users Posts: 4
    Re: Adding Panels to current system

    Yes, my son is planning a string at his home and has another Trina that I could probably acquire. That might be better all around, including to possibly add the 75 watt ones slowly back into the string as niel suggested.
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