Controlling mismatched panels on my RV

Hi, I appreciate the help this forum has provided (I posted a question last week and you resolved that issue. Now I have another stupid question. I currently have 2 85w PV panels mounted on my RV. I want to add 2 120w PV panels (totaling 410w). I am going to replace my controller (currently a Thunderbolt Magnum 30w) as I don't trust it. I am looking at a Schneider 35w PWM. I have received conflicting opinions on controlling mismatched panels. One source says a PWM will control all 4 but will knock the 120's down to about 100. They recommend a MPPT. Another source says an MPPT does not do well with mismatched panels and that I should use a PWM. Any help would be appreciated.

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  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Controlling mismatched panels on my RV
    Hi, I appreciate the help this forum has provided (I posted a question last week and you resolved that issue. Now I have another stupid question. I currently have 2 85w PV panels mounted on my RV. I want to add 2 120w PV panels (totaling 410w). I am going to replace my controller (currently a Thunderbolt Magnum 30w) as I don't trust it. I am looking at a Schneider 35w PWM. I have received conflicting opinions on controlling mismatched panels. One source says a PWM will control all 4 but will knock the 120's down to about 100. They recommend a MPPT. Another source says an MPPT does not do well with mismatched panels and that I should use a PWM. Any help would be appreciated.

    It isn't the Wattage that makes the difference, it's the Voltage.
    If the Vmp of the 85 Watt panels and the Vmp of the 120 Watt panels are within 5% there's no issue: they can be put in parallel.
    Nor does the type of controller matter for this: none can 'correct' a mismatch.

    We would expect a 120 Watt panel to be "12 Volt" just as the 85 Watt ones: the Vmp around 17-18.

    Altogether there would be 410 Watts of panel with a combined output current of about 24 Amps. As such the C35 controller would work.
  • BB.BB. Super Moderators, Administrators Posts: 30,640 admin
    Re: Controlling mismatched panels on my RV

    What are the Vmp/Imp of the various panels you wish to connect?

    Note sure which charge controller... 35 Amp?

    Schneider Electric C35 Solar Charge Controller

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • avbackpackeravbackpacker Registered Users Posts: 3
    Re: Controlling mismatched panels on my RV

    the controller is Schneider Electric C35 35a Solar Charge Controller. The vmp/imp of the new 120w panels is 17/7. I don't that info for the 2 85's I have mounted on the RV. I do know that the max amp rating is 5.34.

    I would really like to use a sunsaver duo with remote meter, but it is only a 25a controller.Adding the remote for the Schneider puts the total cost at about $200 compared to the sunsaver w/ remote is about $150. I am concerned that the sunsaver at 25amps will not have enough capacity for the potential 24.6 amps from the panels. I expect since the panels will be permanently fixed to the roof that they will seldom if ever produce the max output but I don't want to spend $150 and fry the controller. I can live without the remote meter and think the Schneider 35a would be a wiser choice (and a bit cheaper).
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Controlling mismatched panels on my RV
    the controller is Schneider Electric C35 35a Solar Charge Controller. The vmp/imp of the new 120w panels is 17/7. I don't that info for the 2 85's I have mounted on the RV. I do know that the max amp rating is 5.34.

    There shouldn't be much problem then. 17.7 Vmp is about what should be expected, and if you divide 85 Watts by 5.34 Amps (probably Isc) you get 15.9 Volts. Isc is higher than Imp which is what Wattage is based on (Imp * Vmp). Imp is probably just under 5 Amps putting the Vmp in the 17 Volt range.

    So they can all be in parallel on a 35 Amp controller. Just remember that each panel should have its own fuse to protect against short-circuit fires. I'd expect 10 Amps would work for each type.
    I would really like to use a sunsaver duo with remote meter, but it is only a 25a controller.Adding the remote for the Schneider puts the total cost at about $200 compared to the sunsaver w/ remote is about $150. I am concerned that the sunsaver at 25amps will not have enough capacity for the potential 24.6 amps from the panels. I expect since the panels will be permanently fixed to the roof that they will seldom if ever produce the max output but I don't want to spend $150 and fry the controller. I can live without the remote meter and think the Schneider 35a would be a wiser choice (and a bit cheaper).

    There's no reason to use the SunSaver Dual, which is for charging two batteries from one array. Although it would work as the current will be under 25 Amps (410 Watts @ 17.5 Volts = 23 Amps). Another choice is the ProStar 30 with meter with is about $173: http://www.solar-electric.com/ps-30m.html
  • BB.BB. Super Moderators, Administrators Posts: 30,640 admin
    Re: Controlling mismatched panels on my RV

    Ok, one set of panels has Vmp~17 volts (typical is 17.5 to 18.6 or so for "optimum" PWM connection).

    85 watts / 5.35 Imp(?) = 15.9 volts Vmp

    That is a bit on the low side--But for your needs--Simply place all the panels in parallel and connect to the PWM controller.

    There is nothing much else you can do to use all of the panels and keep the system at a low cost. Everything else would be buying new panels, using an MPPT type charge controller, etc. Way more than you need to spend right now.

    The C35 should be able to manage around a:

    35 amp * !7.5 volt Vmp array = 612.5 Watts

    So, as long as you keep the array around 600 watts or less--You should be good to go.

    Things you should do:

    With parrallel connected solar panels, you should have a fuse/circuit breaker per panel to protect against short circuit current. If you do not have the fuse rating for the panel(s), then (roughly) take Imp*1.56 and round up to the next standard fuse/breaker.

    Mount the controller in a location with good air flow... Don't mount it near the top of a hot/unventilated closet (cool electronics are happy electronics).

    Personally, I like to recommend "manual" equalization rather than automatic. Easier on the batteries.

    If flooded cell batteries a good glass hydrometer+thermometer so you can log temperature corrected specific gravity for your cells.

    Somethings to add to your wish list... A DC Current Clamp DMM (very nice for debugging). A Battery Monitor Like a Trimetric or Victron would be really nice (almost a necessity for AGM/Sealed batteries).

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