Parrelling in some smaller panels

pods8
pods8 Registered Users Posts: 16 ✭
in Solar Beginners Corner #1
Base setup: 100watt panel ( Vmp17, Imp 5.8 ), Sunsaver SS-10-12V , strictly charging 12v camper system (no inverter stuff involved).

I was pondering about adding a pair of 35watt panels (Vmp 18, Imp 1.95) with all panels being parallel. Why the 2 35watter's? A combination of the geometry of the space I have and also they'd conveniently fill out the capacity of the sunsaver pretty well. The Isc would be slightly over 10amps at 10.72 but I'm under the impression that is what morningstar means when they say no need to derate due to a 25% design margin (ie you can go up to 10amps Imp), I'll call them to confirm before pursuing though.

From what I gather the smaller panels which have a slightly higher Vmp will have their voltage pulled down to that of the larger panel, however since I'm just using PWM I don't particularly see an issue with that. I'm also under the impression each of the three branches will need a fuse to protect the panel from the combined Imp 9.7, so I would assume the large panel would get a 7.5amp and the smaller 3amp.

Have I understood this all correctly or overlooking anything?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,660 admin
    #2
    Re: Parrelling in some smaller panels

    From what I can see--Just wire the panels in parallel and you should be fine...

    Technically, for safety, the two smaller panels probably need around a 5 amp series protection fuse (in the positive lead of each panel). If one of the smaller panels gets shorted, other two panels can feed over maximum rated series fuse current into the shorted panel and could overheat/start a fire.

    The larger panel does not need a fuse/breaker because its rating is larger than the sum of the two smaller panels Isc ratings.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • pods8
    pods8 Registered Users Posts: 16 ✭
    #3
    Re: Parrelling in some smaller panels

    Good point on the larger panel being rated regardless. Any reason you said 5amps on each smaller panel leg verse the 3amp I was thinking?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,660 admin
    #4
    Re: Parrelling in some smaller panels

    You have to look at the specifications on the smaller panels for the maximum series rated protect fuse (or breaker). You may not find any specifications for that on smaller panels. So--The assumption is around 2x Imp to 2x Isc would be a typical value.

    so, if you cannot find the "rating", then somewhere around 4-5 amps for the fuse is fine (only needed for the two smaller panels for safety--not for normal operation). You don't want a smaller fuse--because fuses and breakers can trip if operated at 80% or more of rated current (i.e., a 4 amp fuse * 0.80 = 3.2 amps maximum continuous current flow).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #5
    Re: Parrelling in some smaller panels

    i agree with bill that you should go ahead and parallel them with fusing only on the small pvs. if buying new fuses go with 4a or 5a as bill says, but if you have 3a fuses on hand and you aren't mounting the fuses in a hard to reach area or unattended for long periods of time i'd say go ahead and use the 3a fuses.
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  • pods8
    pods8 Registered Users Posts: 16 ✭
    #6
    Re: Parrelling in some smaller panels

    Great, thanks for the input.
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  • pods8
    pods8 Registered Users Posts: 16 ✭
    #7
    Re: Parrelling in some smaller panels

    Quick follow up question, morning star advised there is a 25% design margin but only rated 5min at that overload.

    Generally speaking I shouldn't normally have a problem as these are just flat mounted (1/2" airgap under neath edge). My only concern being in Colorado is if I was camping on a cold winter day at altitude with clear sun, is there any realistic possibility that I could actually exceed 10amps?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,660 admin
    #8
    Re: Parrelling in some smaller panels

    Solar panel Vmp/Voc rises when cold... Imp/Isc, very slightly, drops when cold (for all reasonable purposes, you can ignore the temperature effects on Imp/Isc ratings). So, you will not have any "temperature/output" related issues with a PWM controller as long as you meet their requirements (and make sure the PWM controller has good ventilation, and is not installed in the top of a hot/stuffy closet).

    PWM controllers only "pass" the current... They do not care if the Vmp is 15.5 or 17.6 or 22.0 volts--It does not affect their performance (hot panels with very cold batteries can have an issue of insufficient charging voltage--Vmp falls to 15 volts and battery charging set-point rises as they near freezing). It does not hurt anything, it just can limit current/voltage available to "fully and quickly" recharge the battery bank in very hot weather.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • pods8
    pods8 Registered Users Posts: 16 ✭
    #9
    Re: Parrelling in some smaller panels

    Perfect, thanks. I knew output went up in the cold but didn't realized it was the voltage that rose.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,660 admin
    #10
    Re: Parrelling in some smaller panels

    Yea, it gets a bit confusing... Roughly:

    PWM: Imp*Vbatt-charging*~0.95-0.99 eff = Power to battery
    MPPT: Vmp*Imp*0.95 eff = Power to battery

    And remember that Vmp is, more or less affected by temperature (once you have weak full sun on panel) and Imp is directly affected by Watts from Sun on Panel (clear weather, lots of sun, lots of current. Poor weather, shading on panel, not much current).

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