Solar panels with different voltages connected in series using an MPPT Controller

SolarVader
SolarVader Registered Users Posts: 5 ✭
in Off Grid Solar & Battery Systems #1
Hello,

Big fan of this forum, it has been in my “Favorites bookmark” for some time now :cool:.

My question is regarding solar panels with different voltages connected in series.

Some background:

I have a 60A MPPT charge controller, 3 200W 26.6VMP, 7.51Imp panels, 1 Unisolar 136W 33VMP, 4.1Imp panel and my battery bank is at 24VDC. I have two options to add the Unisolar to my current setup:

A) Would be best (I think) but I don’t want the extra connections

I have an extra Xantrex C40 configured to 24VDC. I can use this one to hook up my Unisolar. A VMP of 33V is just shy of 34V, so I think I shouldn’t see large losses form the PWM controller. I’ll then tie my controller’s battery outputs in parallel trough a BUS and feed my battery bank.

B) What I want and would be simplest to do

I’m assuming I can wire all my panels in series and connect them to my MPPT charge controller (I know the controller can handle the extra 136W). My question:
1) What happens if I do?
2) Would my output current be larger than with just my 3 200W panels (all things being equal)?

I don’t mind an efficiency loss due to dissimilar voltages / currents, as long as I can add more energy to my bank.

Thank in advance !!
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Comments

  • jaggedben
    jaggedben Solar Expert Posts: 230 ✭✭
    #2
    Re: Solar panels with different voltages connected in series using an MPPT Controller

    It is not a good idea to put the Unisolar in series with the others. The voltage and current will both be roughly limited to the smaller of the two (26.6V and 4.1A), and thus you'll probably get less power out of the 4 panels than out of the three 200W panels by themselves. Even if the charge controller successfully pushes the panels to an in-between current (say 5A at 30V per panel) you'd be getting no extra power (still 600W), and it might cause the Unisolar to degrade faster from additional heat..

    Is the Unisolar a thin film technology? I think that would make it even less advisable, as it may mess with the MPPT algorithms, although probably the only way to find that out would be the hard way.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #3
    Re: Solar panels with different voltages connected in series using an MPPT Controller
    jaggedben wrote: »
    It is not a good idea to put the Unisolar in series with the others. The voltage and current will both be roughly limited to the smaller of the two (26.6V and 4.1A), and thus you'll probably get less power out of the 4 panels than out of the three 200W panels by themselves.

    Not quite. In series, you will get a higher voltage than their Vmp from the three panels since they are delivering less current but you will get no more than Isc out of the string unless you trigger the bypass diodes in the smaller panel, in which case you will be back where you started with 3 but with a voltage drop from the diodes.

    Just playing with the numbers: (3 x 30 volt + 20 volt) x 5 amps would give you 550 watts. Compared to 600 watts with just the three panels.

    On the other hand, if you were to put them all in parallel (with a fused combiner) you would get roughly 3 x 27 x 7.5 plus 27 x 4.2, for a total of 600 + 113 or 713. But the parallel panel voltage of ~27 volts would not be enough to run a 24 volt battery unless your MPPT also worked in boost mode.

    Summary: If you put them in series, the current is limited by the smallest panel (in terms of current) but may be as much as the Isc of the small panel (with no power contributed by it at all.)
    If you put them in parallel, the currents will add and the voltage will be somewhere between the Vmp of the two types of panels, closer to that of the panels producing the greatest share of the power. (This assumes that Voc of each panel type is greater than Vmp of the other.)
    SMA SB 3000, old BP panels.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #4
    Re: Solar panels with different voltages connected in series using an MPPT Controller

    put the unisolar on the extra controller as a separate system to the same batteries. you will have losses otherwise.
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  • SolarVader
    SolarVader Registered Users Posts: 5 ✭
    #5
    Re: Solar panels with different voltages connected in series using an MPPT Controller

    Thanks for the information !

    From inetdog's comment I thought I could use the 3 like panels in series and then connect the unisolar in parallel, but then the Vmp for the like panel string would be much greater than the Voc of the unisolar.

    I'll go ahead and use my extra controller. The reason I wanted to hook everything up in series is because the bank is in the garage and my panels are in the backyard. So I run 100ft of cable trough my house.

    The unisolar ones are thing film. Pretty nifty they weigh 17lbs (compared to the 200W ones which are 42lb) easier to move around. The only down side for me is the sticky tar they have. I don’t have it glued to anything, my intention is to have it for mobility. Was thinking on gluing it on some very flexible thing plastic, or some other type of flexible material that I can roll. Any idea as to what I can use?
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