Any way to combine these panels?

Desert Rat
Desert Rat Solar Expert Posts: 147 ✭✭✭
in Off Grid Solar & Battery Systems #1
I was asked to check on a neighbor's batteries while he was away for the summer. Battery voltage was 24.0, so I knew something was wrong. Turned out that the Trace C40 controller had died. OK, easy enough. But while inspecting the system I found that he had installed two 120W panels (Vmp 17.6V) in series and two 208W panels (Vmp 28.71) in parallel, with the two pairs paralleled into the controller. Obviously he assumed that the 208W panels were "24 volt" panels and would work. I don't see how he can use all of the present panels together. Ideas?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: Any way to combine these panels?

    Separate charge controllers would be one way. :D

    The 120 Watt panels will have an Imp around 6.8 and the 208 Watt panels will have an Imp around 7.2 so putting them in series of one each would cost some power but could work: 46.3 Vmp @ 6.8 Imp or 315 Watts per string, 630 Watts total.

    This would necessitate an MPPT type controller of course, but that would be needed for those 208 Watt panels anyway as the Vmp is too low for a 24 Volt system.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Re: Any way to combine these panels?

    Well--They will work, as is, as well as they have been... If that was "good enough" with the PWM controller--Then don't worry about it.

    Otherwise, you are looking at adding a second MPPT controller wired up to support the two 208 watt panels (panels in series), and connecting the new MPPT controller directly to the battery bus common connections.

    At this point, the only two "lower cost" options are the:

    MorningStar MPPT 15 amps 12/24 volt
    Rogue MPPT 30 amp 12/24/.... controller that is coming out "in days" (????)

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Desert Rat
    Desert Rat Solar Expert Posts: 147 ✭✭✭
    #4
    Re: Any way to combine these panels?
    Cariboocoot wrote: »
    Separate charge controllers would be one way. :D
    I thought of that, but didn't like the idea. ;)
    Cariboocoot wrote: »
    The 120 Watt panels will have an Imp around 6.8 and the 208 Watt panels will have an Imp around 7.2 so putting them in series of one each would cost some power but could work: 46.3 Vmp @ 6.8 Imp or 315 Watts per string, 630 Watts total.
    I thought that it was unacceptable to combine panels with that much of a voltage difference. Is the loss of power the only reason?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #5
    Re: Any way to combine these panels?
    Desert Rat wrote: »
    I thought of that, but didn't like the idea. ;)


    I thought that it was unacceptable to combine panels with that much of a voltage difference. Is the loss of power the only reason?

    No, you connect them in series: one 120 Watt and one 208 Watt. You get two strings of Vmp 46.3 and Imp 6.8. In series the current is the biggest factor, and it's only 0.4 Amps different here.

    Different Voltages in series can be a problem if one string is shaded and the other produces greater than reverse breakdown Voltage for the shaded string. This is a very unusual situation though.
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  • Desert Rat
    Desert Rat Solar Expert Posts: 147 ✭✭✭
    #6
    Re: Any way to combine these panels?

    OK, thank you Coot & BB.
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  • Desert Rat
    Desert Rat Solar Expert Posts: 147 ✭✭✭
    #7
    Re: Any way to combine these panels?

    I had another thought--What about all four panels in a single series string? 92.6V and 6.8A, right?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: Any way to combine these panels?
    Desert Rat wrote: »
    I had another thought--What about all four panels in a single series string? 92.6V and 6.8A, right?

    Although it would work technically you don't want to move the array Vmp that far away from battery Voltage as it reduces the efficiency of the charge controller. Another consideration is that with such a high Voltage array the Voc could exceed the charge controller's input maximum under cold conditions.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #9
    Re: Any way to combine these panels?
    WayneTPVS wrote: »
    Please provide the equation where a 28.71vmp panel and a 17.6vmp in series produces 46.31 vmp ??

    Panels connected in series. Vmp adds together: 28.71 + 17.6 = 46.31
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  • WayneTPVS
    WayneTPVS Registered Users Posts: 9 ✭
    #10
    Re: Any way to combine these panels?

    I am corrected, was thinking of something else, sorry..
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #11
    Re: Any way to combine these panels?

    It may just work exactly as it was. The 208W panels will only contribute to bulk charging and could bring the battery up to 26-27V. Then the two 120W panels would have to bring the battery up to the absorb voltage of 28.8V and pull it through absorb. Depending on what the loads are and how deeply they discharge the batteries daily, the 2 x 120W might be enough to absorb the battery.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #12
    Re: Any way to combine these panels?

    Don't bet on it.

    Panel Vmp is not the same as Voltage available to charge the batteries. Between loss of Voltage from high panel temperature and further loss through wiring there could be only 23-25 Volts left by the time you get to the battery. As that battery Voltage climbs the current will fall off and the charging will slow, so it could take longer than there is sun to do the job.

    Since the first thing the OP noticed was that the batteries were in fact not charging relying on known low Voltage to do the job wouldn't make sense. Fix it all up right.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #13
    Re: Any way to combine these panels?

    Depends. The OP noticed that the system wasn't working because the charge controller had failed. It may have been working fine up till then. Panel Vmp is also not the final voltage that the system can reach, although there is a steep drop off above Vmp, there is still current available there on those grid tie panels.

    If he only has the grid tie panels present, then I'd say there's no way it'll charge properly, but given that he's got 240W (30% of the total) of proper 24V off-grid panels in addition, it might just make it.
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