PV Output Help/Question

ki7fx
ki7fx Registered Users Posts: 10 ✭
in Solar Beginners Corner #1
Greetings again all...

Now that my system has been in production for about 24 days, I am wondering if I have missed something here.

My single panel is rated at 30.33v and 8.22a (per the mfg's spec sheet).

According to my Tristar MPPT60 controller, the panel is putting out (average) 30.22v and 2.6a in full sun.

I am running 160ft of 6ga copper from the panel to the controller - I expected more voltage loss, but not this much current loss.

Is this normal? I should be harvesting 250 watts, but only getting 77 watts...

thanks for any insight. Mike
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: PV Output Help/Question

    Well, yes it is normal.
    The thing is that the panel will only output full current if it needs to. It does not put out Vmp * Imp when the sun shines brightly on it.
    The MPPT controller will pick a Voltage and current from the panel that it thinks is right to provide the charge for the batteries.
    How deeply discharged are the batteries? If they don't need a lot of current they won't get a lot of current.
    That 78 Watts input (30.2 * 2.6) is 6.5 Amps @ 12 Volts or 5 Amps at Absorb of 14.6.
    For one 75 Amp hour battery you'd want a peak potential of 7.5 Amps, but that doesn't mean it will always need that much current to charge. The most you are likely to see from that 250 Watt panel is: 250 * 0.77 / 12 = 16 Amps. That would depend on the batteries being drawn down to a state of 50% charge too.
    You will only see maximum Voltage drop when current draw is high. Rough calculation: you'll lose about 1 Volt over that distance with 6 AWG running 30 Volts @ 8 Amps.
    Again the big question is: how are the batteries? If they're charged, no worries.
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  • ki7fx
    ki7fx Registered Users Posts: 10 ✭
    #3
    Re: PV Output Help/Question
    Cariboocoot wrote: »
    Well, yes it is normal.
    The thing is that the panel will only output full current if it needs to. It does not put out Vmp * Imp when the sun shines brightly on it.
    The MPPT controller will pick a Voltage and current from the panel that it thinks is right to provide the charge for the batteries.
    How deeply discharged are the batteries? If they don't need a lot of current they won't get a lot of current.
    That 78 Watts input (30.2 * 2.6) is 6.5 Amps @ 12 Volts or 5 Amps at Absorb of 14.6.
    For one 75 Amp hour battery you'd want a peak potential of 7.5 Amps, but that doesn't mean it will always need that much current to charge. The most you are likely to see from that 250 Watt panel is: 250 * 0.77 / 12 = 16 Amps. That would depend on the batteries being drawn down to a state of 50% charge too.
    You will only see maximum Voltage drop when current draw is high. Rough calculation: you'll lose about 1 Volt over that distance with 6 AWG running 30 Volts @ 8 Amps.
    Again the big question is: how are the batteries? If they're charged, no worries.

    That explains it perfectly. The batteries are fully charged, so not much is required of the panel at this point. As I am putting this project together, I am checking each part as I go. Now that I know the panel is working properly, my next step is to swap out the two 75ah batteries with four high quality batteries & add another matching panel. This should get me to where I need to be. As I mentioned in another thread, this is just a hobby/test setup for me, but like most hobbies, it's intended to grow into a full scale project at some point. Just have to work on the next phase.... Thanks a lot for the great info/assistance!!
    Mike
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