panel size vs. controller capacity

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rama
rama Solar Expert Posts: 36 ✭
in Solar Beginners Corner #1
I was chatting with a buddy of mine and found out he's got this problem while testing with his controller. He's borrowed this 2*180w panel (in series) with an mppt controller (max pv input 1800W) and while running a test using these 2 modules with the controller he recorded very low current values. I advised him to test each of the modules out first using a meter to ensure during open circuit conditions he'd get reasonable values of Voc 32.51 and Isc 7.64. Sounded to me he got values pretty close to to the manufacturer's spec (so ruling out module problem), but on the controller LCD he's got readings like 56~58V, but current of only 2.9A, which is just about half of the modules' output power.

I made sure he checked the weather, module voltage/current, tilt angle (=altitude) and array was facing south so these shouldn't be an issue.

I recalled reading somewhere before that if you hook up undersized panels relative to the controller's capacity, the controller would be working at low efficiency and supposedly mppt tracking will also drain away some energy. I know about panel derating, too, but this reading seems terribly low.

Does this seem right and has anyone ever experienced this?

*some background: he's borrowed the modules and controller from a seller as testing sample. Controller is not a major brand (ablerex) but it's supposedly new ...
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
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    Re: panel size vs. controller capacity

    I'm not sure exactly what you're saying the problem is but ...

    The maximum current from the panels is not the same thing as the current the charge controller will supply to the batteries. Especially not with MPPT, which can "down convert" higher Voltage to greater current.

    As the batteries charge, the current goes down no matter what.

    The 32 Voc is wired in series to feed the controller? That's the only way to get 58V output. What values the controller is set at (Absorb and Float Voltages) will dictate how much current flows when that state is reached. If you have two 32 Voc panels wired in parallel to the controller, the output to a 48 Volt (nominal - 56 to 58 V charging) will be nil; you must start out with more than battery Voltage to do any charging. Not even an MPPT will "upconvert" Voltage to higher output.

    I think we could do with a few more system details to know what is going on and what should be going on.
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  • rama
    rama Solar Expert Posts: 36 ✭
    #3
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    Re: panel size vs. controller capacity

    hi coot, at the time of testing no batteries were connected and yes i understand the current going out from the controller would vary depending on battery setup and status etc.. but the controller itself auto-initialize when it reaches 40V and displays all sorts of PV input info automatically, so we didn't even get to entering battery info.

    if we were to leave out the batteries for a sec, should i be expecting such low efficiency from a relatively large controller given an undersized array?
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  • rama
    rama Solar Expert Posts: 36 ✭
    #4
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    Re: panel size vs. controller capacity

    what i find confusing, i guess, is that the modules themselves tested ok, but yet when i plug them into the controller (without battery) the initial readings are so low... so i'm just wondering if this is normal and whether by increasing array size will improve my reading (by increasing controller's working efficiency)?

    i tried contacting the manufacturer for 5 days now but they have not yet responded...
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #5
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    Re: panel size vs. controller capacity

    Charge controllers rarely work when not connected to a battery bank--And probably no MPPT type charge controller will work at all...

    I am confused, where is the power going if there is no battery bank? Did you have other loads connected directly to the controller?

    Is this the Ablerex charge controller you are working with?

    You probably want to be very careful running the charge controller without a battery bank--frequently they configure themselves to 12/24/48 volts based on the input voltage from a "live" battery. Without a stable battery connected--I don't know what output level the charge controller may self configure to...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • rama
    rama Solar Expert Posts: 36 ✭
    #6
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    Re: panel size vs. controller capacity

    hi Bill - ok yeah i see your point. i am not sure what he did with loads when he tested them. i'll see if he can tested it with a working battery and see whether same thing happens again.

    yes the link you sent was the same controller.
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