Confused about VOC & VMP and AMPs

hotdawg23
hotdawg23 Solar Expert Posts: 32 ✭
in Solar Beginners Corner #1
I am now to the point where i can put the finishing touches on my magic test panel. However, i am stumped on the differences and values of VOC and VMP.
Here is where i am on the panel. 3 columns of 13 cells each. Cells are rated at .5V and 1.98 watts, resulting in AMPS of 3.98.
Each column tests out at 6.75 volts on the DVM and the panel tests at 19.75 volts. (I do not know what happened to the other half volt) This test is done without any type of load, resister, diodes etc.
So, What do these volt readings represent? VOC or VMP?
What would the mfg rating be called?
How does one calculate VOC and VMP?

Next question is about AMPS. The mfg rating says one cell is 3.98 amps. So what would the amps be for the whole panel of 39 cells? Is IMP same as AMP?

Thanks to all. Don't know what a fellow would do without this forum.
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  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    #2
    Re: Confused about VOC & VMP and AMPs

    oh my.

    You NEED to know Ohms Law. Know it, not just look it up 2x a day. And understand it, and why it works. Then this other stuff starts to make sense.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #3
    Re: Confused about VOC & VMP and AMPs

    It helps to know what the alphabet soup of terms mean too: http://forum.solar-electric.com/showthread.php?t=6136

    Voc = Voltage open circuit - nothing attached to the panel.
    Vmp = Voltage maximum power - what the Voltage would be under ideal conditions with the maximum output current (good luck measuring this in the real world).
    Imp = Amps maximum power. The 'I' comes from an old term "Intensity" - before Monsieur André-Marie Ampère got his name on it. In theory, Imp * Vmp = panel Watts. There's an awful lot in the real world that can affect that.
    Amp = short for Ampere. Of course if you're a certain foreign manufacturer of cheap charge controllers the long term, it appears, is "Amplifier". :p Haven't stopped laughing at that one yet! Nothing like putting 60 Amplifiers into a battery to charge it up quick, eh? :p
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  • RCinFLA
    RCinFLA Solar Expert Posts: 1,485 ✭✭✭✭
    #4
    Re: Confused about VOC & VMP and AMPs

    You have either got some poor cells or partially shaded sections.

    A PV cell is an illumination based current source capped in maximum voltage by the inherent diode of the cell.

    Short circuited with full sun exposure you will read the full illumination current generated. (Isc)

    Open circuit and all the illumination current is shunted down the cell's inherent diode. For silicon at 25 deg C this should be yield about 0.62v to 0.65 vdc for the inherent diode conduction voltage. (Voc)

    To get maximum power you want to load the cell to allow the maximum voltage with just a small amount of the illumination current going down the cell's inherent diode. This is Vmpp. Impp is the current yielded as output. (lower then Isc)

    Real cells have some series resistance due to current collection to the metal fingers and resistance of the metalization fingers. This causes the Vmpp to pull back to a lower voltage due to the Impp X Rs of cell.

    The real crap shoot, especially when buying cells on internet is the defect density of the cell causing shunt resistance points. All cells have them. Poor quality, reject cells have a lot. This makes a shunt resistance across the cell that can range from over 1000 ohms to less then a few ohms. This will show in low light current output. If you want to measure it, apply a reverse voltage, while cell is covered with no light, each cell individually, up to 10 vdc. Anything with greater then 1 amp at 10 vdc reverse bias is a poor cell. You may feel a hot spot on the cell where worse defect(s) are. You can do this on each cell individually while still connected in series to other cells and panel open circuit. A handheld contactless IR temp sensor is good to check cell temp.

    Your Voc at 25 deg C should be about 0.62 volt X 39 cells = 24.2 vdc.

    Your loaded Vmpp at 25 deg C should be about 0.51 X 39 cells = 19.9 vdc.

    When the cells get hot in the sun the voltage will drop about 0.33% for every deg C above 25 deg C.
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #5
    Re: Confused about VOC & VMP and AMPs
    hotdawg23 wrote: »
    I am now to the point where i can put the finishing touches on my magic test panel. However, i am stumped on the differences and values of VOC and VMP.
    Here is where i am on the panel. 3 columns of 13 cells each. Cells are rated at .5V and 1.98 watts, resulting in AMPS of 3.98.
    Each column tests out at 6.75 volts on the DVM and the panel tests at 19.75 volts. (I do not know what happened to the other half volt) This test is done without any type of load, resister, diodes etc.
    So, What do these volt readings represent? VOC or VMP?
    What would the mfg rating be called?
    How does one calculate VOC and VMP?

    Next question is about AMPS. The mfg rating says one cell is 3.98 amps. So what would the amps be for the whole panel of 39 cells? Is IMP same as AMP?

    Thanks to all. Don't know what a fellow would do without this forum.
    I'll give it a try. Voc (open circuit voltage) and Isc (short circuit current) can be easily measured as long as you ignore temperature effects. Vmp (maximum power voltage) and Imp (maximum power current) cannot. Voc is the voltage you see with a voltmeter across the terminals with nothing else connected. Isc is the current you measure with the + and - terminals connected together and a clamp ammeter around one of the wires or with an ammeter in series. The voltage pretty much is what it is, but the current varies directly with insolation, so you'll also need a pyranometer to measure the strength of the sunlight and normalize your measurement to 1000W/m2, although in "full sun" with the panel pointed directly at the sun, it will be pretty close. Keep your panel out of the sun and cool until you are ready to take your readings, and then do it quickly because the voltage will start dropping when the panel starts to heat up.

    To measure Imp and Vmp you would have to have the panels connected to a MPPT (maximum power point tracking) inverter or charge controller with enough of them in series to get the voltage into the tracking range of the device, and even then if the conditions are not 1000W/m2 and cell temperature 25 degrees C, the readings you get will be off, potentially by quite a lot.

    To get really accurate measurements of these values, you'd really need a test bed with tight control of temperature and light frequency and intensity.
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  • cbonansea
    cbonansea Registered Users Posts: 1 ✭
    #6
    Hi! I'm translating an user's manual of a solar inverter and I've found  the terms Vmpp and Vmp, but I don't understand the difference between them. I've been reading about it and I know that Vmp is voltage maximum power, so what does Vmpp mean? This is the sentence I'm having trouble with: "Max Power Voltage (Vmpp) of PV modules should be close to best Vmp of inverter or within Vmp range to get best performance".
    Thank you in advance!
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  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    #7
    I think the two are similar. Vmp is voltage at maximum output under specific test conditions (eg STC). This number can vary a bit under different conditions though. For example, voltage at max power can be higher at lower temperature. An mppt controller can find the best combination of voltage (Vmpp) and current to maximize output wattage under these varying conditions.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
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  • jonr
    jonr Solar Expert Posts: 1,386 ✭✭✭✭
    #8
    As far as I know, the terms are interchangeable.  But in your case, they are differentiating between the panels and the inverter (you want to operate at a voltage that is good for both).

    I am available for custom hardware/firmware development

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