Measure DC voltage and current

hanz
hanz Registered Users Posts: 2 ✭
in Solar Beginners Corner #1
hello..i want to know the right way to measure Vdc and Idc from pv panel..i attach the picture so that it can help more explanation..what is difference between voc and vdc??it is same??from the picture, am i right??thank you
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #2
    VDC = Voltage Direct Current. Simply the voltage measured with a volt meter set to DC scale.
    Voc = Voltage Open Circuit measured on solar panel under sun (no loads connected).
    Vmp = Voltage Maximum Power is voltage measured when the solar panel is under "optimum load".
    Isc = Short Circuit Current. The amount of DC current measured when the panel is short circuited under "noon time" sun.
    Imp = Maximum Power Current. The optimum Pmp=Vmp*Imp under "noon time" sun.

    Note that Solar panels are not "voltage sources" like a lead acid battery (battery tries to hold "12 volts" no matter the load).

    Solar panels are (more or less) a "current source". The solar panel will (under noon time sun) output a near constant current between Vmp and Zero volts. The exact output current and voltage will also vary based on PWM (pulse width modulation) or MPPT (maximum power point tracking) solar charge controllers (and battery voltage, amount of sun,  of/solar cell, amount of charging current the battery needs, etc.).

    The panel's output current is proportional to the amount of sun hitting the panel. First sun (very weak) is very little current. Under noon time sun, then you have Imp and Isc current rating.

    The second drawing would normally have a solar charge controller between the solar panel and the battery. Because you have no charge controller in your drawing, the solar panel could over charge the battery (take it over 100% full and damage the battery). In drawing 2, the solar panel voltage will be Vbattery (whatever the battery voltage is at that moment in time) and roughly Imp (if under full noon time sun).

    And the AC inverter would be connected directly to the battery terminal/battery bus.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • hanz
    hanz Registered Users Posts: 2 ✭
    #3

    thanks..

    i attach here a switch box of standalone system of my pv panel..

    firstly, i want to measure vdc..right now, i take the terminal + and - from the pv(at incoming part below) and measure it..i get about 18 v dpend on sun..from this terminal, i connected to voltage divider so that it will drop to 5v and  that value  the arduino can read it..so far, the matlab software can read this value..i still confuse this way i get value vdc or voc??..i will take the sensor value and measure value to make analysis..

    secondly, how i can get value of idc??it is simple just i take that terminal + and - and connected to current sensor acs 712(30amp)  and to the arduino?? 

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  • BB.
    BB. Super Moderators, Administrators Posts: 33,642 admin
    #4
    Voc is what the solar panel outputs with no load (volt meter directly connected to solar panel--no other loads/connections).

    Vdc is simply the measurement of the panel voltage by a volt meter (loaded/unloaded/etc.). No other meaning in "solar" terms.

    There are other panel voltage terms... Vmp (voltage maximum power) and such. But those depend on amount of sun/type of solar charge controller you have, etc. PWM controllers will have one Vdc measurement. MPPT type charge controllers will look different yet. And what the solar charge controller "demands" from the panels (based on loads/amount of sun/battery state of charge, etc.) also will affect the instantaneous VDC readings of the panel.

    Measuring current flow in DC power systems is usually done with a large precision resistor that gives you xyz millivolts per amp. And you use a precision volt meter (100 to 500 mVolt full scale) to measure the voltage drop:

    V=I*R
    Deltec 100 amp 100 millivolt current shunt
    Deltec 100 amp, 100 millivolt current shunt

    Connect your negative battery cable to one of the large studs on top. And your loads/charging source to the other stud. Connect your millivolt meter to the two screws on the side (Kelvin Contacts).

    https://en.wikipedia.org/wiki/Four-terminal_sensing

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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