Solar Panel Voltage

keyturbocars
keyturbocars Solar Expert Posts: 375 ✭✭
in Solar Beginners Corner #1
On a typical solar panel, does the voltage and/or the amperage vary as the amount of solar energy hitting the panel varies?

In other words, let's say I have a solar panels with Vmp=18.3V and Voc=22.8V. If there are some clouds that block the sun, or the panel angle on a fixed array is not ideal for the sun's position, will the panel voltage drop or does the current drop (or both)?

I know that the power output drops (P=IxV) but I wasn't sure if it was the V or the I that drops when power output on a panel drops.

Just trying to better understand solar panels.
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #2
    Re: Solar Panel Voltage

    If you watch in the morning (dawn) you will first see V rise slowly and finally when the sun is high enough A will start to kick up to MPP. As you are aware V can go higher than MPP in cold weather.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,639 admin
    #3
    Re: Solar Panel Voltage

    More or less--Vmp, once you have enough sunlight, is dependent on temperature. And Imp is dependent on the amount of sun.

    A PWM controller runs Varray to ~Vbatt-charging. And Imp ~ Amount of sunlight (watts/sqmeter) falling on panel. As battery reaches full charge--The PWM controller starts turning "on and off" the arrray to battery connection.

    MPPT controllers, when battery is low, attempts to control current from the solar array and maximize Pmp=Vmp*Imp. You will see Vmp, more or less, vary with temperature of array. And Imp will, more or less, follow the amount of sun. Once the battery nears full charge, the MPPT controller behaves like a PWM controller instead (turning on and off current to the battery bank (or averaging lower current flow).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #4
    Re: Solar Panel Voltage

    Thanks for the response guys. I'm playing around with the idea in my mind of adding a small array of 920W (4 x 230W) to my existing solar and wind system. I've found some panels that have Vmp = 30.0V. I was thinking of wiring 2 panels in series and then the 2 sets of 2 in parallel for a Vmp of 60.0V on the array. I was contemplating running this small array straight to my 48V battery bank with no controller. My system is set up to divert all excess power to an auxilliary water heater. Essentially, I keep my battery bank in float (53.6V) and all power is used to heat water. So, my existing diversion system would be able to regulate the incoming power from the additional 920W array.

    The question I had in my mind would be whether or not the Vmp = 60.0V would be enough under most conditions. As far as battery bank voltage goes, it will just float at 53.6V most of the time. I've got Lifeline AGM's. If I needed to go off grid and run off battery power, and then later charge up my battery bank, I'd have a maximum of 57.6V charging voltage. No equalization planned on these AGM batteries. So, I'm thinking that the Vmp of 60.0V could work in theory, unless the panel voltages will be lower than I'm thinking.

    Assuming I was thinking about not using a controller (PWM or MPPT), I figured this could be most efficient if the difference in voltage between the array voltage and battery bank voltage was as close as possible, but still having enough headroom on the array voltage to ensure charging. I'm just concerned that this is too small of a delta V to ensure good charging under most conditions. What do you think?
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #5
    Re: Solar Panel Voltage

    What do the manuals say about supplying > 60 volts to those batteries, I wouldn't want to do in 16 of them.

    Personally I would not do it without a CC. I asked the same Q about using a small 25w panel on my camper battery and ended up getting a PWM CC based on the feed back http://forum.solar-electric.com/showthread.php?11704-Charging-without-a-Charge-controller&highlight=Charge+controller
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #6
    Re: Solar Panel Voltage

    Thanks for the feedback, Eric. The batteries would not see 60 volts. The Classic 150 that controls my wind turbine is also set up (through it's AUX output) as a diversion controller to pulse (PWM) a set of 2 100A solid state relays that sends power to my auxillary water heater. So, using my Classic being set up as a diversion controller is what will limit the voltage that my battery bank sees. The Classic and my diversion system is what keeps my battery bank in float mode (53.6V) under normal conditions.

    PS. I should clarify that when I say "connect directly to the battery bank", I do mean that I would still have a circuit breaker and a blocking diode.
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #7
    Re: Solar Panel Voltage

    The batteries would hold the voltage down to whatever the battery voltage happened to be, and the current the panels would dump into the batteries would depend on the amount of light hitting the panels.
    However, with the batteries at 54 volts, and only 60 volts available from the panels, you may not get a lot of current flow.
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #8
    Re: Solar Panel Voltage

    You are way ahead of me, I thought that one Classic could only handle one source, Wind OR Solar...http://midnitesolar.com/smf_forum/index.php?topic=100.0
    Must be missing something in your setup...??
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    #9
    Re: Solar Panel Voltage
    keyturbocars wrote: »
    I was contemplating running this small (940 watt) array straight to my 48V battery bank with no controller. What do you think?

    This is why I answered as I did. But I assume from other info, (My system is set up to divert all excess power to an auxiliary water heater.) that you have a controller hooked up for diversion control to prevent excess battery voltage.
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #10
    Re: Solar Panel Voltage
    westbranch wrote: »
    You are way ahead of me, I thought that one Classic could only handle one source, Wind OR Solar...http://midnitesolar.com/smf_forum/index.php?topic=100.0
    Must be missing something in your setup...??

    You are right, that the power input to the Classic can only be wind, solar, or hydro. However, the Classic has AUX1 and AUX2 that can be used to perform other functions. I use AUX2 on the Classic to control my diversion system to keep battery voltage at a preset float level under most conditions. It works very well.
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #11
    Re: Solar Panel Voltage
    waynefromnscanada wrote: »
    This is why I answered as I did. But I assume from other info, (My system is set up to divert all excess power to an auxiliary water heater.) that you have a controller hooked up for diversion control to prevent excess battery voltage.

    Yes, I do use my Classic 150 AUX2 output as diversion control. I think you are right that a Vmp=60V might be shaving it too close to the battery bank voltage. If I was confident that I would have 60V most of the time, then I think it could work with my float of 53.6V and 57.6V maximum charging voltage, but I suspect that the voltage would not always be at Vmp, and so I would risk not being able to charge under some conditions. I'm just trying to figure out a way to do this addition as inexpensively as possible, but obviously I'm not saving anything if it doesn't work as planned.

    Below is a picture of my Classic that handles my wind turbine input and also my diversion solid state relay (SSR) that sends power to a water heater. I originally had a single 100A SSR as seen in this picture at the bottom of the large heat sink, but since then I have added a second 100A SSR wired in parallel, because I wanted a greater factor of safety. When the Classic AUX2 PWM pulse width is at 100% duty cycle, then I am diverting right around 102A. This is too close to the 100A limit for a single SSR, so I divided the load up over 2 SSR's.

    home-wind-turbine-control.jpg
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  • ggunn
    ggunn Solar Expert Posts: 1,973 ✭✭✭
    #12
    Re: Solar Panel Voltage
    keyturbocars wrote: »
    On a typical solar panel, does the voltage and/or the amperage vary as the amount of solar energy hitting the panel varies?

    In other words, let's say I have a solar panels with Vmp=18.3V and Voc=22.8V. If there are some clouds that block the sun, or the panel angle on a fixed array is not ideal for the sun's position, will the panel voltage drop or does the current drop (or both)?

    I know that the power output drops (P=IxV) but I wasn't sure if it was the V or the I that drops when power output on a panel drops.

    Just trying to better understand solar panels.
    In general, and discounting temperature effects, it's almost entirely the current that varies with the intensity of solar radiation.
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  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    #13
    Re: Solar Panel Voltage

    Key, I assume you have that 'nude' classic 'defrocked' for ambient cooling.

    Do your kids ever get close to it? (safety?)
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #14
    Re: Solar Panel Voltage
    ggunn wrote: »
    In general, and discounting temperature effects, it's almost entirely the current that varies with the intensity of solar radiation.

    Thanks for confirming that ggun.
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  • keyturbocars
    keyturbocars Solar Expert Posts: 375 ✭✭
    #15
    Re: Solar Panel Voltage
    westbranch wrote: »
    Key, I assume you have that 'nude' classic 'defrocked' for ambient cooling.

    Do your kids ever get close to it? (safety?)

    Eric, That's an older picture that I snapped while I was working on the system. The front cover is normally in place. The Classic does very well at keeping itself cool. Even when the wind is roaring and it's handling 2000-4000+ watts, the main fans only cycle on/off periodically. Once in a while if the winds are really strong, the turbo fan will keep in to give some extra cooling to the FETs. Good design.
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  • Plucka
    Plucka Registered Users Posts: 130 ✭✭
    #16
    Is the answer to the original question YES or NO.
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  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,113 ✭✭✭✭✭
    #17
    Plucka said:
    Is the answer to the original question YES or NO.
    YES, current is more affected than voltage but voltage will vary with orientation and solar intensity.  Add cell temperature effects and you will get a pretty good variation in voltage output.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

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