Voltage numbers don't seem to match between array and charge controller

HairfarmHairfarm Solar Expert Posts: 225 ✭✭✭
Greetings,

I've attached an image of my panel array config, my charge controller and my Kyocera specification chart. Please correct me if I'm wrong but illustration of my panel configuration shows that my array makes 70.8 Volts @ 15.2 amps for 1080 watts. My panels are Kyocera KD145.

My question is if my array makes 70.8 volts why does my charge controller show 87 volts coming in? What am I missing?


thanks!


Comments

  • HairfarmHairfarm Solar Expert Posts: 225 ✭✭✭
    I used to be able to insert images in my post but now I'm only able to attach them as separate documents.
  • littleharbor2littleharbor2 Solar Expert Posts: 1,405 ✭✭✭✭
    edited January 2017 #3
    You're floating and only consuming 0.3 amps. Very near open circuit voltage of 88.4.

    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, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • westbranchwestbranch Solar Expert Posts: 5,142 ✭✭✭✭
    edited January 2017 #4
    70.8V is its VMP voltage at its (optimal) power point, it IS producing more voltage than MPP as it is in FLOAT and the CC is reducing the amps going into the battery...  VOC,  over current voltage, is 22.1 x 4 = 88.4V

    Also it is winter and panels normally produce more V than the VMP says when they are below 25*C
     
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  • HairfarmHairfarm Solar Expert Posts: 225 ✭✭✭
    edited January 2017 #5
    Thanks littleharbor2 and westbranch! Looks like I have to go back to solar school again. 

    Another question: So open circuit voltage (VOC) is when there is NO load on the panels and maximum power voltage (VMPP) is when there IS a load on the panels? Is that too simplistic?

    And does the charge controller basically lower the array output to the batteries when in float mode causing VOC to happen instead of VMPP?

    Hope that makes sense.

    thanks


  • BB.BB. Super Moderators, Administrators Posts: 29,521 admin
    Sort of... Vmp is panel voltage at rated Imp (current maximum power).

    Solar panels are really current sources--Not voltage sources (like a battery).

    More or less, Imp is proportional to the amount of sunlight hitting the panel.

    So, if your current draw is less than [email protected] the panel voltage will be between Vmp and Voc.

    However, if current draw from the panel is over [email protected] you are simply drawing more current than the panel can produce at that moment in time, and the Vpanel will drop (and could collapse to zero volts).

    In reality... You have the I*V curve of the panel and the I*V curve of the load. As the panel voltage drops, the load will eventually draw less current.

    So, the panel operating voltage between Vmp and Zero volts is dependent on the load. For example, PWM solar charge controller and battery bank--The panel voltage will be pretty close to the battery bank voltage (when the PWM controller is passing maximum current to a partially charged battery bank).

    With MPPT controllers--More or less, the controller figures out the Vmp voltage of the solar array for that (say) five minutes in time. And the controller will vary the current to keep the solar panel voltage at Vmp.

    When charge controllers start to cut back on current/power draw from the solar array (battery is in absorb or float), the Vpanel will slowly rise as less current is drawn. Note--That charge controllers are "switching power supplies"--You may see "wondering" voltage readings on the panel voltage (like, Voc=20 volts, and Vmp=18 volts; or Vbatt~14.5 volts) as the digital volt meter "samples" and averages the measured voltage readings (which are changing 10's to 100's of times per second).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • HairfarmHairfarm Solar Expert Posts: 225 ✭✭✭

    And the controller will vary the current to keep the solar panel voltage at Vmp.
    It sounds like my MPPT CC has way to resist/release the array current when the batteries are charged/depleted. Interesting.

    When charge controllers start to cut back on current/power draw from the solar array (battery is in absorb or float), the Vpanel will slowly rise as less current is drawn.
    Thanks. This answers my question about my reading of 87 VOC at float.

    Thanks again B.B.!

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