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

Hairfarm · January 2017

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!

Hairfarm · January 2017

I used to be able to insert images in my post but now I'm only able to attach them as separate documents.

littleharbor2 · January 2017

You're floating and only consuming 0.3 amps. Very near open circuit voltage of 88.4.

westbranch · January 2017

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

Hairfarm · January 2017

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. · January 2017

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 Imp@sunlight--Then the panel voltage will be between Vmp and Voc.

However, if current draw from the panel is over Imp@sunlight--Then 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

Hairfarm · January 2017

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.!

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

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