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pablo

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Posts: 7
Reply with quote  #1 
Hi everyone,

I am new to solar power.  I think I have a misunderstanding on how everything works.  

Thank you all for any insight(s).  I'll apologize in advance for subsequent dumb questions I am sure I'll be asking!  [smile]
-pablo

Summary
Why doesn't my Open Voltage tests come close to what the MPPT's status display reports?

My Open Voltage test shows roughly 32+ VDC/ 6.8 amps (the sun wasn't great) yet the MPTT's status display only reports 31+ VDC / 2 amps.

Details
I have a cabin (cottage) which we only use in the summer. 

I have the following set up:
  • Two Costco Golf Cart batteries connected in series for 12v/220 AH.
  • A 40 amp controller[2]
  • Two 280 watt solar panels[1].  Due to minimal sun, they are connected in parallel.
  • 100' 12g run between the solar panels and the controller
By design there is no inverter as all my needs are 12VDC.

Performing an Open Voltage test near the controller (literally 12" away) I see roughly 32+ VDC/ 6.8 amps (when the sun was so-so).  However, the MPPT's display shows considerably less:  31+ VDC / 2 amps.  

I would have expected roughly 200+ watts but I'm not seeing that.  If I generate more than 62 watts of load, I start to draw from the battery.

Questions

  • Given the above, do I have a problem with my controller?
  • Do I have a misunderstanding of how things work?  (Quite possible!  [smile]
  • Other[smile]

References

[1] - Canadian Solar - http://napelem.net/napelem_arak/adatlap/Canadian_Solar-CS6K-M_en.pdf
[2] - http://hespv.ca/all-brands/epsolar/eps-mppt-4215bn

Rick H Parker

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Posts: 725
Reply with quote  #2 
My Open Voltage test shows roughly 32+ VDC/ 6.8 amps (the sun wasn't great) yet the MPTT's status display only reports 31+ VDC / 2 amps.

T
here is something not right about Open voltage 32V @ 6.6A. I suspect at least some of your problem lies in your understanding. How did you measure 32V @ 6.8A?
The term "open voltage" refers to the voltage of a open circuit. That is the voltage when no current is flowing. Therefore an open voltage of 32V @ 6.8A is no open voltage.


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Rick H Parker
Kansas, USA
Electronics Engineering Technologist
pablo

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Posts: 7
Reply with quote  #3 
Hi Rick,

I measured Open Voltage by following the manufacture's process.
  1. Test the panel in isolation with a volt meter
  2. Measure voltage - set my volt meter to 200 DC max
  3. Measure amperage - set my volt meter to 20 amps DC max
Thx!
-pablo
Rick H Parker

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Posts: 725
Reply with quote  #4 
You are measuring open voltage and short circuit current. Totally different condition then what your controller's panel is measuring, you're comparing apples to oranges. That explains the difference between the difference between 32+ VDC/ 6.8 amps and 31+ VDC @ 2 amps.

31V @ 2A = 62 Watts ... right in line with your other observation.

How do you have the panels mounded?

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Rick H Parker
Kansas, USA
Electronics Engineering Technologist
pablo

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Posts: 7
Reply with quote  #5 
Hey Rick,

The question in my mind is why is there such a huge disparity between the Open Voltage/Short-Circuit tests versus what the MPPT is reporting?

As for how the panels are mounted, are you asking something other than what I originally posted?  "Two 280 watt solar panels.  Due to minimal sun, they are connected in parallel"

My apologies if I'm being dense!  [smile]

Thx!
-pablo

Rick H Parker

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Reply with quote  #6 
The question in my mind is why is there such a huge disparity between the Open Voltage/Short-Circuit tests versus what the MPPT is reporting?


Most likely is is an issue of available power. If you had done the short circuit current test with two meters, one measuring current and the other voltage, you would of seen the voltage dropped from 32V to 9.4V. 9.4V * 6.6A = 62 watts. 

As for how the panels are mounted, are you asking something other than what I originally posted?

The word Mounted does refer to the physical arrangement (Mechanics) not the electrical wiring.




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Rick H Parker
Kansas, USA
Electronics Engineering Technologist
pablo

Registered:
Posts: 7
Reply with quote  #7 
Quote:
Most likely is is an issue of available power. If you had done the short circuit current test with two meters, one measuring current and the other voltage, you would of seen the voltage dropped from 32V to 9.4V. 9.4V * 6.6A = 62 watts. 


As I have two volt meters, I'll be up this weekend and I'll run the above test

Quote:
The word Mounted does refer to the physical arrangement (Mechanics) not the electrical wiring. 


Gotcha.  I have them mounted on custom frames.

Thx!
-pablo
stmbtwle

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Reply with quote  #8 
Bear in mind also that it is a charge CONTROLLER. It will not force full power into a fully charged (or nearly so) battery. If it did it would destroy your batteries in short order. Load your batteries overnight, the next day you should see a higher output.

If you install a couple more batteries, you will get more out of your panels as there is more capacity.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
pablo

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Posts: 7
Reply with quote  #9 
Hi Willie,

Thank you for your information.  Yes, you're right and I've observed that:  with little demand on the system, I see a reduction on what the controller tells me is available on the PV side (of its display).

The part that I'm not 100% is why if I increase the demand (load + battery), I don't see the full wattage as I measured in the Open Voltage/Short-Circuit test.

What Rick is saying is if I use two meters during my tests, I'll see the real wattage coming from the panels.  I plan on doing that test this weekend.

Cheers!
-pablo
Rick H Parker

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Reply with quote  #10 
I don't see the full wattage as I measured in the Open Voltage/Short-Circuit test.

You did not measure the wattage in that test. To measure the wattage you need to take a voltage and current measurement SIMULTANEOUSLY then calculate the wattage. That is why I am directing you towards doing the the short circuit current test with two meters.  

For the sake of clarity, the open voltage wattage is always zero because the is no current in an open circuit. When you switch the meter to current mode it forms a closed circuit.  Because the open voltage is from one circuit and the short circuit current is from another circuit, one cannot use the two to calculate wattage. 

Gotcha.  I have them mounted on custom frames.

Elaborate .... which direction do they face, angle from the horizon.

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Rick H Parker
Kansas, USA
Electronics Engineering Technologist
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