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### Topic: PV cell energy monitoring (Read 2494 times)previous topic - next topic

#### dominacos

##### Jul 24, 2012, 12:50 am
Hi all, im planing to build a simple solar tracker connected to load/batteries, just to measure the efficiency of the system, so i need to measure the energy.
The thing is that i got to a solution, in which i'd rather use a voltage divider (measuring the voltage) and a simple resistor (shunt) for measuring the current, since the output of the panel will be DC.

The specs of the cell are as according :
voltage: 5v
current: 150mA
Power: 0.75W
Conversion rate: 17%

Mainly i have 2 problems.
The first is based on how can i calculate the values (resistors/caps) used to make the voltage divider.
And the second is how shoulda connect them with the arduino pins.

Any schematic would be more than appreciated.

#### WillR

#1
##### Jul 24, 2012, 03:28 am
I posted some links to basic electronics tutorials in the electronics forum:
http://arduino.cc/forum/index.php/topic,114925.0.html

The last post has specific references to links that have lessons on how to make a voltage divider.
Just another Hacker

#### dominacos

#2
##### Jul 24, 2012, 04:31 pm
Well the basic formula is Vo = r2/(r2+r1) x Vi.
But if i have Vo equal to 5V then voltage divider isnt necessary to measure the voltage. I can directly connect the positive wire to A0 for instance; am i thinkin' rite?

#### wildbill

#3
##### Jul 24, 2012, 04:50 pm
Yes, as long as you're absolutely certain that it won't exceed 5V.

#### wanderson

#4
##### Jul 24, 2012, 05:44 pm
Solar cell specs are a little tricky.  That spec might refer to the voltage output when the supply is providing 150ma in which case when the draw is less than that the voltage will be higher (possibly much higher).  Or it might be referring to the open (no load) supply voltage, in which case the voltage will be much lower as the load approaches 150ma

Solar cells do not supply regulated voltage
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#### dominacos

#5
##### Jul 24, 2012, 08:40 pm
@wanderson : well i'm kinda aware of the fact that pv cell is behaving as a current source rather than a voltage one.
So i was thinking of supplying a nominal load, in my case 150mA.

#### wanderson

#6
##### Jul 24, 2012, 08:52 pm
If you understand that the voltage will vary, not only with current demand but also available light thats good.  It also means that you can't use a simple resistive load for measuring the solar output.  You need a constant current load to do that.  This can be as simple as a lm317 wired up as a constant current load--see data sheet, but the following may give you some ideas on how to wire up the arduino to do both the constant current component as well as the voltage measuring; http://www.eevblog.com/2010/08/01/eevblog-102-diy-constant-current-dummy-load-for-power-supply-and-battery-testing/
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#### mauried

#7
##### Jul 25, 2012, 03:07 am
Are you trying to measure the maximum power output of the cell , or are you just trying to measure the variation over time .
Solar Cells only deliver maximum power at one point on their V / I graph, and using a simple load like a resistor or current sink
will not give you the absolute maximum power output.
It will work if all you are looking for is relative changes over time.
To extract the maximum power from a Solar cell you need some kind of peak power point tracking.
This explains what the problem with Solar cells is and why peak power tracking is needed.
http://en.wikipedia.org/wiki/Maximum_power_point_tracking

#### dominacos

#8
##### Jul 29, 2012, 07:18 am
@wanderson, tho i liked that dummy load i think its a lil' expensive for my prototype, and i'll look around for cheaper solutions therefore.
@mauried, regarding to the measuring of the power, i need the maximum available as u've highlighted it.
So i think im gonna need something like a mppt charger,which actually could be used both for driving any load e.g. motor/led array/whatever [so i wont need dummyload anymore - rite?], as well as for collecting the data (voltage,current) to form a graph later.

#### PeterH

#9
##### Jul 30, 2012, 11:59 pm

Hi all, im planing to build a simple solar tracker connected to load/batteries, just to measure the efficiency of the system, so i need to measure the energy.

The power output you're measuring is very small. This may seem a daft question, but have you realised that the Arduino will likely consume more power than your cell is producing?
I only provide help via the forum - please do not contact me for private consultancy.

#### wanderson

#10
##### Jul 31, 2012, 12:24 am

@wanderson, tho i liked that dummy load i think its a lil' expensive for my prototype, and i'll look around for cheaper solutions therefore.
...
So i think im gonna need something like a mppt charger,which ...

If you think that dummy load circuit is expensive (could be done for less than \$10), you might want to reconsider the whole project. Developing  MPPT charger circuits would be orders of magnitude more expensive...
New true random number library available at: http://code.google.com/p/avr-hardware-random-number-generation/

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