Wednesday, April 3, 2013

Chart Your Solar Panel Voltage

A friend of mine gave me some of his old photovoltaic, (PV), solar panels.  The frame they were on had disintegrated and they had not been used in a long time.  I was thrilled to have them.  I decided it would be good to characterize them simply by following their voltages over a few days.  I laid them out in the yard and ran a cable back to my shop a few feet away.  The series connected voltage measured with a DVM showed it to be in the expected range, never to exceed about 20 volts. 

A new life for some old PV cells.

An old, junker laptop was available, so I combined it with a LabJack U3 - LV  analog to digital converter (ADC)  and my Radio-SkyPipe data logging program. The LabJack is really more than just an ADC. While here I am only interested in measuring the voltage from the panel the LabJack U3 can detect digtal states on some of its inputs and can output both digital and analog signals.    The input range for the U3-LV (Low Voltage) is 0 to 2.4V in the singled ended configuration so I needed to reduce the voltage from the panels to fit in this range.  A resistive voltage divider was called for.  I decided to just use a 5K pot (variable resistor) instead of a calculator and a couple of fixed resistors.  Using the pot has the advantage that it can be changed if you would like to alter the range or compensate for a impedance other than that presented by the LabJack analog input. 

LabJack U3-LV fed from a pot used as a voltage divider attached to the PV panel output.
Before hooking anything to the LabJack input:  The outer terminals on the pot should temporarily go to the positive and ground of a regulated variable DC power supply.  I adjusted the power supply to 20V and then the wiper arm of the pot so that it read 2.4 V.  Only then connect the wiper arm to the LabJack analog input. You may need to slightly retweak the pot after the connection is made.

This arrangement means that when read by the ADC,  20V will be produce a reading of 4095.  Each step from zero to the full input voltage is 20/4096 = 0.0048828125 Volts.  Forgive the ridiculous precision.  I wanted to read the voltage in volts, not the number of 0.0048828125ths of a volt, so I used the Equation feature of Radio-SkyPipe that allows you to apply a function to the data read from the ADC. The Equation function is simply X*0.0048828125 .

It would be a simple matter to extend the functionality of this arrangement so that it could function as a Battery Charge Controller.  I mentioned above that the LabJack U3 has a number of digital ports.  We can use one these output ports to toggle a relay that connects the panels to the storage battery.  Hidden away in the Radio-SkyPipe program on the Misc. tab of Options, you will find the Triggers button.  You may configure triggers that are activated by incoming chart data (in this case the PV output voltage).  These triggers effect real world outputs, like the LabJack output we are using to control the battery charging relay.  When the PV voltage drops below some minimal charging voltage, the relay opens disconnecting the voltage source from the battery. The photocells actually can draw current away from the battery when they are not adequately illuminated. A diode in series with the + line from the panels can also do the job at the sacrifice of the 0.7V or so that drops across the diode junction.  A second Trigger may be configured to remove the PV charging voltage when it exceeds a limit corresponding to overcharging.  Overcharging can seriously affect battery life.  I am no battery expert, so I will leave it to you to research the voltage thresholds you want to use.

Whether or not you use RSP as a charge controller, it can be informative to monitor the voltages in your PV system over days or even over seasons.  This information can be helpful in making adjustments that optimize your energy collection and usage.