Logging harvested solar power using $15 Scorpion Board

It’s undeniable that South Africa is experiencing a critical electricity crisis. On the positive side, it’s forcing us to conserve and consider alternative sources of energy. My mom sponsored our household with a cheap Chinese solar panel with battery pack and LED lights to use during load shedding (we live in Cape Town zone 6 and you can find the schedule here).

This made me wonder: how much solar power does this system harvest in one day? Enter my handy $15 Scorpion Board. I built a cheap current sensor board (using a Diodes ZXCT1051 low side current sensor IC). Schematic:

Then I populated the Scorpion Board with the optional Lithium battery connector and charger, connected the current sensor board and calibrated it using a bench supply and electronic load. This part is easy. Using the command line ADC Data Logger App I set the ADC scale to 1.0 and the offset to 0.0 of each channel and set the board to sample each channel continuously:

>log fit 0 1.0 0.0

>log fit 1 1.0 0.0

>log fit 2 1.0 0.0

>log fit 3 1.0 0.0

>log s

The output values correspond to the raw ADC values and I noted the ADC value for different voltages and load currents. This data was captured in a spreadsheet and a linear fit performed:

The voltage measurement is super accurate (<0.3%) and the current measurement accurate enough (<2.5%). Now I have the calibration values and can set them as follows (ADC#0 is used for current and ADC#2 is used for voltage; I’m not interested in ADC#1 and ADC#3 so the scale and offset is set to zero):

>log fit 0 1.100399391 -5.277095671

> log fit 1 0 0

> log fit 2 35.4527418 50.83118688

> log fit 3 0 0

Nothing left but to set the date & time, the logging interval to 60 seconds, and start logging:

>rtc w 15-05-27 06:30:00

>log samples 60000

>log s

Here is a picture of the solar charger system with the power logger, before I placed it on my roof:

At the end of the day, I stopped logging by pressing the user button and dumped the data:

>log ls

File 1 [15-05-27 06:30:37]

>log dump 1

I imported the data into a spreadsheet and here’s the resulting graph:

The solar panel specs are:

• Peak Power: 2W
• Maximum Power Current: 0.23A
• Maximum Power Voltage: 9V
• Short Circuit Current: 0.267A
• Open Circuit Voltage: 11.25V

The system harvested 528mAh (at about 7.4V), or more accurately 4.1Wh (V x I x t) with mist in the morning and some clouds during the day. The peak power harvested was 1.73W. It can be observed that the system was not harvesting from the solar panel at it’s Maximum Power Point (MPPT).

It is the start of winter and rain is forecast for the whole week, so I will have to wait some time to repeat the run on a perfectly sunny day. Lesson learned: as an engineer, be realistic / pessimistic about the power that you can actually harvest versus the maximum / idealistic figures advertised for the product. To measure is to know…

The Scorpion Board was perfect for the job. It’s low price, small size, low power consumption and big ecosystem of software makes it the go-to board for jobs like these.