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:Logging harvested solar power

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.

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