Solar + super cap power source testing

In my last post, I described our solar cell + super-capacitor power module. I did some more testing and graphed the voltage and light intensity recorded periodically over time. WiSense nodes have an on board light sensor (the TSL45315) which outputs the light intensity in “Lux”. The super-capacitor voltage (which is the voltage provided to the WiSense node) is measured using an ADC10 channel on the MSP430. The MSP430 has one ADC10 channel dedicated to measuring the supply voltage without using an extra pin.

The test setup had these components

  • One WiSense coordinator node
  • One WiSense node configured to operate as a reduced functionality device (RFD).
  • One Solar + Super-capacitor power supply (powering the RFD above).
  • One multi-meter to monitor the RFD voltage

The super-capacitor voltage was around 0.3 V at the beginning of the test.

The test started when the Solar + Super-cap power supply was connected to the RFD and placed in direct sun light. A multi-meter was setup to monitor the super-cap voltage.

The multi-meter showed the super-cap charging towards 2.6 Volts at which point the voltage monitor IC switched on the load cell, thus powering up the RFD.

The RFD monitored the supply voltage (every 5 seconds) till it rose above 2.8 Volts.  The RFD blinked an on board LED to indicate that it was up and running and monitoring the supply voltage.

When the supply voltage rose above 2.8 V, the RFD started the association procedure and eventually registered with the coordinator.

After association, the RFD periodically (every 5 seconds) sensed the power supply voltage and ambient light intensity and reported both to the coordinator which pushed the data to the WiSense gateway software running on a laptop. In between the RFD entered and remained in deep sleep.

The test ended when the RFD lost power because the super-cap voltage fell to 2.5 V and the load switch got turned off by the voltage monitoring IC.

The test lasted around 6.5 hours.  You can see two graphs below depicting change in voltage / light intensity with time.

jan_9_solar_sc_test_graphs

The bottom graph indicates that starting from 9:46 am, the solar cells stopped receiving any sunlight till the end of the test. The RFD remained powered by the energy stored in the super-capacitor for the next 4.5 hours till the super capacitor voltage dropped to 2.5 Volts.

The top graph shows that the first voltage value reported by the RFD to the coordinator is around 2.67 V even though the RFD waited for the super-cap voltage to climb above 2.8 V before it started the association procedure. This is because the association procedure requires the radio to be switched on for a considerable time since a joining node has to send beacon requests in all the channels (9 in the 865-867 MHz band) and wait for beacons in each channel. All this microcontroller and radio activity drained the super-cap from 2.8 V to 2.67 V even though the cells were producing power at the same time. The energy consumed by the node was more than the energy provided by the cells leaving the super-cap to pick up the slack. If the RFD firmware did not wait for the voltage to rise above 2.8 V and instead started the network association procedure as soon as the node was powered on, the node would never have been able to associate since the super cap voltage would drop below 2.5 V, thus switching power off to the node. This process will repeat endlessly as long as the solar cells are able to generate power.

Once the node started sending periodic sensor data (every 5 seconds), the super-cap voltage started rising up to a max of 3.481 V since the RFD spent the 5 second interval in deep sleep.

I used plot.ly to get the two graphs shown above. I just uploaded a text file containing time, voltage and light intensity data. “plot.ly” took the comma separated data in each line of the uploaded file and plotted a line graph. It is a really nice and extremely useful website and it just took me 5 minutes to get my graphs with no prior experience.

Posted on January 10, 2015, in Uncategorized. Bookmark the permalink. Leave a comment.

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