A few years ago I installed a wireless, self-powered light sensor in my garden to determine when it is time to close down our electric blinds. It uses ultracapacitors to store energy during the day to be able to continue transmitting sensor status at night.

I monitored the capacitor voltage to be able to track the performance of the ultracap over time. The question was how it would behave during cold winter nights and hot summer days, and also if and how much the capacity of the component would be influenced by aging.

The first months the system worked fine, the ultracap got charged by the solar panel when the sun was shining, and the sensor node would happily bridge the night until sunrise, with even some juice left in the ultracap (the capacitor voltage never went below 3V). Over the time of a few months, the voltage was dropping further and further every night. A sign that the capacity of the ultracap was decreasing, as I assumed that the power consumption of the electronics would not change (the electronics are mounted in a water/airtight enclosure and were not touched).

After a year, I decided to add a second ultracap to help bridging nights. This helped temporarily, but after a few months the system voltage reached the minimum working voltage (2.5V) again every night.

After three years and after posting a message on jeelabs.org on the influence of aging on an ultracapacitor, I decided to dismantle the sensor node to actually measure the remaining capacitance of the ultracap.

I determined the capacitance by discharging the cap over a known resistor. Assuming that the RC time constant equals approximately 1.4 x the time it takes to discharge the ultracap from 80% to 20%, I determined the remaining capacity of the first ultracap to be 2F. Spot on the specs of the ultracap that was used. Hmmm, a little weird, given what I was seeing in the measurement results.

On to the second cap. Oops: apparently this second cap went faulty. I could not charge it any more and the output voltage was 0V. I carefully removed the heat shrink from the ultracap and tadaa: the connections from the two separate ultracap cells that make the pack to the equalizer electronics were cold solder joint that only gave contact now and then. I assume this is caused by the influence of changing temperatures in the sensor box. The cold joints were quickly fixed with some fresh solder. A measurement on this fixed ultracap yielded the same remaining capacity as the first one.

So: I need to revise the statements I made on the capacity decrease of ultracaps. The decrease I noticed was due to a faulty ultracap cell. As the cells we not connected to the equalizer electronics, the electronics tried to do what they were designed for: to equalize the cells, thereby pulling energy from the second ultracap…

I fixed this now, and added both ultracaps back as power storage to the wireless sensor node. Working like a charm again…

So I take back the statement I made on the aging of the ultracaps. After 3 year deployment in an outdoor environment the caps are still working fine with no noticeable capacity reduction.