I have been building connected sensors for us in our local parks. Often, there is no access to WiFi so, I have been using the Adafruit Fona with a custom carrier board to build my Internet of Things nodes. The issue is that even with good battery management, and only reporting to Ubidots (my IOT back end service) hourly, I only get about seven days on a 2000mAH LiPo battery. This is because the Fona GPRS sensor draws some significant current (about 180mA from the board since there are some large capacitors on the breakout) while transmitting. In order to put one of these sensors in a park for an extended time, I needed to use Solar power for recharging.
I went back to Adafruit for their Solar Charger board and large panel but ran into a couple issues when I tried to use them for my device:
1) The voltage varied from 3.3 to 6 volts
2) I could no longer monitor the battery voltage level from the Fona
I decided to build a small board to sit between my device and the Adafruit Solar Charger to do two things: Provide a consistent 4V and monitor the state of charge for the battery. To do this, I built two circuits on a single board: a TI switched buck-boost power supply based on the TPS63020 and a fuel gauge monitoring circuit based on the Maxim 17043.
The results? With only three hours of sunlight a week, I can maintain a full charge on my LiPo battery and my circuit does not require and I can get by with a simple linear regulator on the device to step down to 3.3V for the Arduino. The Fona happily accepts 4V straight and the power surges during transmission are not an issue.
Here are a couple shots from my Ubidots dashboard showing almost a month of continuous operation. Notice that even with days of rain or cloud cover, the battery charge never stopped below 87%.
The board is available on OSHPark here.
The EAGLE Files are hosted on my GitHub
If you want to learn more about the Fona-based IOT device, leave a comment with contact information.
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