I’ve built a small solar-powered weather station that connects to my home system using a Jeenode. I’m using three AA NiCds that will, at full charge, deliver around 4v. As this is a Jeenode, I’m running my ATMEGA328 at 3.3v. I’m sending three pieces of data to Xively: Battery/Solar Voltage, Temperature and Humidity. I was running it indoors for a couple of months, just to make sure it worked, then I placed it in my freezer to test its behaviour in low temperatures, and it seemed to work as expected.
I’ve placed it on the side of my house, above a south facing roof section, where it now gets very warm in the early afternoons: above 48C these past couple of days. While indoors in test mode, the battery level would top out around 4.4v depending on how much sun it would get, and drop to about 3.7v just before sunup. But now, the level crashes from about 4.4v to 3.8v around 1pm, and then slowly recovers to 3.9v later in the day, then dropping at night to 3.6v.
I’m confident that the solar panel is delivering enough juice to the NiCd’s, but I’m curious about the apparent voltage crash: I don’t know if
1/ the level is really going down (I don’t want to have to sit on a hot section of roof with my voltmeter to confirm this)
2/ if there is something in the circuit design or
3/ if the components themselves are contributing to the problem.
I’ve shared a 1-day graph of Temperature and Battery Voltage along with a schematic of the charging circuit: