Back to my original thread with an interim update. We are sanding and re-staining our log cabin this week. Because I have an aerial man lift on property, I decided to inspect my ISS. It was full of dirt so I'm glad I took a look. My ISS battery clocked in at 3.1 volts. It has been installed for 4 months now. I think the threshold for replacement is 2.8 volts, so I am well within range for good operation.
One idea, which I have not tried yet, for checking to see if the capacitor is working is to take the battery out of the ISS and see how long it will continue to transmit readings. On a new wireless transmitter board, on a sunny day, that capacitor should continue to operate for several hours. Davis says it can go for up to eight hours, but should go at least two to four. Of course, you have to ignore the low battery voltage warning, which removing the battery will generate, but that should give me an idea of the solar panel/capacitor generating and storing power.