This project first version uses a MCP73871 as Adafruits original solar design, and incorporates a INA219 to measure the current and power from the sun. In addition, a fuel gauge is measuring the battery voltage so that you can estimate how much time there is left for normal operation. This way, you can better control sleep time for all of y'alls project. All data sent on I2C. All libaries readily on hand at the github.
My high school students are going to use these in several solar science projects.
So we've managed to put a couple of first complete prototypes together. The first attempt failed, so we just put the MCP73871 on for testing the charging only in version two. Unfortunately this competition is in the middle of our very gray winter in Sweden when we very rarely get direct sunshine, and then not for very long. But we actually got one chance to put it in the sun, with code to report batteryvoltage via AIO. It had positive results, the voltage increased slightly, but we didnt have the Huzzah32 in deep sleep during that only run. Weve since have put deep sleep on, but no sun. We are suspicious though, because you would think that a 2 W solar panel would give something held very near an artificial light source, even though most of the light sources nowadays might not have the right wavelengths for producing current. That is, if i remember correctly, something ive seen before.
Also, we have a drop from 6 V no load to 3,8 V after the diode, when plugged in. we would expect it to average 4,5 V which the voltage divider to VPCC is set to, just like Adafruits own design. We havent been able to measure the same experiment on their solar charger that uses the same chip.
So, inconclusive as for now, but the design should work, albeit that there are possibly some redundancies with the fuel gauge and the INA. Maybe there is a component that does both of their jobs. Or some other combination of fuel gauge and charger. I have found one that is usable for solar though. Tips are welcome!
So we have made the first prototype. We ordered the wrong formfactor for the MAX1704X chip and forgot to order a 270k resistor. But we were able to bypass the MAX1704X chip and find a replacement resistor. Below are a few pictures of the progress.
Here is my attempt at making and adapter from the formfactor of the MAX1704X chip. It didn't work out in the end and we bypassed it.