Recently, I developped several projects based on solar power. By definition, the sun is not predictable and it can take hours and days to validate some design concept.

My current project is to develop a solar power supply for my Nixie watch.
https://www.elektormagazine.fr/labs/nixie-watch-1

I had several design difference and wanted to see how it can really recharge the battery in real time, on 2 days basis. I modify the SW of my watch to create display every 3 min and I have to monitor the battery level of 2 different design, plus one without.


For this purpose of slow voltage measurement, I use the famous ESP32, adjust to read the 3 channels every 10 min and upload to the cloud www.thingspeak.com
I use a lot this cloud service, for another project of weather station or here, to measure the battery level of my watches.

The hardware is quite simple. I connect the 3 different channels to the ESP32 through a resistor divisor bridge. Even if the ESP32 looks easy to use, I struggle to find the right A/N channels. seems many can be use, or conflict with Wifi. So, every 10 min, the voltages are measured and upload to the thingspeak. The link is there:
https://thingspeak.com/channels/329068

For the ESP32, I m using the version with the serial/USB link integrated. The overall consumption is not very good, like 14mA, so, the power is by power bank. A small one is full time connected and a big one is recharging the small one. Almost every day, the big power bank is swap with another one. I need use powerbank because the only window it can have 6h of sunshine has no main power close.

For the software, I tried several environment for PC and ESP32. Many was too complex, or never work. Finaly, I develop in C on Arduino 1.8.4. a very good tutorial was found for the install and it is working well now.

Some more pictures of the simple multimeter hereafter.

Laurent