After a small break, more progress. Wifi is working and can be updated OTA. The ADC is working. Biggest progress is that the charging circuit is build and working. To find a good PWM frequency and duty cycle for the current goal of 1.5 to 2.0 A I set. I created a small test setup using a frequency generator as PWM source.

With the test setup I was able to determine that a frequency of 5 KHz with a duty cycle of 50% would result in a current draw around 1.7A. Higher frequency would charge slower and a lower frequency would increase the current without increase the charging speed much. Since the circuit is switching a coil my guess would be that it would generate some noise on the power supply. Oh boy what was I right...

The source is dipping below 9V. To smooth this a bit I added a 470uF cap on the power connector.

That's better

Next up finishing up the code, make the controller board.