Day 6: Time to fix the battery

More about the analogRead() function: 

https://www.arduino.cc/reference/en/language/functions/analog-io/analogreference/

More about the Serial Monitor: 

https://docs.arduino.cc/software/ide-v2/tutorials/ide-v2-serial-monitor/

More about unsigned integer:

https://www.arduino.cc/reference/en/language/variables/data-types/unsignedint/

Constants:

https://www.arduino.cc/reference/en/language/variables/variable-scope-qualifiers/const/

Byte:

https://www.arduino.cc/reference/en/language/variables/data-types/byte/

Static variables:

https://www.arduino.cc/reference/en/language/variables/variable-scope-qualifiers/static/