INA219 Voltage/Current monitor

This small board is available from most places online and I have found it to be easy to use and provide a wealth of information about your battery voltage and the number of amps being pulled. Its interface uses i2c and if your using python the following library makes reading the information easy : https://pypi.org/project/pi-ina219/

There is a great simple intro with circuit diagram on on www.hackster.io

https://www.hackster.io/chrisb2/raspberry-pi-ina219-voltage-current-sensor-library-f3bb54