I was wondering if there was a way I could get the battery level and have an indicator (LED) let me know that it's time to replace them?
This will give you the basic idea. Simplest option would be to use an Analog pin in the board to read the voltage. Which would need to be done very carefully because anything over 3.3v could cause the board to loose it's magic smoke, but the power supply can take significantly more. Also a work of warning this method is not very accurate, The closer the battery gets to zero the voltage will be steady for a while then when it drops it will drop fast. https://autonomoushobbyist.wordpress.co ... ery-meter/
