This is a basic battery voltage meter that we made to quickly test our batteries between competitions. It uses four op-amp comparators to turn on LEDs at specific (and customizable) voltage steps.
We made this tool because we found that it's really annoying to check batteries on the robot. You have to plug the battery into the robot and then find the driver station phone to actually see the voltage, and when there's maybe ten minutes between each competition and a two-minute walk between the pits and the competition fields, you don't have much time to be checking all of the batteries this way. This meter is small and readily accessible so that you can check all of the batteries very quickly.
Make it Yourself
You can find the files to make one of these yourself on GitHub. This is an open project, so feel free to make suggestions for improvements!
Just note that this is a soldering project, and if you don't know how to solder, you should probably practice on something less densely populated than this board.