ROS Battery Status

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There's a pretty substantial gap between writing software and checking the state of your batteries, and you'll need support from your hardware to cross that gap.

At the most basic level, battery monitoring is generally done by measuring the battery voltage, measuring and integrating the current through the battery, or both. This is an entire field of study with chemistry and is a bit too broad to cover here, but it's important to remember that the relationship between battery capacity and voltage can be highly non-linear depending on your battery chemistry.

I'd suggest that you buy (or possibly build) hardware that can measure the SOC (state of charge) of your batteries, and then connect that hardware to your computer and write a ROS node that translates that data into a sensor_msgs/BatteryState message. I believe there are dashboard widgets that can consume and display this message in a reasonable way.

P.S. - your question makes reference to a demo, but doesn't link to it. ROS is a very broad community, and most of us aren't familiar with all of the demos, so a link would be helpful.