Milwaukee M18 batteries and chargers communicate via digital signals, which Milwaukee calls Redlink Intelligence. A USB logic analyser, some leads, and a few resistors is all you need to listen in on these signals. I tested 2 chargers and several batteries and listed my best interpretation of what the messages mean and what each byte in the messages means. 

There's still a few parts that I'm unsure of or have no idea of their meaning. It'd be great if others can follow my guide and record the signals of their batteries and chargers to see if they can uncover any new information.

This video owes a lot to Buy It Fix It who solved a lot of the basics of the protocol and definitely saved me a lot of time. They don't have a video on this topic, but they've got lots of repair videos on their channel, so go check them out: https://www.youtube.com/c/BuyitFixit

REFERENCES:
Buy It Fix It (Reddit post): https://www.reddit.com/r/MilwaukeeTool/comments/108d3j9/m18_fuel_battery_protocol/
Quagmire Repair: https://quagmirerepair.com/milwaukee-m18-battery-reverse-engineering

CREDITS:
Matrix Raining Code: youtu.be/KCVN-SuBBWM
Explosion: youtu.be/Q7KmAe8_jZE


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