Does your Milwaukee M18 battery pack only have 3 bars when fully charged? That's usually a sign that the cells are not balanced. All lithium-ion cells need to be at the same voltage or the battery pack can't fully charge. This cell rebalancing procedure will also work for DeWalt, Makita, or any brand of power tool battery packs. See links below for the parts and tools I used:

Battery Charger:

https://amzn.to/47bQpOl (affiliate*)

Security Bit Set:

https://amzn.to/48wcKIH (affiliate*)

Slim Multi-Bit Screwdriver:

https://amzn.to/42K6c5u (affiliate*)

*Purchasing from these Amazon affiliate links earns my channel a small commission. Think of it as a tip that doesn't cost you anything. Thanks for the support!