Can You Leave Your Milwaukee Charger Plugged In All The Time?
The various rechargeable power tool battery systems from Milwaukee require proper care and handling to function safely. This is why, for example, it's generally considered a good practice to take your Milwaukee batteries off of the charger as soon as they're finished charging in order to ensure their long-term efficacy. For the sake of convenience, you might consider leaving both the charger and batteries plugged in all day.
Technically, this is fine. According to the operator's manual for the Milwaukee M18 & M12 Multi-Voltage Charger, neither the charger nor the batteries will experience any immediate damage or danger if you leave the charger plugged in while not in use or after charging is finished. However, the manual also notes that it's generally good practice to unplug your charger and batteries when charging is finished and store them in a safe place, away from moisture and extreme temperatures. This ensures the charger doesn't quietly steal power from your home and that it can't be a potential conduit for a sudden fire.
It's safe, but it would be better to unplug it
While leaving your Milwaukee charger plugged in all day won't endanger it, unplugging it is more a matter of basic electrical safety. Leaving unused electrical devices plugged in all the time, even if they're not in use, can cause small passive energy drains, subtly increasing your overall electrical consumption. Additionally, unattended chargers in particular can be minor fire hazards, as an errant power surge or ambient heat can cause them to spark up. It's the same reason you should unplug your phone charger when you're done with it.
A Milwaukee charger can become unexpectedly warm even when it's functioning normally, which can both siphon power away unnecessarily and potentially lead to electrical fires. As convenient as it is to have a charger ready and waiting for your Milwaukee batteries, there's no real need for it to be plugged in if it's not charging anything, so you might as well set it aside for a little peace of mind.