This is a multi use accessory for the milwaukee m12 battery.

It's a power tap to use the m12 batteries 12vdc power for other uses, mobile lighting, convert to USB as a powerbank, powerwheels or other purposes. all you have to do is take one of those blue insulated spade connectors, take off the blue plastic insulated part, insert and solder a wire (i used 16awg, but depending on the thickness of the wire insulation, you may need to experiment).

It's also a cap for your battery, since the m12 doesnt have a power gauge unless you install it on a tool, i print these and (without adding the wire connectors) put them on my batteries when they're full, that way when i use it i obviously take the cap off, so if there's no cap i know the battery is partially or fully used, if there's a cap, i can be confident that the battery is fully charged without having to check.

and lastly i put a screw hole in the center, this can be used with a #8 screw to secure to a wall to store the batteries, i wouldn't recommend this in something that sees a lot of movement like a van or truck, but i have used these to mount to my garage walls for years and they work just fine.