I had a 16-Volt Max Cordless Lithium DustBuster Hand Vacuum and the batteries were wearing out. I wanted to see if I could convert it to run on an auto cigarette plug. I used a PH1 screwdriver to remove the 6 screws that hold the clamshell case together. And after tracing the wiring, figured out you can remove the connector from the battery pack and install pins to connect the power switch and 12V power, and it works.
I found some solid wire that fit the snug in the connector, made a jumper that connects the wire from the switch to the motor, and soldered pig tails to the incoming 12V power. I drilled a hole in the area where the battery is, tied a knot and drilled a hole for a zip tie to reduce the movement of the incoming wire. Then I zip tied the 12V wires to the connector so the wire pins I used should stay steady. I put 12V on the new wire and turned the switch on, and it works.
While they claim it is a 16V Vacuum, I found that when you turned the power on, the lithium battery voltage dropped from 16V to the mid 13V volts. When I ran the unit off my new wire at 12.5V the motor drew about 6A. I used a 15V fuse in the auto power plug I used. Now my vacuum with warn out batteries has a second life running off car power or my 12V power packs.