In this one I’m just playing around in the shop and trying to make something that doesn’t really exist: a dummy Milwaukee M18 “battery” that lets me watch both voltage sag and tool current draw while the tool is actually running. I started with a knockoff 9Ah M18 kit battery case (no cells) and used it as a housing, then stuffed it with a pair of DC Hall-effect current transformers (CTs) and a voltage/current display. I also showed the old-school way to do it with a DC shunt, but the CT method is way easier here because you just pass the conductor through the sensor. The big gotcha with M18 is you need the right resistor values so the tool will behave, so I stole the resistor pack out of an M18-to-DeWalt adapter and built an ABS shim/block so everything would mount correctly. I ran 10 gauge wire to keep voltage drop down, cut the meter openings with a band file and oscillating tool, and packed the whole mess into the case. It’s clunky, it’s a shop tool (not a jobsite accessory), and the cheap kit screws feel like the weak link—but it works. On a blower I was seeing about 20.3V, and on a heat gun I saw a noticeable sag and roughly 16.5A, with about a half-amp disagreement between the two CTs because centering and “chinesium accuracy” are real things.