V: 18

AH: 5

Wh: 90

A: ???

The problem: figure out maximum discharge amps. Potential solutions include: splice in a multimeter, run the drill until the battery dies and do math. My initial solution was to take apart the battery pack to see if the cells indicate anything. They were all soldered together inside of an internal plastic housing. I did not want to destroy this battery because it is still functional.
"What's Inside Tool Batteries? Milwaukee FLEX DeWalt Makita Snap-On [18650 21700 ??]" - youtube channel that did exactly what I just tried to do
spreadsheet with many battery specs included (change /pub to pub?output=ods to download it)
The power supply should provide:

 

V: 18

A: 20

W: 360

It would be wise to have the option to slowly titrate up the amperage to avoid an electrical fire. I'm sure I'm not the first person to do this.