Would you believe it if I told you the DeWALT still sells NiCad batteries? They even still NiCad tool kits. A huge chunk of the DeWALT user base is still or was on the 18V XRP battery platform. If the tool still works why upgrade, right? DeWALT did offer a stick pack lithium that worked with most 18V XRP tools. This was called the Nano which was their first foray into lithium. It worked well, but soon it got over shadowed by the all new 20V Max platform.
The 20V MAX had a better ergonomic grip, lighter weight it was smaller and a more aggressive look. Keeping their core 18v platform users happy they continue to sell and support NiCad. The 18v and the 20v are the same voltage. They both read 20V when fully charged. This adds a bit of confusion to the consumer but it helps to separate the 2 platforms. All the 12V Max platforms do this also, they are really 10.8V.
So now they have adapter where you can use 20V batteries with your 18V tools. This is huge! Suddenly your tired old DCD950 drill is turned into a powerhouse with massive run time! Going from NicCad to Lithium ion is like going from 1988 Ford Astro van to a 2016 Porsche 918 Spyder. It is an amazing experience. The DCA1820 doesn't add anymore bulk are weight than a standard DeWALT 18V battery.