Hi,
To start, I assume you took the "good" battery from the working drill and tested the inoperative drill.
Good for doing that.
The other causes I know of in Drills that turn them into paper weights are as follows:
1 Worn Brushed: You can attempt to test this by squeezing the assumed good trigger button and
Taping the drill or lightly banging it on a bench top or something else. If it is brushes, it will often but no necessarily all the time, start to run for a moment, or just running. Since you use the drill and "use it a lot" That will be the first thing I would look for.
2 Trigger Button.
I would bring it to a tool service place or send it back to R
idgid.
Try this link
http://www.ridgid.com/Tools/Technical-Services/index.htmHope this helps, Happy New Year
Mark