I was still on the 20v rating for ACE mention outright. 18v would be enough for near max speeds on a 20v motor, I'd guess is would like 10-15v normally.
Rated at 12vdc I'm usually at 6-9v. And 6v at 1.5-3, 3v at 1.5max, and a 1.5v motor on a plastic 0-4-0 fame(L) that I'm having trouble with; in slowing it down enough to stay railed