To make a DC generator more than a toy, you'll need to have machine techniques that are reasonably far along. Looking at the instrument makers of the 18th century, with the right flash of insight (thanks, Mark E.) I'd guess it's not impossible by the time of the American Revolution. The question then becomes: what does one do with the power thus generated? You'd need a transmission network, meaning larger scale copper mining and wire manufacture for openers. And then what happens at the other end? I'll grant that if one could make a generator, making a motor is equally possible--but it would be stationary, probably providing belt power for (say) a machine shop, cabinet maker, or some similar enterprise. In that day of localized commerce, it would have been simpler/cheaper to locate such an enterprise at the source of power; i.e., on a flowing stream with a millrace. IMO, you'll need the Industrial Revolution to get applications to make hydro-generated electric power more widespread--or for that matter, steam-generated electric power after about 1804.