As mentioned many moons ago... I modified a Dremel rotary tool to use Bosch batteries. Bosch includes a real charger, and the batteries last longer per charge. The battery plugs and unplugs into a wire that goes to the rotary tool. So the weight of the battery is not felt at the tool. It's like both corded and cordless, the battery is carried and must rest on something or sit in my pocket.
Have used it many times. Works great. They rotary tool is extremely light and easy to handle/control, without the battery stuck into the case. Much easier to control than using a flex shaft.
I'm not suggesting the idea is novel concept. Long ago, my dad stuck a light on the barrel of a rifle. The battery was kept in an Army belt, with a wire going to a plug in the stock of the rifle.
Any cordless (but wired) tools offered like that? Might not be a big deal when using a power drill, when you might be concerned about sudden torque damaging your wrist or arm. But it's great for a rotary tool.