I haven't taken a dynamo flashlight apart yet, but I believe that the dynamo itself is actually shaped like a?

I took a dynamo flashlight apart, it used a permanent magnet motor as a generator. The hand crank had 3 rows of gears to increase the crank speed to the armature shaft. A bridge rectifier is used to allow the crank to be turned either clockwise or counterclockwise and still charge the metal hydride battery. A voltage regulator is used to prevent overcharging the battery. The brushes in the dynamo are what will fail. They are cleverly designed to be made cheap. Even if the dynamo were made better the battery would be the limiting element. They can only be recharged so many times. There is nothing in the design of the one I took apart that has any overcharge protection.