Every now and then, I’m asked about the suitability of small DC motors for use as a generator. The fundamental statement is that motors with low power losses are equally efficient when operated as generators. The basic calculations between speed and voltage as well as current and torque are very simple. In the following, a few rules for a successful selection.
Rule 1. For the generation of DC voltage select a brushed DC motor or use a brushless EC (BLDC) motor with voltage rectifier.