1. Batteries: Batteries are chemical energy storage devices that convert chemical energy into electrical energy. They provide a direct current (DC) voltage with a relatively constant potential difference between their terminals.

2. Generators: Generators are mechanical devices that use electromagnetic induction to convert mechanical energy (like rotation) into electrical energy. They typically produce an alternating current (AC) voltage, but can also be modified to produce DC voltage.

Let me know if you'd like to know more about these sources or other types of voltage sources!