A bottle dynamo or sidewall dynamo is a small electrical generator for bicycles employed to power a bicycle's lights. The traditional bottle dynamo (pictured) is not actually a dynamo at all (which creates DC power), but a low-power magneto that generates AC. Newer models can include a rectifier to create DC output to charge batteries for electronic devices including cellphones or GPS receivers. Named after their resemblance to bottles, these generators are also called sidewall dynamos because they operate using a roller placed on the sidewall of a bicycle tire. When the bicycle is in motion and the dynamo roller is engaged, electricity is generated as the tire spins the roller. Two other dynamo systems used on bicycles are hub dynamos and bottom bracket dynamos.

Advantages over hub dynamos

Disadvantages over hub dynamos