The Reelight SL100 Cycle Light set for hub geared bikes is magnetic powered and uses no battery power. The Reelight SL100 Cycle Light is powered by magnetic induction from 2 wheel mounted neodymium magnets which rotate past the frame mounted cycle light and power the flashing LEDs. Unlike conventional cycle lights the Reelight SL100 will always run with no power failure and you don't need to worry about changing or re-charging batteries.