This taillight is based on the principle of electromagnetic induction, so it needs no charge and no battery. The taillight lights up automatically during riding. The faster the speed, the brighter the taillight will be. The whole taillight is very light and portable, so it doesn't add extra weight to you. Sealed with highquality materials, waterproof in daily life. It can be installed horizontally or vertically in various ways. Specifications: Name: Selfcoming electromagnetic tail lamp Charging: Magnetically generated electricity Specifications: 53mm* 19mm*32mm Installation position: bicycle seat tube Applicable models: round spoke wheel and flat spoke wheel Lamp bead: highlight LED lamp *2 Material: high quality engineering plastics Waterproof: waterproof for life