Lithium-ion batteries have a high energy-to-weight ratio and energy-to-volume ratio, have no memory effect, can be recharged multiple times, have a long service life, and are getting lower and lower prices. These characteristics of lithium-ion batteries have promoted the development of portable products in a smaller and lighter direction, and more and more products are powered by single-cell lithium-ion batteries. The requirements for the charger are becoming more and more strict. This charger can quickly charge the constant current and constant voltage to protect the battery.