Simple lithium battery charger circuit principle

The picture shows the circuit principle of a simple lithium battery charger . After the 220 V AC voltage is stepped down by capacitor C1, rectified by diodes VD1~VD4, and filtered by capacitor C2, the charging current is limited to about 70 mA. When the battery voltage is lower than 4.2 V, ZD is turned off and all current is charged into the battery. When the battery voltage rises to 4.2 V, ZD begins to conduct and play a shunt role. Since the charging current is smaller, the charging time is longer. In the circuit, the resistance accuracy of resistors R2 and R3 is required to be very high, with an error of 1%.