A serial charger with the function of automatic change of charging speed for NiCd/NiH batteries is provided where the different charging modes for multi-speed (four speed) charging process, two speed charging process or two speed charging process in two charging phases can be achieved by use of the speed-changing principle with a low power consumption of an exchange power unit. The finished charging time of each battery varies inversely as the number of the detected batteries to be charged. No matter whether the charging voltage is variable or constant, the charging current varies automatically to achieve the speedy charging process or the normal charging process even under low power consumption. Accordingly, an exchange power unit can be reduced in size and built in the charger so that the charger is convenient to carry and the finished charging time can be adjusted automatically according to the demand of battery consumption of customers.