A battery system includes a unit battery module, a current and coulomb measurement circuit and a master control circuit. The unit battery module stores electricity and calculates battery information of the battery set according to a system current value, a system coulomb value, a cell voltage and a cell temperature of the battery set. The current and coulomb measurement circuit is coupled to the unit battery module, generates the system current value according to the current flowing though the battery set, generates the system coulomb value by integrating the system current value, and provides the system current value and the system coulomb value to the unit battery module. The master control circuit is coupled to the unit battery module, receives the battery information from the unit battery module, generates a system battery information according to the battery information and provides the system battery information to an external device.