The BMS of the lithium battery pack adopts a two-level / three-level protection architecture. The slave control (BMU) Lithium-ion battery pack for Solar System and UPS system consists of lithium battery cell acquisition, battery balance control circuit and communication circuit. All lithium battery cell data status information is uploaded to Cluster control (BCMS) by the communication bus. BCMS performs high-speed calculations on battery operation data to control high-voltage relays to achieve overcharge, overdischarge,overtemperature, short circuit and other protection of the battery pack, and achieve highly reliable operation of the battery pack. Through the first-level master control (BAMS) and UPS to realize RS485 communication, and to realize the double protection action of UPS and lithium battery pack, which is more safe and reliable.