Overview: The upper computer, lower computer and BMS are interconnected in the lithium battery management system (BMS), forming a complete management, monitoring and control framework. Through their respective role divisions and collaboration, they ensure the safe, reliable and efficient operation of the battery system. 

First、 Basic Relationship: 
1. Upper Computer
‌    The upper computer is the central control and management device of the system. It is mainly responsible for monitoring, analyzing overall data, and issuing operation instructions. The upper computer communicates bidirectionally with the BMS through standard communication protocols (such as CAN bus, RS485, UART or Ethernet), obtaining the operating data of the battery pack, including voltage, current, temperature, SOC (charge status), SOH (health status), and other key parameters. The upper computer analyzes and stores these data, and issues operation instructions based on the battery status, such as initiating battery balancing, adjusting charging strategies, and performing protection actions, etc. Additionally, the upper computer can also monitor multiple BMS modules, enabling remote monitoring, centralized control, and maintenance management. 

2. Lower-level Machine
The lower-level machine is responsible for interacting with sensors or actuators, collecting sensor data or performing specific control tasks. It transmits the data to the BMS and executes tasks related to battery management. In some systems, the lower-level machine is also responsible for specific execution tasks, such as controlling the temperature regulation device of individual battery cells. The hardware platform of the lower-level machine typically includes PXI chassis, real-time processors, and I/O boards, etc., and is responsible for sequence execution and device invocation. 

3. 3. BMS
BMS is a device that directly manages the battery module. It is responsible for monitoring, protecting and controlling the battery. BMS communicates with the upper computer to report the battery status and receive instructions, and also communicates with the lower computer to obtain sensor data at the bottom level or control specific actuators. BMS acquires sensor data at the bottom level through the lower computer, such as the voltage, current, and temperature of each battery cell, and processes the key data locally before sending it to the upper computer. BMS also has various functions, including data collection, balance management, protection functions and fault diagnosis, charging management, thermal management, and alarm notification, etc. 

Second、 Interaction among the Host Computer, BMS, and Lower-Order Computer 
1. Data Flow:
From the lower-level machine to the BMS: The lower-level machine collects battery data and transmits it to the BMS for centralized processing and management.

From the BMS to the upper-level machine: The processed data by the BMS is uploaded to the upper-level machine, which then conducts further analysis and storage, and generates reports, predicts battery lifespan, etc. 

2. Instruction Flow:
From the upper computer to the BMS: The upper computer, based on the battery's operating status or the operator's instructions, sends operation instructions to the BMS, such as adjusting the charging and discharging strategies, performing equalization or protection actions.
From the BMS to the lower computer: The BMS sends instructions from the upper computer to the lower computer to carry out specific operational tasks, such as starting or stopping the battery protection The BMS sends instructions from the upper computer to the lower computer to carry out specific operational tasks, such as starting or stopping the battery protection device, adjusting the temperature control, etc. 

Three.、Communication and Protocols 
1. Communication protocol between the host computer and BMS: 
Common communication protocols include CAN bus, RS485, Ethernet, UART, etc. These protocols provide reliable data transmission channels, ensuring that the upper computer and BMS can exchange data and instructions in real time. 

2. Communication Protocol between BMS and Lower-Level Computer: 
The communication between the lower-level machine and the BMS may employ internal communication protocols such as I2C or SPI, which are used for the rapid exchange of sensor data and control commands. These protocols are typically employed in short-distance and real-time-demanding communication scenarios. 

Third、 Hierarchical Structure of the System Architecture 
Hierarchical architecture: The upper computer, BMS and the lower computer form a hierarchical structure: The upper computer is at the top level and is responsible for overall monitoring, data processing and system control. 

As an intermediate layer, BMS not only performs specific battery management tasks but also reports data to the upper computer and executes the instructions given by the upper computer. 
2. The lower-level device is at the very bottom and directly interacts with sensors and actuators, providing local data collection and control functions.