Improve the safety of lithium-ion battery packs for electric vehicles?

　　In the management system, the battery monitoring circuit board uses two key subsystems to reliably monitor the battery status and provide digital results to the main control processor in charge of the operation of the control system. In order to separate these subsystems, the high-voltage battery sensing circuit and the circuit board Optically isolated signal interfaces are used between the communication devices to ensure that high voltages will not affect the digital subsystem.

　　At present, in all-electric or hybrid vehicle applications, the management of high-voltage lithium-ion battery packs faces many challenges. In addition to monitoring the charge and discharge cycles, it is also necessary to isolate battery packs that provide hundreds of volts for safety reasons. This article specifically discusses the needs of lithium-ion battery monitoring, and discusses the architecture and components used in battery monitoring systems, digital communication systems, and isolation interfaces.

　　In the management system, the battery monitoring circuit board uses two key subsystems to reliably monitor the battery status and provide digital results to the main control processor in charge of the operation of the control system. In order to separate these subsystems, the high-voltage battery sensing circuit and the circuit board Optically isolated signal interfaces are used between the communication devices to ensure that high voltages will not affect the digital subsystem.

　　Li-ion battery characteristics

　　The complex electronic system that must meet the performance, safety and reliability requirements of electric vehicles is directly affected by the characteristics of lithium-ion batteries. When lithium-ion batteries are discharged, lithium materials are usually ionized at the graphite anode, and then these lithium ions are used The electrolyte moves through the separator to the cathode to cause the charge to flow. The charging process reverses the entire process, bringing lithium ions from the cathode back to the anode through the separator.

　　The performance and reliability of this chemical reaction program is controlled by the temperature and voltage of the battery cell. At lower temperatures, the chemical reaction is slower, making the battery cell voltage lower. As the temperature rises, the reaction speed will increase until lithium is reached. The ion cell begins to collapse. When the temperature exceeds 100°C, the electrolyte begins to decompose, releasing gas that may cause the pressure of the battery cell without a pressure relief mechanism in the design. At a sufficiently high temperature, the lithium ion battery cell may be caused by oxides. Decomposition faces thermal runaway and releases oxygen, which further accelerates the temperature rise.

　　Therefore, maintaining the optimal operating conditions of lithium-ion batteries is a key requirement of the battery management system. The main challenge in designing the control and management system is to ensure reliable data collection and analysis to monitor the status of the lithium-ion battery in the car. , And this is the characteristic problem of the lithium-ion battery itself.

　　In the ChevyVolt electric vehicle, the battery pack contains 288 prismatic lithium-ion batteries, divided into 96 battery groups, which provide a DC system voltage of 386.6V through connection. These battery groups combine temperature sensors and cooling units to form four main batteries Module, the voltage sensing line connected to each battery group is terminally processed when it is connected to the top of each battery module, and connected to the battery interface module above each battery module through the voltage sensing strap combination connector, 4 of them are used The color-coded battery interface modules operate in different positions of the battery pack, corresponding to the low, medium, and high voltage ranges of the four modules' DC voltage offsets.

　　The data provided by the battery interface module will be sent to the battery energy control module, and then this module will provide fault conditions, status and diagnostic information to the hybrid control module as the vehicle diagnostic main controller. At any time, the entire system will Run more than 5000 system diagnostics, of which 85% of the diagnostics focus on the safety of the battery pack, and the others as the target battery performance and life control.

　　Multilayer circuit board

　　The battery performance analysis starts with the battery interface control module used in the ChevyVolt electric vehicle. Please refer to Figure 1. The design is specifically for high signal integrity. The four-layer design of the circuit board uses trace layout technology, isolation technology and ground plane To help ensure signal integrity in such a challenging environment, the top layer contains most of the components, including optical isolators, ground planes, and signal traces with multiple vias, providing access to the lower layer The second layer uses power and ground planes to be distributed under the high voltage area of the circuit board. The third layer contains signal traces that pass through these areas. The other side of the printed circuit board, that is, the fourth layer As a ground plane and signal routing, and contains some additional components.