Analysis of battery failure of 48V power bus of electric vehicle

“Electric mobility (e-mobility) is growing faster and faster, thanks to the avalanche of cars, buses, cargo trucks and scooters with electric vehicles. This has also led to the rapid development of EV battery and powertrain manufacturing technology with innovative solutions. These all increase efficiency and reduce operating costs. The gradual transition to the vehicle’s 48V power bus and the introduction of high-voltage batteries requires proper thermal management techniques. Continuous monitoring and control of the temperature of the most critical components, such as the battery and charging system, improves vehicle reliability, increases driving range, increases driving comfort and reduces charging time.

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Author: Stefano Lovati

Electric mobility (e-mobility) is growing faster and faster, thanks to the avalanche of cars, buses, cargo trucks and scooters with electric vehicles. This has also led to the rapid development of EV battery and powertrain manufacturing technology with innovative solutions. These all increase efficiency and reduce operating costs. The gradual transition to the vehicle’s 48V power bus and the introduction of high-voltage batteries requires proper thermal management techniques. Continuous monitoring and control of the temperature of the most critical components, such as the battery and charging system, improves vehicle reliability, increases driving range, increases driving comfort and reduces charging time.

Thermal management in electric vehicles is more complex than in conventional internal combustion engine vehicles. The electric motor must be continuously cooled, and the battery must be cooled or heated depending on the ambient conditions. Also, unlike conventional vehicles, there is no immediately available wasted heat to heat the cabin. Therefore, it is necessary to provide appropriate energy saving measures such as heat pumps. To keep the electric motor and battery at the right temperature, the necessary cooling circuits can be used flexibly to dissipate heat inside the car.

As the cooling circuit absorbs heat, its temperature rises, requiring the presence of a heat exchanger in which a liquid or gaseous refrigerant circulates. The refrigerant must have a high heat capacity in order to absorb as much heat as possible within the same footprint. Through the process of refrigerant evaporation (transition from liquid to gas), the battery can be cooled to temperatures even below ambient temperature. The heat generated during condensation (transition from gaseous to liquid) can alternatively be used to heat the passenger compartment during cold periods. Efficient thermal management solutions enable greater autonomy to meet current and future demands for electric vehicles.

Electric Vehicle Battery Monitoring

A battery pack installed on an electric vehicle consists of several battery modules connected in series and parallel. The Electronic circuit required for battery module management is called BMS (Battery Management System). A BMS includes one or more power conversion stages and a microcontroller-based embedded system that handles all aspects related to the power subsystem. During EV battery charging or discharging, the status of each battery cell belonging to the battery pack must be monitored.

EV batteries can gather a lot of energy in a small volume. If left unmanaged, overvoltage or undervoltage conditions can cause thermal runaway that can damage the battery. For this purpose, a special circuit called BMIC (Battery Monitoring Integrated Circuit) is introduced to monitor the voltage and temperature of each battery. This information is sent to the battery management controller (CMC) and, depending on the complexity of the system, to a higher level battery management controller (BMC).

The BMC aggregates information about the battery voltage monitored by the CMC to calculate the current state of charge (SOC) of the battery. SOC is the basic parameter for evaluating the remaining capacity of the battery to determine when a new charge is required. Another parameter is the state of health (SOH), which provides important information from which remaining battery life can be derived. Thermal runaways are especially deceptive, and these runaways are triggered by different types of faults, including performing a charging or discharging process too quickly. To avoid these phenomena, the communication between BMS, CMC and BMC must be done with as little delay as possible.

business solutions

Solutions exist for monitoring electric vehicle battery performance, which are commercially available from corporate organizations such as STMicroelectronics, Analog Devices, and NXP.

STMicroelectronics offers a broad portfolio of EV battery monitoring solutions that provide high-accuracy measurements in 48V and high-voltage battery packs. Figure 1 shows a block diagram of a typical BMS architecture, where several battery management ICs are used to sense the voltage, current, and temperature of each battery pack cell. An example of an AEC-Q100 qualified IC for EV battery management is the L9963, a Li-ion battery monitoring and protection chip for high-reliability automotive applications and energy storage systems. Up to 14 stacked cells can be monitored to meet the requirements of 48 V and higher voltage systems. This information can be transferred via SPI communication or an isolated interface. Multiple L9963s can be daisy-chained and communicate with a host processor through a transformer-isolated interface, featuring high speed, low EMI, long distances and reliable data transmission.

Figure 1: Block diagram of the battery management system

Analog Devices offers a broad portfolio of battery management system devices that flexibly support virtually any EV battery system architecture. The LTC6810 (Figure 2), for example, measures up to 6 battery cells connected in series with less than 1.8mV total measurement error. The battery measurement range of the LTC6810 is 0V to 5V, making it suitable for most battery architectures. Multiple devices can be connected in series, allowing the cells of long high-voltage battery packs to be monitored simultaneously. Each LTC6810 has an isoSPI interface for high-speed, RFI-resistant long-distance communication.

Figure 2: LTC6810 block diagram

NXP offers powerful, secure and scalable BMS ICs for a variety of automotive applications. An example is the MC33771, a Li-ion battery cell controller IC designed for automotive applications such as HEVs, EVs, e-bikes, and e-scooters. The device features ADC conversion of differential battery voltage and current as well as coulomb counting and temperature measurement. It also supports standard SPI and transformer-isolated daisy-chain communication with the MCU for processing and control.