Current and future BMS design considerations for electric vehicle manufacturers


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High Voltage Battery Management System (BMS) technologies are developing rapidly. To get more mileage from a single charge, reduce charging times, and lower the overall cost of electric vehicle battery packs, designers are adopting new battery chemistry and experimenting with new architectures. Intelligent battery junction boxes (BJBs) and a field-controlled building management system are the next evolutionary steps for EV BMS architecture, providing greater design flexibility, reduced software load, and higher battery pack performance. Smart BJBs A traditional BMS has three main subsystems: a battery management unit (BMU), a BJB, and a cell monitoring unit (CSU). The BMU contains the main BMS MCU, which is responsible for the calculations of the state of charge (SoC) and state of health (SoH) of the battery pack. Accurate SoC and SoH metering is key to lowering cost and providing an accurate representation of battery life and driving range. In addition, most of the electronics needed to monitor voltage and current, measure insulation resistance, and actuators of circuit breakers and thermal fuses are in the BMU. The CSU contains the electronics to monitor the cell voltage and temperature, and the BJB basically acts as an electromechanical box where the transformers, conductors, and thermal fuses are located. TI’s latest battery monitors and balancers, such as the BQ79616-Q1, support a wide range of battery chemicals, including LiFePO4, to improve accuracy of cell voltage monitoring and enable accurate SoC and SoH measurements. The traditional BMS architecture requires several cables running between the BMU and the BJB that take up a lot of valuable real estate in the battery pack and add weight to the vehicle. In response, manufacturers are increasingly transferring electronics such as the UIR sensor (for pack voltage, current, and insulation resistance measurement), conductor drivers, and thermal fuse drivers to the BJB. These smart BJBs greatly reduce the amount of cabling between the BMU and BJB while providing greater flexibility for locating the BMU and BJB in the battery pack. The BMU becomes a low voltage-only board, which reduces its complexity and cost. This new design architecture presents a new challenge: the intelligent BJB and BMU still need to communicate with each other through a Control Area Network (CAN) bus or an isolated daisy chain, among other options. The CAN approach requires that you place a secure MCU, CAN transceiver, and associated power circuits on the BJB. On the other hand, the insulated daisy chain offers a simple twisted-wire protocol that does not require any MCU or associated software, serving OEM requirements for reduced complexity and lower bill of materials costs. On smart BJBs, the UIR sensor communicates with the BMU as well as all other circuits on the BJB, without the need for an MCU. The UIR sensor uses the same isolated daisy chain bus as the cell displays on the CSU, enabling the BJB and all CSUs to sit on the same isolated daisy chain bus and communicate with the BMU. This figure shows the transition from a traditional BMS (A) to a BMS with a smart BJB (B). Domain Controlled BMS The next step in the evolution of BMS architectures is to take the BMU out of the battery pack and integrate it into the powertrain domain controller to create a field-controlled BMS. The main enabler of this architecture is the smart BJB and BMU’s low voltage panel. Such an architecture greatly reduces the complexity and cost of designing a BMS for both hardware and software. This figure shows the transition from a BMS with an intelligent BJB (A) to a field-controlled BMS (B). The domain-controlled BMS architecture requires a communication interface between the domain controller (DCU) and the battery pack. Adopting a unified interface allows OEMs to use off-the-shelf DCUs and reduce their dependence on any specific semiconductor supplier. TI’s suite of high-performance, fully scalable BMS solutions enable OEMs and Tier 1 manufacturers to meet their performance and cost goals as they electrify their fleets. Written by Ankush Gupta, Product Line Manager, Texas Instruments Please read the e-book for the full article.


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