To achieve the maximum driving range and efficiency, electric vehicles (EVs) have high voltage (up to 900 Vdc) and high energy density (up to 250 Wh/kg) battery packs. Battery management systems (BMS) are needed to ensure optimal operation and reduce safety concerns. This FAQ reviews the functions of a BMS, how the BMS integrates with the overall EV control system and closes with a look at emerging wireless BMS designs.
A BMS performs several functions (Figure 1). These include monitoring battery operating parameters is the basic function of a BMS. Those parameters include the voltage, current, temperature of individual cells, modules, and the overall battery pack. Monitoring and controlling the temperature is particularly important to ensure safety. Liquid cooling systems are most common in EV battery packs. The BMS monitors the temperature and flow of coolant and its speed and adjusts cooling system performance as needed to support the thermal loading of the pack.
Data acquisition in the BMS feeds the state determination engine with data from the voltage, temperature, and current sensors. That data is used to calculate numerous cell and pack parameters like:
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- kWhr of energy delivered since the last charge.
- Current in Ah delivered or contained in cells and modules.
- Total lifetime energy delivered.
- The state of charge in Ah.
- Number of charge and discharge cycles, including depth of discharge and amount of charge for each cycle.
- In addition to those basic parameters, algorithms in the BMS are used to estimate the state of health (SoH) that compares the current capacity to the original capacity. The state of power (SoP) estimates the amount of power available based on current usage, temperature, and other factors to estimate the remaining driving range. State of Safety (SoS) provides an early warning of potential safety issues with cells and modules.
Finally, a BMS often includes two communication links. One for connecting with the main EV control system and a user interface for maintenance personnel.
Figure 1. Typical BMS architecture (Image: Cyient).
Communication issues
Communication is a critical and complex issue when designing a BMS. Monitoring cells, modules, and the overall battery pack involve connecting to sensors at different voltages. The cells are less than 5 V, the main power bus is hundreds of volts (as high as 900 Vdc), and the modules are somewhere in between. The number of cells varies widely. EV battery packs using cylindrical cells have between 5,000 and 9,000 cells. Packs using pouch or prismatic cells can have a few hundred cells. In every case, the wiring for monitoring the cells and modules is extensive, complex, and heavy.
The complexity of BMS wiring harnesses makes them a reliability concern. Their weight reduces the EV range. They are expensive and time-consuming to install. One way to address these numerous challenges is to use a wireless BMS (WBMS).
No wires needed
WBMS are just emerging and have not been standardized. There are several possible communication approaches being deployed or considered including Bluetooth, Zigbee, and Wi-Fi, as well as proprietary protocols. In a WBMS, all the information exchanges between each sensor node, the master controller, the onboard display unit, the data storage unit, and other control and management units are implemented wirelessly. WBMS implementations are inherently flexible, lighter-weight, more reliable, and more easily integrated with other battery pack systems like the thermal management system and charger (Figure 2).
Figure 2. Typical wired BMS system (left) and a proposed WBMS implementation (right) (Image: Analog Devices).
Summary
BMS are ubiquitous features of all EVs. They are needed to deliver maximum battery pack performance and ensure safety. Common functions across all BMS include sensors for cell and module monitoring, a state engine for determining the current state of the cells and modules, and a communications interface. Today’s wired BMS harnesses are beginning to be replaced with WBMS designs for weight, improved performance, and lower cost.
References
Automotive Battery Management System, Infineon
Automotive Battery Management System, STMicroelectronics
Battery Management System in Electric Vehicles, Cyient
Battery Management Systems Connectivity, TE Connectivity
wBMS Technology: The New Competitive Edge for EV Manufacturers, Analog Devices
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Filed Under: Batteries, BMS, FAQs, Featured