Battery Management System: Reference Design for High Voltage systems

Introduction: Battery Management system (BMS) is an electronic system considered to be the brain of the battery packs. The two main functions of the BMS are safety protection and energy management. These are essential to manage the operational & safety parameters of the battery pack over the expected lifetime. The BMS estimates the state of health (SOH) and state of charge (SOC). The BMS also continuously monitors and provides protection during high charging current in coordination with charger. During the discharging phase, the BMS monitors and communicates with the Inverter.  In brief, the BMS is critical for battery pack operation. The following key functions are incorporated in the BMS design.

• Parameter Estimation: Estimates the cell level and pack level SoC, SoH and communicates with controllers via CAN.
• Data Storage: Records the pack & individual cell level signals and stores it in an inbuilt memory.
• Control: Measures temperature, voltage & current and controls them to achieve cell balancing.
• Diagnosis: Predicts faults, monitors cell level variations, senses errors, identifies safety risk and sends inputs to drivers for decision making

Opportunities for BMS industry The Indian Electric vehicle industry has witnessed a phenomenal growth. Reflecting its commitment to the Paris agreement, the Indian Government had unveiled “National Electric Mobility Mission Plan 2020” that aims to bring in a major shift in Electric Vehicles by 2030. This brings a plethora of opportunities in the BMS space. Moreover, the global landscape shows that the Battery Electric vehicles market is expected to reach 95.1 Mn by end of 2030 compared to 7.97 Mn by end of 2020. Considering every EV will need atleast one BMS, we are looking at a phenomenal growth in the current decade. Key functional requirements to be addressed in BMS design The battery is the primary source of energy for Electric vehicle. The energy density, durability of battery and proper utilization of energy determine the performance of the vehicle. A reliable BMS is responsible to ensure efficiency and safety of EV batteries. Based on our assessment, following are the technical gaps that need to be addressed in the growth of BMS market,

• Accurate estimation of SOC, SOH for Dynamic conditions
• Accurate Impedance measurement of cells
• Cell to Cell high differential voltage

Syrma SGS – Reference design:

Figure 1: High Level System Blocks of Slave BMS

 

Figure 2: High Level System Blocks Interface of Master BMS

  Syrma SGS has developed the reference design of BMS for automotive quality of highest safety (ASIL – C). The system comprises of approx. 200 cells in series to get an output voltage of approx. 750V. A single BMS AFE (ADBMS6830) can monitor upto 16 series connected battery cells. The cell measurement range of 0V to 5V makes the ADBMS6830 suitable for most battery chemistries. Multiple ADBMS6830 devices are connected in daisy chain, permitting simultaneous cell monitoring of long, high voltage battery strings. isoSPI transceiver (ADBMS6821) interfaces the AFEs in daisy chain to the host microcontroller enabling bidirection operation and communication integrity. Redundant communication transceivers address any communication failure in the Daisy chain. AFEs have passive balancing for each cell with individual PWM duty cycle control. Thermistor over each balancing resistor monitors the temperature. Other features include an onboard 5V regulator, where overall current consumption is reduced to 1mA. The system has been designed to ensure accuracy for voltage measurements, safety mechanisms, open wire detection and comprehensive diagnostic features. The AFE of the BMS communicate over isospi transceiver (ADBMS6821), that drives differential signals using matched source and sink currents, eliminating the requirement for a transformer center tap and reducing EMI. The drive currents and the comparator thresholds are set by a simple external resistor divider, allowing the system to be optimized for required cable lengths and desired signal-to-noise performance. Customers can minimize their time to market by leveraging Syrma SGS’s BMS that is ASIL – C reference design. Further, the design is imbibed with CAN connectivity, Automotive Bluetooth wireless connectivity, memory for data logging and safety mechanisms for thermal runaway. Much Needed Government support for development of indigenous BMS ecosystem:

• Need for accessible, affordable compliance & validation test centres: Currently, the compliance tests (EMI/EMC) are expensive and need to be done in specific labs. As the EV BMS ecosystem is relatively new and burgeoned with startups, financial assistance in any form will help them to do more compliance tests
• Need for financial incentives to compete with the global players: More than 70% of BMS deployed in India are imported from China. The Chinese exporters get a credit of 13% which is passed on to Indian importers. This further puts the Indian manufacturers at a disadvantage in terms of cost competitiveness.
• Incentive programs for next generation technology development support

Conclusion: The EV market in India is poised to grow in a structural and consistent manner. Recent incidents like EV 2 wheelers catching fire put a lot of focus on safe charging and discharging of the EV. There could be multiple reasons ranging from poor cell quality or battery design and improper BMS. Challenges such as these have opened opportunities for companies like Syrma SGS to promote their safe and automotive qualified BMS designs. Our comprehensive reference designs enables customers to launch their products quickly in the market.