How to Design a Custom BMS for Li-ion Battery: Complete Engineering Guide

Designing a custom Battery Management System (BMS) for Li-ion batteries is a critical engineering challenge that directly impacts safety, performance, and longevity of battery packs. The battery management systems monitor the individual cells working status and provide advanced safety features to prevent overcharging, over-discharging, overheating, and short circuit protection. Understanding the fundamentals of custom BMS design is essential for creating reliable and efficient battery solutions.

Understanding BMS Fundamentals

A Battery Management System serves as the "brain" of lithium-ion battery packs, orchestrating complex monitoring and control functions to ensure safe operation. Battery management system (BMS) is technology dedicated to the oversight of a battery pack, which is an assembly of battery cells, electrically organized in a row x column matrix configuration.

Core BMS Functions

Cell Monitoring: Real-time tracking of individual cell voltages, temperatures, and current flow provides the foundation for all BMS operations. This continuous surveillance enables early detection of anomalies and prevents dangerous conditions.

Protection Systems: Comprehensive safeguards against overcharge, over-discharge, overcurrent, short circuit, and thermal runaway ensure battery pack safety under all operating conditions.

Cell Balancing: Active or passive balancing circuits maintain uniform charge distribution across cells, maximizing pack capacity and extending service life.

State Estimation: Accurate calculation of State of Charge (SOC) and State of Health (SOH) enables intelligent power management and maintenance scheduling.

Key Design Considerations

Battery Chemistry and Configuration

Different battery chemistries require specific BMS designs. Li-ion cells demand precise voltage control, with fully charged cells reaching 4.2V and requiring immediate charge termination at this threshold.

Series Configuration: Increases total pack voltage but requires careful voltage monitoring of each cell to prevent imbalances that could damage the entire pack.

Parallel Configuration: Increases capacity and current capability while maintaining cell voltage, but requires current sharing analysis to ensure balanced loading.

Series-Parallel Combinations: Provide optimal voltage and capacity combinations but demand sophisticated monitoring strategies to track individual cell groups.

Electrical Protection Requirements

Lithium-ion rechargeable cells have the highest energy density and are the standard choice for battery packs. While they perform superbly, they can be rather unforgiving if operated outside a generally tight safe operating area (SOA).

Voltage Protection: Implement both hardware and software-based overvoltage and undervoltage protection with appropriate hysteresis to prevent oscillation.

Current Protection: Monitor both continuous and peak current levels, incorporating time-based integration to allow brief high-current events while protecting against sustained overloads.

Temperature Protection: Deploy multiple temperature sensors throughout the pack, implementing both charge and discharge derating based on thermal conditions.

Circuit Design Architecture

System Architecture Options

Centralized BMS: Features a single control unit managing the entire battery pack through extensive wiring harnesses. This approach offers cost advantages for smaller packs but becomes complex for large installations.

Distributed BMS: There is also a distributed BMS design solution available that features high connection reliability, no overly long cables, and tight integration of battery and measurement circuits to help reduce interference and errors.

Modular BMS: Combines elements of both approaches, using standardized monitoring modules that can be daisy-chained together for scalable solutions.

Essential Circuit Components

Microcontroller Unit (MCU): It gathers and processes current signals to monitor the voltages and temperatures of the cells. Besides, it controls safety protocols to protect the battery pack from overcharging, overheating, and over-discharging. Select MCUs with adequate processing power, memory, and built-in safety features.

Analog Front End (AFE): The analog front end (AFE) is a key component in BMS. This monitoring chip has multiple sampling channels and can monitor voltage and temperature to keep the battery healthy. Modern AFE chips integrate multiple functions including voltage measurement, temperature sensing, and cell balancing.

Current Sensing: Implement high-precision current measurement using hall effect sensors or shunt resistors with differential amplifiers. Accuracy is critical for SOC estimation through coulomb counting.

Protection Switches: Use appropriately rated MOSFETs or contactors for charge and discharge path control, ensuring adequate current handling capability and fast switching response.

Voltage Monitoring Implementation

Multi-Cell Voltage Measurement

For the measurement of different cell voltages individually, we need to introduce some circuits to perform a differential operation. The reference point as the ground remains the same in the measurement of all the cell voltages.

Differential Amplifier Approach: Use operational amplifiers configured as differential amplifiers to measure voltage differences between adjacent cells while maintaining a common ground reference.

Multiplexed ADC System: Implement analog multiplexers to sequentially measure multiple cell voltages with a single high-resolution ADC, reducing component count while maintaining accuracy.

Precision Requirements

Resolution: Aim for voltage measurement resolution of 1mV or better to enable accurate SOC estimation and early detection of cell imbalances.

Calibration: Implement software-based calibration routines to compensate for component tolerances and temperature drift.

Filtering: Design appropriate analog and digital filtering to reject noise while maintaining adequate response time for safety functions.

Temperature Management and Cell Balancing

Temperature Monitoring Strategy

Cell-Level Monitoring: Place temperature sensors in direct contact with battery cells to monitor core temperatures and detect thermal runaway conditions.

Pack-Level Monitoring: Monitor ambient temperature, coolant temperatures, and thermal gradients across the pack to optimize thermal management strategies.

Multi-Level Protection: Implement graduated responses including charge/discharge current reduction, active cooling activation, and emergency disconnection based on temperature thresholds.

Balancing System Design

Passive Balancing: Simple and cost-effective approach using resistors to discharge higher-voltage cells, though energy is wasted as heat. DW01-A is a 1 cell Li-ion/ Polymer battery protection IC responsible for all protection features.

Active Balancing: Use capacitors, inductors, or DC-DC converters to transfer energy between cells, improving efficiency compared to resistive methods and enabling balancing between non-adjacent cells.

Communication and Safety Implementation

Communication Protocols

CAN Bus: Communication Protocol includes TCP, UART, CAN (250k-1MB), and RS485. CAN is widely adopted in automotive applications, offering robust communication in electrically noisy environments.

UART/RS485: Suitable for simpler applications or point-to-point communication with host controllers.

Wireless Options: Bluetooth, Wi-Fi, or cellular connections enable remote monitoring and over-the-air updates for advanced applications.

Safety and Protection Circuits

Hardware Protection: Implement failsafe hardware circuits that operate independently of software control, ensuring protection even during system failures.

Software Protection: Develop robust algorithms for monitoring and controlling battery parameters, with appropriate fault handling and recovery procedures.

Redundant Systems: For critical applications, implement redundant monitoring and protection circuits to prevent single-point failures.

Testing and Implementation

Development Validation

Simulation: Use circuit simulation tools to validate protection circuits and control algorithms before hardware implementation.

Prototype Testing: Develop comprehensive test protocols covering all operating conditions, fault scenarios, and environmental extremes.

Compliance Testing: Ensure designs meet relevant safety standards such as UL, IEC, and automotive functional safety requirements.

Best Practices

Safety-First Approach: Prioritize safety over performance or cost considerations, implementing multiple independent protection mechanisms.

Modular Design: Develop modular architectures that can be scaled and adapted for different applications while maintaining core safety functions.

Quality Assurance: Conduct thorough design reviews at multiple stages, involving cross-functional teams to identify potential issues.

Cost Optimization and Manufacturing

Component Selection Strategy

Standard Components: Use widely available, automotive-grade components to ensure long-term availability and competitive pricing.

Integration: Select highly integrated components like AFE chips that combine multiple functions, reducing board space and component count.

Design for Manufacturing: Optimize PCB layouts for automated assembly and testing, reducing production costs and improving quality.

Conclusion

Designing a custom BMS for Li-ion batteries requires careful consideration of safety, performance, cost, and regulatory requirements. Success depends on thorough understanding of battery chemistry, robust circuit design, comprehensive testing, and adherence to industry best practices.

The investment in proper BMS design pays dividends through improved battery performance, extended service life, enhanced safety, and reduced warranty costs. Whether developing systems for electric vehicles, energy storage, or portable electronics, the principles outlined provide a solid foundation for creating effective custom BMS solutions.

Frequently Asked Questions

1. What are the main components needed for a custom Li-ion BMS design?

Essential components include a microcontroller (MCU), analog front end (AFE) for voltage monitoring, current sensing circuits (hall sensors or shunt resistors), temperature sensors, protection MOSFETs, and communication interfaces. The AFE is particularly critical as it monitors multiple cell voltages simultaneously with high precision.

2. How do I choose between centralized and distributed BMS architecture?

Centralized BMS suits smaller battery packs (under 20 cells) with cost advantages and simpler design. Distributed BMS works better for large packs (50+ cells) as it reduces wiring complexity and electromagnetic interference. Consider pack size, cost constraints, and reliability requirements when choosing architecture.

3. What voltage accuracy is required for effective Li-ion cell monitoring?

Aim for voltage measurement accuracy of ±1mV or better. This precision enables accurate State of Charge estimation and early detection of cell imbalances. Use 16-bit ADCs with proper voltage references and calibration routines to achieve this accuracy across temperature variations.

4. How do I implement effective cell balancing in my custom BMS?

Start with passive balancing using resistors for cost-effectiveness in smaller packs. For larger packs or higher efficiency, implement active balancing using capacitors or DC-DC converters. Activate balancing when cell voltage differences exceed 10-50mV, typically during charging phases.

5. What safety certifications should I consider for custom BMS design?

Target UL 2271 for energy storage systems, IEC 62133 for battery safety, and ISO 26262 for automotive functional safety. Also consider regional standards like UN 38.3 for transportation. Start compliance testing early in development as safety certification often requires design modifications.