Battery management systems (BMS) are safety devices integrated into battery packs to monitor the cells during charging and discharging phases. These systems can activate safety controls when problems arise while logging data that you can use to evaluate the cell's stability. When functioning properly, the BMS can improve the efficiency and longevity of the battery pack. Unfortunately, BMS design flaws may cause serious risks and failures to cells.
When tackling design flaws, most failures deal with improper calculations with cell balancing functions, parameter limit miscalculations, temperature sensor misplacement, CAN/UART/RS485 timing glitches, and outdated firmware. Avoiding those mistakes requires careful testing and validation to ensure full operability when the BMS is in use.
Cell Balancing Issues
Not every cell in the battery pack may have the same capacity. When there are different energy levels, it causes overcharging and deep discharging issues that can damage cells and lead to a loss of capacity. A BMS can be designed with cell balancing features to improve battery pack longevity and prevent degradation.
Custom battery pack with BMS designed for cell balancing.
It does this using active or passive balancing. Active balancing involves transferring energy from higher capacity cells to lower capacity cells using transformers, capacitors, and inducers so all cells have the same state of charge (SoC). Passive balancing uses resistors to burn off the excess energy in high-capacity cells until all cells have the same voltage.
To identify cell imbalances, the BMS will monitor every cell's SoC and voltage. Limit thresholds are established based on the type of balancing used: active or passive. Active balancing is usually set at a limit of 6 amps (6A) or lower. Passive balancing is set to limits of up to 0.25A of current.
To determine the SoC between cells of differing capacities, you need to figure out the delta state of charge. Delta SoC takes the maximum capacity and subtracts it from the minimum cell charge. Then it is divided by the maximum capacity. An ideal Delta SoC would be 0%, as it indicates there is little capacity difference between cells. A higher level of delta SoC signifies cell imbalances that must be rectified.
A common mistake with BMS design is not setting the right limits based on the cells. You end up not transferring enough or too much energy between cells for active balancing. The BMS may also burn off too much of the excess capacity for passive balancing. Another issue is selecting a resistor that cannot handle the energy burn-off temperatures without becoming damaged.
Redundant Critical Protection and Firmware Updates
The BMS relies on sensors and ICs to monitor and activate safety controls for the battery pack. Any problem can come in an instant that can compromise the safety and integrity of the cells. Yet these controls cannot be a single point of critical protection. If a malfunction occurs in a sensor or monitor, there is nothing there as a failsafe to take appropriate action.
Redundant critical protections are secondary controls in case the primary monitoring systems fail. These protections are independent ICs that evaluate current limits, temperatures, and the BMS operation to detect anomalies. If sensing an issue, they activate safety controls and log errors so you can determine the BMS failure. Redundant critical protection is a necessity that should not be overlooked.
In addition to redundant protections, firmware updates should also be available for the BMS and components during field service. Outdated or incompatible updates can make the BMS inefficient. The system may fail to read errors, determine current limits, or activate controls. During the planning and designing phase, determining the right components that allow for regular updates and maintenance helps to optimize the system while keeping operation costs low. Field engineers will not have to constantly replace legacy components when they can instead simply update firmware.
Temperature Sensor Misplacement
Thermal management is crucial for a BMS. Cell overcharging can cause temperatures to rise exponentially, leading to thermal runaway. The components inside the pack can also overheat, leading to swelling and cracking. Temperatures can also become too cold inside the device when used for certain applications, such as aerospace. A BMS sensor tracks the temperature and will control heating and cooling measures.
However, these sensors must be placed in ideal locations to monitor temperatures. If they are placed in a perpetually cool or hot spot in the pack, this factor will lead the BMS to log an inaccurate reading. The system may activate heating and cooling measures when not required or fail to activate controls when necessary. Evaluating the placement and operating temperature of components inside the battery pack allows the proper placement of sensors for the BMS.
CAN/UART/RS485 Timing Validation
A major requirement for a BMS is effective communication protocols. Sensors, controls, interfaces, and other components are communicating with each other, sending and receiving transmission data. Any type of hiccup or glitch in this communication can cause systems to react slowly or poorly, which hampers the safety of the battery pack and the device. CAN/UART/RS485 are serial communication standards using timing (data bits and baud rates).
With all the components sending and receiving signals, the communication relay can experience heavy bus loads that can impact the CAN/UART/RS485 timing, causing timing errors, corrupted data, and signal reflection. Using validation testing, such as signal integrity analysis, load testing, and termination testing, can check signal quality and maintain a high signal transition. This validation ensures that, even during heavy transmission loads, communication signals still reach components when required.
Summary
A BMS needs careful design consideration to safely protect the battery while also implementing safety protocols to maintain operability. Adopting validation techniques, thermal management systems, and correct cell balancing offers a robust BMS system that can make your battery pack efficient.
Key Takeaways
- Correct cell balancing limits are essential for pack longevity: Active and passive balancing must be matched to the specific cell chemistry and capacity differences. Poor limit settings or undersized components can accelerate degradation instead of preventing it.
- Redundant safety protections prevent single-point failures: A BMS should never rely on one sensor or IC for critical protection. Independent backup monitoring ensures faults are detected and mitigated even if primary controls fail.
- Firmware strategy matters as much as hardware selection: Designing for field-updatable firmware helps prevent long-term compatibility issues, improves safety performance over time, and reduces maintenance and replacement costs.
- Temperature sensors must reflect real cell conditions: Improper sensor placement can produce misleading data, causing unnecessary thermal control actions or missing dangerous conditions. Sensor location should be validated against actual heat sources and operating environments.
- Communication timing must be validated under real loads: CAN, UART, and RS485 communication errors often appear only under heavy traffic. Signal integrity, load testing, and proper termination are critical to ensuring reliable BMS response when it matters most.
