Mitigating Battery Management System Failures: Preventing Thermal Runaway in Li‑Ion Batteries

Understanding Thermal Runaway in Li‑Ion Batteries and How BMSs Safeguard Them

Li‑ion batteries are considered safe when operated within well‑controlled parameters. Yet no battery management system (BMS) or manufacturing process is flawless. By engineering more robust BMS designs, we can counteract the inherent physics of Li‑ion technology and enhance safety for critical applications.

This article builds on our previous introduction to BMSs, delving into potential failure modes and the strategies to mitigate them. We also preview emerging BMS components that will support the next generation of battery technology.

Thermal Runaway in Battery Management Systems

Thermal runaway is a well‑known failure mode in power systems and is closely linked to fire hazards. In a BMS malfunction, it can be triggered by hardware faults or firmware bugs. For instance, if a stop command in the cell balancer is forgotten, a cell may continue to discharge beyond its safe limits. Even if the system detects the anomaly and blows a fuse, the cell can still be over‑discharged, leading to separator degradation and the formation of an internal short circuit during subsequent charging attempts.

Figure 1. Formation of internal copper shorts due to over‑discharge. Image courtesy of Xuning Feng

Such a short may initially present a high resistance that keeps the external voltage above the nominal level while drawing an enormous self‑discharge current—often invisible to external current sensors or voltage monitors. The resulting internal heat raises the cell temperature above 60 °C, causing the cell to burst and ignite. The heat then propagates to adjacent cells, triggering a chain reaction that can lead to catastrophic failure.

Figure 2. Burnt high‑energy battery pack from a 2011 Chevrolet Volt. Image from the Chevrolet Volt Battery Incident Overview Report

Failure Mitigation

One robust solution to unforeseen bugs is the inclusion of an external watchdog that monitors the microcontroller unit (MCU) for fatal errors, as illustrated in Figure 3.

Figure 3. Typical BMS block diagram with MCU watchdog implementation

If the MCU is operational but a command is inadvertently omitted, the cell monitor can enforce a watchdog mechanism, shown in Figure 4.

Figure 4. BMS block diagram with complete watchdog implementation

In rare cases—such as latch‑up caused by electromagnetic interference (EMI) or radiation—a watchdog designed to issue a full power cycle rather than a simple logic reset can be lifesaving. This architecture is less common but provides an additional layer of protection.

Additional Solutions for Mitigating BMS Failure

As energy densities climb and power demands rise, the tolerance margin for battery cells shrinks. Consequently, even more precise state‑of‑charge estimation is required, with cell impedance playing a pivotal role.

Direct, real‑time impedance measurement is highly valuable. Panasonic, for example, has pioneered a localized AC stimulation technique that monitors electrochemical impedance without the need for an unloaded voltage reference or extensive calibration. Other methods exist, but they often require more complex setups.

FRAM (Ferroelectric RAM) is another promising technology. Because FRAM retains data after a power cycle, it can securely store the last valid Coulomb counter sample, reducing the risk of data loss during sudden resets.

Ultimately, the chemistry of the cell itself determines safety margins. Beyond Li‑ion, chemistries such as Li‑S, Li‑FeS₂, and solid‑state variants are being explored to further enhance safety and performance.

If you’d like to dive deeper into battery system design or have specific questions, feel free to leave a comment below.