The role of battery management systems in battery fires

Sometimes batteries catch fire even when the battery is not charging. What role does the battery management system play in such cases? Cell balancing is one of the most underestimated aspects of lithium-ion battery pack designs. 

Cell balancing done wrong can lead to thermal events. It might sound odd, because the most obvious causes are overloading during operations, bad thermal design, passive cooling, poor or slow BMS decision-making, etc. But cell balancing should not be overlooked to understand and prevent thermal runaway.

‍

Why investigate cell balancing as a cause of thermal runaway

Cell balancing is a process whereby the cells are charged (or balanced) to the same level. Imagine two cups, one filled to the brim, the other filled to 95% of its capacity – how can we get these two cups to have the same percentage of capacity filled? Balancing! Let us have a closer look:

• Cell balancing can be executed in multiple ways: passive, active, or hybrid. All these balancing methods are monitored and controlled by the battery management system. Active balancing, as the name suggests, can actively pump in current. Passive balancing, the most used method, uses resistor-based balancing where the cell with excess charge will balance by ohmic losses. Hybrid uses both active balancing and passive resistors. Regardless of the balancing method, choosing the right balancing current is crucial. 
• If the balancing mode (software, hardware selection, designing the balancing modes) is not meticulously designed and validated, this can lead to instances where the balancing mode pumps in too much current (overcurrent) or too little, leading to irregular aging. 
• Usually, automotive implementations are safeguarded by ISO 26262 guidelines and audits. However, non-automotive applications are not necessarily governed by stringent regulations. Overlooking this quality-assurance step or cutting corners in testing can lead to thermal events during operation.
• Balancing is heavily dependent on the cells. Every cell has its SoC-OCV (state of charge – open circuit voltage) behaviors. So, taking an existing BMS and balancing circuit, and implementing them in a new cell is not going to work as intended. 
• Balancing is done at rest. In some cases, between charging cycles. This means the battery is not being discharged. So, if the balancing does lead to an overcharging event that the BMS is not able to monitor quickly enough, it will be too late, leading to a thermal event. 

I could author a small book on how not to design a balancing circuit. But to summarize, every circuitry design is unique due to the cell selection and BMS implementation strategy.  

Learn more: How to Avoid Thermal Runaway

‍

What factors affect BMS design?

Well, many.  But, to simplify the story, let us start with these four:

1. Quantity and quality: Quantity and quality of the sensors which are used to monitor the cells. Several factors could affect such a decision. So, just listing component names will not get us far. Instead, key things to consider are inertia, tolerance, range, and sensitivities of the sensor itself.
2. Frequency of data uploads: How often do I want to read out the values? The frequency determines the data volume. Deciding on data frequencies is system specific and needs careful consideration. For example, cell voltages are usually measured more often than system ground-fault detection.
3. Algorithms + cell data + system specification: These are crucial. Deep understanding of the cell and the algorithms designed to handle the system specifications is essential. You could get away with poor or low-traffic data with clever algorithms, but the other way around is disastrous! Imagine a truckload of quality data the algorithm cannot make sense of in the given time. Recipe for failure!
4. Balancing the processing speed with bandwidth: You could increase the processing power to handle data bandwidth (CAN traffic). But too much data is also not productive. The frequency and magnitude of data coupled with the nuance of the BMS processing power to pick up crucial data will have to be a balance between safety, ISO 26262, component availability, and maturity of the algorithm to identify culprits and react. 

As “the brains” battery management design is crucial and plays a significant role in battery safety, particularly to stop overcharging and as part of cell balancing circuits. If the BMS is supposed to monitor battery safety, who is monitoring the BMS to ensure that it is doing its intended job? That is where cloud-based battery analytics come in, which I will cover in my next article.

Stay tuned and sign-up for the Battery Knowledge newsletter to get articles sent straight to your inbox!

‍

^{1 https://batteriesnews.com/7-battery-electric-cars-day-catch-fire-china-most-involved-brands/}

^{Article retrieved: September 26, 2022}