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Acting as the brain of any battery storage unit, a battery management system (BMS) is responsible for battery safety through decision-making.
Two-thirds of electric vehicle (EV) battery fires happened when the vehicle was parked. That is 66% without any mechanical impact! In the first quarter of 2022, about 86 cases were disclosed to the media, of which 27.5% caught fire while charging and 38.5% while parked.1
Scary, but not surprising! Why?
Let me explain.
When a vehicle is charged, the battery should be safeguarded by the battery management system. But still, sometimes fires happen. How can that be?
To understand the role of battery management systems in battery safety, we need to understand what a BMS is. The battery management system is an integral part of all high-voltage battery systems. Think of it as the brain of any battery storage unit. It measures, monitors, controls, communicates and, most importantly, is responsible for safety through decision-making. The quality of these decisions has a direct impact on system safety.
So, the battery management system makes critical decisions to ensure system safety and deals with lots of data to do its job. Surely, that means the more data, the merrier, right? Not really. Throwing in additional sensors, additional algorithms, and complex systems logic might sound logical to make better decisions, but these can sometimes be counterproductive.
Decision-making should be a meticulous process of data handling and analyzing. Data needs to be acquired, stored, processed, sorted, and finally used as the base for decisions. To understand this, let us look at the following challenges:
If I were to choose, I would choose less but more strategically clever data. How much is too much is subjective to individual design, so there is no one-solution-fits-all. But, in simple words: too much data is sometimes worse than poor data!
Certainly not. It is fair to point out cell quality or go a step further and point out the chemistry or type of cell. But if the fire starts spontaneously while charging, it is usually the battery management system that needs to be addressed. Because regardless of the cell type (cylindrical, prismatic, pouch), if you keep pumping in current beyond a certain limit, you will hurt the cells. So, what is supposed to prevent this from happening? Usually, the BMS – in some cars, the Vehicle Control Unit (VCU) also has a part to play.
It is easier (mostly fair, too) to blame the cell. But the cell is just an “energy storage unit.” Think of it as a water tank and your current as water. If you keep filling the tank beyond the point of capacity, it is going to overflow. It is the job of the control unit—the brain—to monitor the water level, communicate with the water pump, and to stop pumping water into the tank if required. To do so, it must consistently and diligently measure the water level.
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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.
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:
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
Well, many. But, to simplify the story, let us start with these four:
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.
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Article retrieved: September 26, 2022
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Ramesh is our Customer Engineering Specialist and battery systems expert at ACCURE. Ramesh helps automotive clients to understand their technical and user requirements to find the right product fit. Over the past decade, he has held different roles in the automotive space including: Battery Cell Engineer, Battery Thermal Safety Expert, Lead Engineer for Battery Systems, and Engineering Manager for the North American Battery Market.