Why BMS only goes so far in addressing thermal runaway

As the energy storage industry works to increase safety, battery fires still remain a challenge to overcome. Advanced battery management systems (BMS) aim to decrease that fire risk.   

Designed to monitor voltage, current and temperature, BMS helps regulate a battery’s charging and discharging. Though the technology is improving and has begun incorporating AI and predictive analytic technologies, it’s not infallible: recent high-profile failures have highlighted that BMS can only go so far in preventing thermal runaway events before they escalate. 

Ionel Stefan, the chief technology officer at Amprius, said that though BMS is critical for monitoring battery health, its ability to prevent failures is constrained by the parameters it tracks.   

“BMS is more or less a detection point,” said Stefan. “Sometimes a spark can be detected and stopped before it turns into an event, but sometimes that heat release is enough to trigger a fire.”  

And, not all potential problems cause immediate changes in voltage or temperature, meaning they can develop unnoticed until a runaway reaction is already underway.   

It’s a particular challenge as storage systems continue growing bigger and more energy-dense. Large battery installations can contain thousands of cells tightly packed together, which can increase the risk that a failure in a single cell will jump to those around it.   

Some BMS include ceramic separators that keep the cells isolated enough to significantly reduce the chances of a chain reaction, if not fully prevent it. Others include a metallic film that impedes dendrite formation, which is a main cause of sparks inside a battery.   

“The most basic role of the BMS is to keep the cells balanced and keep them at the same state of charge,” said Stefan. He explained that as cells age and go through charging cycles, their chemistry can alter. In practice, this can mean that two battery cells in a pack can have vastly different strengths.   

“Batteries are like people,” he said, adding that the BMS protects the weakest cell from being overcharged. “They like to be treated mildly and not abused in any way.”  

The divergence rate also varies between different types of batteries. For instance, lithium cells diverge quickly, as they don’t contain stable anodes or host materials.  

Every cycle contains a redeposit, Stefan noted. The lithium deposits, and then redissolves, making the battery chemistry entirely different from cycle to cycle.  

That’s why, he explained, it’s tricky for lithium-ion batteries to have long cycle lives. Another material may not have that same problem, though it would likely come with its own unique set of challenges.    

“To work effectively in a variety of batteries, the BMS has to know how each type of chemistry behaves before it can monitor the cell,” Stefan said. “They have to be trained in advance on a certain set of data and keep the batteries within safe temperature, voltage and current ranges.”