51 Soteria e-bike project | EVD E-Mobility Engineering | July/August 2024 somewhere inside the battery that generates heat, usually within a single cell. What happens next depends on how well the cells are thermally isolated from each other. With good protection around the cells, an individual cell failure will lead to a battery fire about once in five million cases, which, with 50 cells per pack, would lead to a fail-safe rate of about 99.999%. This, he says, would make battery fires 100 times rarer than they are today. Morin cites four main causes of e-bike battery failure. The first is cell ageing, where a cell that is weaker than the others is overworked, and eventually overheats and consequently enters thermal runaway. The second is mechanical damage to the pack through, for example, rough usage, resulting in damage to internal connections, which, over time, cause the cell to be overworked or lead to heat build-up outside it. In either case, the cell overheats and ignites. Thirdly, some packs provide the cells with little or no protection from shock and vibration, so an individual cell can be damaged, again leading to overheating and eventual thermal runaway. Electrical damage is the fourth major cause. “Charging cold or bad chargers, or a weak cell, can cause the cells when charging to not be able to accept the lithium into the anode, which will cause it to plate on the surface. This can cause internal short circuits, which, again, will lead to overheating and thermal runaway,” Morin explains. Counterfeit cells and packs Another serious problem upon which Soteria’s investigative work shed light is the proliferation of counterfeit batteries and cells. “The biggest thing is that the cells potentially do not match the performance of the copied cells, so they are going into packs that are designed for a different cell, “Morin says. “Temperature ranges, voltage ranges and other features can be different from one cell design to another.” Counterfeit batteries will almost certainly be made of cheap materials and lack the safety mechanisms engineered into high-quality ones, and quality is likely to be of the minimum necessary to prevent immediate failure. What’s more, they are extremely unlikely to be assembled in tightly controlled environments, such as the expensive clean rooms in which reputable manufacturers put cells together, with the risk that dust, moisture and other contaminants will get in and cause early failures. As considerable effort goes into making fake cells resemble price of an equivalent OEM pack, unsurprisingly used lower-quality components and had fewer safety features. However, some encouraging results also emerged, with many e-bike riders putting fire-protection measures in place while charging, and a significant majority (67%) exclusively using OEM packs. Among these packs, most included some form of temperature sensor, and one pack stood out for having the safest design, more of which later. Commenting on the early results in late June 2023, the group’s CEO, Brian Morin, noted that two of the packs dismantled by the team appeared to contain counterfeit Samsung cells, adding another layer of risk. In terms of design, most packs showed inadequate or zero spacing between the cells, increasing the risk of cell-to-cell propagation of a thermal runaway. They also lacked any type of insulating material between them that could stop or slow such propagation. What’s more, he said several had a battery management system (BMS) that was either explicitly not designed to balance the charges between cells or no mention was made of cell balancing in the documentation. Low probability, high consequence We caught up with Morin in May to bring us up to date with Soteria’s work and to gain his perspective. The vast majority of batteries that fail do so without catching fire, he points out. “The fail-safe rate is about 99.9%, which at first glance looks like a good number. However, when you have 300,000 e-bikes in New York City, it means six new fires a week, and each is a building fire because of the ferocity and immediacy of the fires,” he says. “There is no chance to put it out before it ignites the building.” Morin believes the overarching cause of a battery failure resulting in thermal runaway and a fire is a weakness Charging e-bikes indoors is risky, and faults in the batteries are a leading cause of building fires in major cities (Image courtesy of Merseyside Fire & Rescue Service)
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