52 high-quality ones from reputable manufacturers, they are not easy to spot. “Counterfeiters are really, really good, so it’s only possible to tell by taking a cell of known manufacture and comparing it to the potentially counterfeit cell, and then looking at specific features such as the size and shape of the button, the type of casing materials and marking, the size and features on the can, and other features,” he says. To avoid the fakes, Morin advises battery builders buy cells either directly from a manufacturer or from a trusted company with a good reputation as a distributor of that manufacturer’s cells. Unfortunately, it is almost impossible for consumers to distinguish counterfeits from the genuine article. “The cells are on the inside of the battery and impossible to identify from the outside,” Morin says. “For now, the likelihood of counterfeit cells is highest from the very lowest-price packs, but there are high-quality replacement packs out there from reputable companies.” Safety, present and absent Modern e-bike battery packs usually have a range of safety features, including protection against overcharging, overdischarging and excessive currents. They are also structurally stable, and usually sealed against the ingress of moisture and contaminants. “There are also several safety features that we’ve identified that were not present in any of the e-bike batteries we examined,” Morin says. “We sat our committee down and asked them, ‘what is the minimum, clearly defined feature set that will give a very safe battery?’.” They came up with five features that happen to be standard in batteries for larger EVs. The list begins with cells that are certified to an international safety standard and manufactured under a well-established quality assurance programme, such as UN 38.3. Next, each battery pack should be fitted with a vent that keeps moisture out, but allows the hot gases from a failing cell to escape without creating excess pressure inside the pack or heating up the rest of the cells. Although fuses are relatively common, Soteria recommends there being a fuse between each cell and its neighbours in a module, and between each module and its neighbours in the pack; the idea being to prevent a damaged cell from drawing excess current that could damage the surrounding cells or battery system components, and generate heat inside the pack. The fourth and fifth features concern thermal management. Each cell should be connected to a thermal conductor that will transfer heat away from it and to the outside of the pack. “This, by itself, will prevent many battery pack fires,” Morin says. Also, each cell should be thermally insulated from its neighbours. And, lastly, the BMS should – in addition to providing the standard overcharge, over-discharge and overcurrent protections – receive data from a set of temperature sensors and be able to shut the pack down if a high temperature is detected. Rider input Soteria’s study identified a number of operational factors associated with battery failure in e-bikes, such as riders using them for deliveries. Morin stresses there is nothing special about any individual trip for deliveries that is different from, for example, commuting, but delivery riders do experience higher failure rates for several possible reasons. “If I had to guess, I would say they just use their batteries to a much higher and longer degree, not throwing them away when they get worn, and it is the old batteries that have weak cells in them or have had some level of mechanical damage.” Riders carrying out their own maintenance also seems problematic. While working on mechanical systems such as chains, sprockets and tyres is fine, Morin points out that anything related to the electrical components should only be handled by a trained technician. EVD | Soteria e-bike project July/August 2024 | E-Mobility Engineering Penetration tests are one means of discerning lithium-ion cells’ susceptibility to thermal runaway and taking fire-prevention measures (Image courtesy of Soteria) Packs that fail often show design faults such as poor thermal insulation and inadequate spacing between cells (Image courtesy of New South Wales Fire & Rescue Service)
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