54 “Under no circumstances should someone replace the cells on the inside of a battery,” he stresses. “There is just too much that can be different about the new cells, and some of the things that need to be done to take the old cells out are irreversible and will leave the battery in a vulnerable state.” Morin also recommends that refurbished batteries should not be used, given the standard of many on the market. He explains that taking a battery apart and replacing the cells, even with identical ones, will leave it in a vulnerable state. Any connections and welds done by hand will tend to be of variable quality and greatly increase the failure rate. The final operational factor identified by Soteria was riding mountain bikes in the way they are designed to be ridden. “Here, the use patterns are more aggressive, with far more vibrations from general use, and more shocks from wipe-outs that are a normal part of mountain biking,” Morin notes. “That’s not to say don’t ride electric mountain bikes, but plan to replace the batteries more frequently just to be safe, and check them for any mechanical damage before every ride. Never use a pack that has more than cosmetic damage to the surface of the case.” The safest pack that Soteria examined had several key features that were missing from the others. It was of very high quality in its materials and construction, revealing particularly good welds, for example. It also had a custom BMS; an approach that typically provides more sensors and software to monitor cell quality and ageing through a battery’s service life. In this case, it had several wellplaced temperature sensors within the pack. The cells in this pack were also of a very high quality from a reputable manufacturer and they were manufactured to UN 38.3 or an ISO quality standard. It still left something to be desired, however. “Even the best packs were missing some of the five requirements we plan on having,” adds Morin. Technological progress Several emerging technologies could improve the safety of e-bike batteries, according to Morin, and are likely to be used in combination with the abovementioned fuses, vents, thermal conductors and thermal insulation. These include composite current collectors made from combinations of polymer and metal, which are designed to be fail-safe to give cells localised protection from excessive currents. Thermally stable separators are another emerging technology for use inside cells. They are under development to replace conventional, plastic separator membranes with alternatives that remain stable at temperatures that are hundreds of degrees higher, preserving the barrier that prevents short circuits between the anode and the cathode. Morin points out that several companies now offer non-flammable electrolytes, which significantly reduce the risk of a battery catching fire. In larger EV battery packs, sensors are increasingly being used to detect overheating and alert the BMS to the presence of the gasses produced when a cell starts to emit them in the early stages of a thermal runaway, enabling the BMS to cut the battery off from further loads or currents. The application of such devices to much smaller e-bike batteries requires miniaturisation and a reduction in cost. As they get bigger, e-bike batteries of the future could benefit from welldesigned cooling systems, with both passive and active options under development, Morin says. He is critical of both industry and regulators for not agreeing on standards and testing procedures that are good enough to ensure the safety of batteries. “While each existing standard is a step in the right direction, compromise between the component manufacturers who make up the committee can lead to missing features representative of higher safety,” he says. New standard, political action Soteria is planning to publish its own standard this summer with what Morin calls a comprehensive list of necessary design features enabled by the above technologies. “We have to recommend using this, or a standard that includes at least all the features included in our own standard,” he says. “If nothing else, put in abundant active temperature monitoring and passive cooling for every cell in the pack. This isn’t enough, but it is a big step in the right direction.” The industry is also collaborating on political action to establish and uphold July/August 2024 | E-Mobility Engineering Electric offroad bikes used as their designers intended should have their battery packs replaced more frequently than others as a precaution (Image courtesy of Pexels)
RkJQdWJsaXNoZXIy MjI2Mzk4