Last week, there was a fire at night in an e-bike shop in New York City, and four people in the apartments above died of smoke inhalation while two others were hospitalized. This marks the 13th death so far this year, more than twice the count for all of 2022. The number of fires is over 100 in New York City alone, and globally, lithium-ion battery fires topped more than 500 through May of 2023.
As the readers of this newsletter already know, Soteria has started a project to understand how e-bike batteries are designed and what measures are used to ensure they can be used safely. We have now completed our survey and interviews of e-bike riders and have disassembled eight new e-bike battery packs. The results reveal a stark reality that is unfortunate for those who have become its victims.
Yesterday, we issued a press release with a summary of those results. It is sobering to think that a community’s safety depends on the broad display of engineering errors that we found. Please refer to the press release for a detailed list of the findings. Here, I’m going to discuss the significance of a few technical findings from the inside of the battery pack.
Counterfeit Cells
Two of the battery packs appeared to have counterfeit Samsung cells. Please see the picture attached to this newsletter, which shows the real cells and the counterfeit cells we pulled from two of the battery packs.
Companies like Samsung have decades of engineering that has been poured into their manufacturing, quality and inspection processes which ensures a consistent, high-quality cell. Companies that make their own cells and relabel them as Samsung cells are almost certainly cutting corners to achieve low cost and improved profit margins. Likely, they do not have the economies of scale that Samsung does, and so are using 2nd grade materials and poorly designed and refined processes. At the end of they day, if a company is willing to lie about the product on the outside, how much can you trust what is on the inside? Also see the section below on cell balancing to understand how this can play out when the rest of the battery pack does not accommodate it.
Inadequate Spacing Between Cells
Most of the battery packs had no spacing at all between the cells, and in some cases the cells were glued together. But not a single battery pack had more than 1 mm of spacing. Our tests, plus a conversation with the CTO of a very large, small-device battery pack company, confirm that a minimum of 3 mm is needed to have a low probability of cell-to-cell propagation of thermal runaway. In all these battery packs, if a single cell goes off, it is almost certain that the rest of the battery pack will also go into thermal runaway, which makes the fire 50 – 100 times bigger. Which is enough to ignite the room, and building, as has happened so many times in the e-bike fires that have been reported.
Inadequate Cell Protection
Another way to protect one cell from another is to place insulating materials between the individual cells. Again, none of these packs had any protective materials between the battery packs. This can prevent or slow down the cell-to-cell propagation of thermal runaway, and there is an abundance of materials being sold for this application. This is another relatively easy protective mechanism that has not been adopted into any of the battery packs we’ve disassembled so far.
Lack of Cell Balancing
Four of the battery packs had a battery management system (BMS) that specifically stated that it could not handle cell balancing, while three others did not mention cell balancing as part of the capabilities of the BMS. Only one had verified cell balancing.
Cell balancing—that is bringing the cells to the same state of charge at the end of each charge cycle—may not be necessary in a battery pack that is built intelligently, relatively small and used in a controlled environment. For battery packs in which the cells are not sorted prior to assembly, or which see extremes in moisture, temperatures, vibration, or abuse—all highly likely for e-bike batteries—cell balancing is critical to make sure there is not a weak cell that is either overcharged, over discharged or over heated. The lack of cell balancing is magnified if the quality of cell is lower—in which case the variation in properties like capacity and internal resistance from one cell to another can be relatively high, and this variation is magnified as the battery pack is used and cycled.
UL Certification
One of the battery packs was certified to the standard UL 2271 for battery packs for light electric vehicles, including e-bikes, while two others were built for bikes that were certified to the standard UL 2849, which is for e-bikes but has a component for the battery packs.
A cursory ready of UL 2271 shows it to be a relatively large collection of tests that are very mild compared to the abuse that e-bikes receive. The standard involves a single 1-meter drop test, overcharge to 10% above the stated voltage, some mild vibration and submersion for 2 hours, temperature cycling and a few other tests. However, the standard doesn’t address any of the above deficiencies. We’ll do a full review when we have the project completed later.
The Project’s Next Phase
The next phase of the project will involve three different experiments:
- New Battery Pack Disassembly: Take apart another 10 – 12 new battery packs, to get a more complete picture of what we’ve presented above.
- Used Battery Pack Disassembly: Take apart used battery packs, noting points of wear and abuse, and measuring the variation and state-of-health of the cells in the battery packs.
- E-bike Ride Measurement: Place measurement devices on e-bikes as they are in-use in various scenarios, to measure actual temperature, vibration, shock and other activities to compare to the designs and standards.
How Do We Fix the Menagerie?
These key deficiencies were relatively widespread—counterfeit cells, inadequate cell spacing, inadequate cell protection, and lack of balancing—and can dramatically affect the safety of the battery packs. I expect the next phases will generate equally interesting information.
Soteria will be holding a meeting of the project participants to review all of the results and findings on October 16-17 in Greenville, SC, on the Clemson University International Center for Automotive Research (CU-ICAR) campus. After this meeting, we’ll be putting together a set of recommended best practices to design a safe e-bike battery, which likely will be able to be adopted with a relatively low cost increase.
To join us, please sign up for the project by visiting https://soteriabig.com/ebike-project/.