Though it was first patented around 1880, it was not until the 1990s that the e-bike surged in popularity reaching more than 36 million units sold in 2022, according to data research body Statista. That success has brought a raft of safety challenges, as a study by Soteria Battery Innovation Group highlights. The company’s CEO, Brian Morin, provides more context in this article.

If you have not ridden an e-bike, you should. Every hill is downhill. You push gently on the pedal, and the battery and motor move it for you – you can almost feel the pedal pulling away from your foot. In some cases, the bike does most of the work. The wind blows by as you sail past car after car after car, oblivious to traffic, confident you will get to your destination in reasonable time. It is mild exercise, exhilarating freedom, and the enjoyment of being outside in the fresh air.

In New York City, approximately 65,000 e-bike delivery workers make over 300,000 deliveries a day. This is a massive industry, and one the city has come to rely on. In addition, there are approximately 60,000 daily commuters using their e-bikes. All of these riders reduce the number of cars on the roads while also opening up public transportation seats.

While this amounts to more than 100 million deliveries per year, it comes with a cost. In 2023, there were almost 300 fires, more than 125 injuries and 18 deaths related to e-bike fires. This is up from 220 fires and six deaths in 2022.

The numbers only tell part of the story. What is missing is the speed, ferocity and intensity of the fires. We are used to wood fires that start small, and then in the presence of more fuel and oxygen can, in many minutes or a fraction of an hour, grow large. They generate smoke from the beginning, and so the smoke alarms go off sometimes when the fire is still quite small.

E-bike fires are different

With e-bike fires, it is different. The heating happens inside the steel can of the 18650 cells. The cells will vent when they get hot enough, but the vented gases are not detectible by smoke detectors, so no warning is given. The heat builds enough to cause a large internal short circuit, and the whole battery will erupt, shooting flames to fill the battery casing.

Since most of these batteries have no insulation between cells, within a few seconds many of the other cells also explode and it cascades. The fires are so ferocious, they ignite everything around them within a few seconds. Only after this intense conflagration does a smoke alarm go off. Many of the fires are three-, four- and five-alarm fires, requiring dozens of trucks and hundreds of firefighters to put out.

Understanding e-bike fires

To address this, Soteria initiated a project to understand e-bike fires, both by studying the habits of riders as well as understanding the batteries.

We worked with market research company TestedHQ to survey more than 1,000 e-bike riders, which gave us a comprehensive overview. The survey resulted in five personas that enabled us to understand differences between rider personas. Some of the more striking results are shown in Fig 1 and include:

• #1 consideration when buying a bike was safety
• 18% have experienced an e-bike fire
• 54% worry about an e-bike fire
• of those who experienced an e-bike fire, 83% use a refurbished battery (only 24% of those who have not experienced an e-bike fire do so).

People enjoy the convenience and freedom of e-bikes, but still have safety concerns. The most shocking and striking result was the number of people who have experienced an e-bike fire. If it had matched the number of fires in New York City – about one fire for every 1,000 e-bikes, then we would have had one positive response.

But to get 157 responses was so far out of our expectations that we double, triple and quadruple checked it. But that is the number.

These surveys enabled the team to identify five e-bike rider personas:

• recreational riders: typically aged 35–44, been riding 1–3 years, ride 3–5 miles: 16% have experienced an e-bike fire
• daily commuter: as above, but 17% experienced an e-bike fire
• health and fitness: typically aged 25–34, been riding 1–3 years and do 3–5 miles per trip: 18% experienced an e-bike fire. Interestingly, 88% use a non-refurbished battery
• touring and off-road: same characteristics as for health and fitness, but 22% had experienced an e-bike fire. 89% use a non-refurbished battery
• delivery: same again but 33% had experienced an e-bike fire. 58% use refurbished batteries, reversing the trend.

We learned that delivery riders experience the highest level of fires, while recreational, health and fitness, and commuters the lowest. This most likely coordinates with the use level but not equipment, as they all report spending the same money on their e-bikes.

Pack disassembly

We disassembled more than a dozen packs, measuring each cell and each component, and noting things such as weld quality and waterproof capability. Key findings:

• counterfeit cells: One pack had cells that were labelled as Samsung, but hardly resembled the authentic cells, either in form or function
• inadequate spacing between cells: Many packs had no spacing between cells, or less than 1mm. This is not nearly enough to prevent one cell from sending an adjacent cell into thermal runaway
• inadequate cell protection: None of the packs had any protective material between the cells, further exacerbating the possibility of adjacent cell thermal runaway
• lack of cell balancing: Only a couple of the packs had any balancing of the cells. Coupled with finding non-uniform cell decay in the used packs, this bodes poorly
• not waterproof: Several packs allowed significant water ingress when they were submerged, one even letting out a stream of bubbles. Water can corrode circuits, welds and other electrical components inside the pack
• no venting: There were no vents on any of the packs, so whatever water got in was there to stay for a long time, potentially corroding the inner workings of the packs. Also, any leaked electrolyte would be trapped, vulnerable to a spark
• poor thermal monitoring: Only one of the packs had more than one thermocouple, and several had none at all. This would keep the pack from shutting off during a thermal excursion
• poor welding: Several packs were hand-welded, with some of the welds easy to pop off by hand. Loose welds can cause a spark
• unsorted cells: Cells in the packs were not sorted by quality to match like cells in each module, potentially leading to non-uniform cell ageing
• non-uniform ageing of cells: In the used packs, the cells showed significant variation from cell to cell than in the original packs. In extreme circumstances, this can cause a single cell to grow weaker over time, eventually igniting either during charge or use.

It should be easy to see that a standard requiring these features will produce significantly safer battery packs. None of these solutions is difficult or expensive to integrate.

Manufacturer interviews

The e-bike manufacturer interviews were largely conducted in Europe but with several from the US. Parties interviewed included battery and e-bike manufacturers, e-bike retailers and rental companies.

The surprising and universal result of the interviews was that, while all of the parties were aware of the problem, each had a very logical and reasonable explanation as to why it did not apply to them, and did not affect their business, batteries or products.

When asked whether they had any experience with battery fires, there was universal hesitation. It was not a “no,” but rather they each firmly believed the problem belonged to someone else.

If you were to compare the conversations with these companies to ones we have had with the heads of fire departments from New York City, San Diego, London and elsewhere, it was as if you were not talking about the same topic in the same language, or on the same planet. Simply stated, the industry is very comfortable ignoring the issue.

Next steps

To understand what Soteria is going to do with this information, it’s helpful to understand the company. It is dedicated to inherently safe batteries everywhere. While we have our own technology to achieve this and it does a good job, we also recognise safety requires a full-stack solution.

Consortium for battery safety

To help facilitate this, we launched a consortium of companies to work together for battery safety, now with more than 125 members including NASA, Mercedes-Benz, Motorola, DuPont, Teijin and many more. The goal is to propagate safety technologies throughout the industry as quickly as possible, saving lives, reducing injuries and reducing property damage.

With the information in hand, we plan several distinct actions to help the industry align on a solution:

• report: A comprehensive report will be made available for free to fire departments and city, state and national agencies, and to anyone in the industry for a small fee
• best practices: The group of companies participating in the E-bike Battery Safety Project is meeting in January to review the information and put together a list of best practices for building a safe e-bike battery. Different from a test standard, this will tell you how to do it. Packs built according to these best practices will be better than any that we disassembled, containing significant safety advantages over those in the market today. These best practices will be provided free to e-bike manufacturers, and to the rest of the industry for a small fee
• design standard: From the group-produced best practices, Soteria will author a design standard, similar to a military detailed specification. E-bike battery packs can be audited to it and the design certified. Companies wishing to design their batteries according to this design standard will be given access to the standard
• certification process: Working with consortium members, we will set up a process for certifying a battery to the design standard. Companies can have their final packs audited to the standard, and once audited, can mark them as such and use the Soteria logo to show that they have been certified
• batteries: Voltaplex will offer a line of batteries built to the design standard and certified. It will offer these for sale as replacement packs to consumers, and will also begin working with e-bike OEMs to build certified packs for them.

E-bike industry largely ignoring massive problem

The e-bike industry is a massive industry with a massive problem, and they are largely ignoring it, despite protestations from fire departments around the world. E-bike batteries are catching fire at a rate that is at least 100 times more than it should be – perhaps much higher than that, if our survey can be believed.

The batteries themselves are a comedy of small errors and carelessness, coupled with relatively poor design. Unfortunately, there is not a single culprit that can be pointed to. It was not refurbished batteries, Chinese, or inexpensive replacement batteries. OEM batteries were no better or worse in design. The worst ones were bad, but the best still had much room for improvement.

Soteria will be publishing the results. A solution will be easily implementable and Soteria will share this with the industry.

Through its subsidiary Voltaplex, Soteria will offer a line of batteries built according to this specification, and will also offer a certification service for those other companies who would like to communicate with confidence that their batteries have been made to a safe standard.