It was weirdly coincidental, maybe ominous? Right around the time that I launched Direct Current, a few terribly tragic fires happened in American cities. Their cause: E-bikes, or more specifically, their batteries.
The New York State Attorney General issued a consumer alert in June 2023, stating that e-bike batteries had caused more than 100 fires to date, which resulted in 13 deaths. Some searching on Google yields plenty of horror stories, media reports, and viral videos that might make you think twice about a bike with a battery.
Uhhh… Should I be worried? After all, I’ve got two e-MTBs in my garage right now (a Cannondale and a Specialized).
Instead of wading through the Internet for an answer, I called a friend, who is in the battery biz. He’s an engineer at a well-known consumer brand that makes lots of battery-powered things. He’s a PhD. The only catch is, we have to keep him anonymous, so he doesn’t get in trouble with corporate. But trust me, his insight helped me understand batteries better than I’d even dreamed possible.
What Are Lithium-Ion Batteries and How Do they Work?
We never used to worry about battery fires with the AAs in our old GameBoys and flashlights, did we? Maybe they’d leak a little acid and corrode things, but Tetris never burnt down a house. That’s because those are alkaline batteries.
Alkaline batteries rely on a potassium hydroxide paste to move ions around and produce electricity. It’s a water-based medium that eventually decomposes and has a limited amount of voltage. Less power, not truly rechargeable because the components degrade by design.
These batteries are fairly safe, so long as you don’t eat them, but they aren’t powerful enough to power an e-bike, let alone a car.
Enter the lithium-ion battery. This battery relies on reversible chemical reactions to produce electricity, so these batteries can be recharged many, many times before they’re done. These batteries shuttle lithium ions from the negative side, where they’re stored in graphite, to the positive side, where you find a lithium-containing metal compound, such as nickel or cobalt. This is the cathode material. When everything works right, these ions move from negative to positive to give you the juice; then, you plug in your device and the ions move back to the negative side.
Most importantly, lithium-ion batteries produce way more voltage than alkaline batteries, making them crucial for all of our electrically motorized conveyances.
What Causes E-Bike Battery Fires
Unlike alkaline batteries and their water-based medium, lithium-ion batteries are filled with a special type of electrolyte, which is carbon-based. If you remember your basic chemistry, carbon is flammable.
Along with this very flammable liquid, the battery’s cathode materials contain oxygen. They become unstable when overheated, and that oxygen fuels combustion, which can lead to a runaway chemical reaction. As any chemistry teacher (or firefighter) will tell you, that adds up to two points on the fire triangle: fuel and oxygen. Complete the triangle with an ignition source, and you’re off to the races.
In most lithium-ion battery fires, that ignition source is a short circuit between the positive and negative sides of the battery. When you bridge the positive and negative terminals, the battery rapidly discharges voltage, producing a lot of heat, and eventually igniting a fire.
Scenario #1: Battery Damage
This short circuit could occur if the battery is damaged. If you don’t have AirPods in, you’ll hear flight attendants on a plane tell you not to move your seat if your phone gets dropped. They don’t want you to crush that lithium-ion battery and start a fire.
Should we worry?
Probably not. Most e-bike batteries are housed in frame tubes, especially high-end e-MTBs like those in my garage. They’re shielded from all but the most extreme impacts.
Scenario #2: Poor Manufacturing
The battery can also short-circuit if its tolerances are off — and tolerances are very tight in these units. Manufacturers pack a bunch of individual cells into a larger battery unit. Each cell holds electrolyte, cathode, graphite, and all the other good stuff that makes voltage (or fire). If any of the internal materials are out of place, they might bridge the positive and negative terminals of a cell, resulting in a rapid discharge of voltage, heat, ignition, and fire.
Should we worry?
A battery from a large, reputable manufacturer is unlikely to be out of spec. If you’re buying a battery off of Ali Baba for $50, then you’re playing with… well, you’re risking it.
Scenario #3: Extensive Wear
While lithium-ion batteries can be recharged far more times than Alkaline batteries, over time, the electrolyte material breaks down and starts to create build-up, leading to more resistance for those lithium ions to move back and forth. In some cases, the ions won’t return to the graphite material on the negative side, and will instead form a sharp growth outside of the graphite. Eventually, if the battery keeps getting used and recharged, this tiny dendrite could puncture the membrane separating positive from negative, which would connect the poles and likely result in a fire.
Should we worry?
Maybe. But there are some ways to be a little gentler on our e-bike batteries to avoid unwanted wear and the risk of a fire.
How to Avoid E-Bike Fires
We can make some choices about charging our e-bikes to extend the life of their batteries and reduce the likelihood of failure. Most of it comes down to temperature. It’s best not to charge a battery in extreme heat or extreme cold.
Also, it’s best not to charge a battery to 100% and leave it plugged in for a long time. Thankfully, some e-bike drive units have safeguards that make it easy to avoid this mistake. I’ve noticed that the Bosch battery and charger on my Cannondale usually tops out around 98% charge. In the Specialized app, you can switch on Charge Limit to stop charging at 80%.
Filed under good common-sense advice, you also should keep your e-bike away from any flammable materials in the garage. If something were to go wrong, it would be a whole lot worse if the lawnmower’s gas can was right next to that lithium-ion battery.
And finally, don’t cheap out on some off-brand bike or battery. While price doesn’t equate to safety 100% of the time, it’s usually a safe bet to buy from proven brands.
So Don’t Worry, Just Be Smart
What I gathered from my chat with my PhD pal is that, in general, I don’t have much to worry about. My e-MTBs are made by reputable manufacturers. I don’t overcharge the batteries. I unplug them if I’m not riding for a while. And I keep them separate from the HazMat in my garage.
Better still, now I know what’s going on with these batteries. They don’t like extreme temperatures. They need to be protected from damage. And they’re designed to extremely tight tolerances.
The benefits outweigh the risks. Lithium-ion batteries are tremendously powerful — about 4x what you get from an alkaline battery. They can be recharged many, many times, so you get more bang for your buck, and aren’t throwing away batteries on a regular basis. With great power comes great responsibility.