Edward Benjamin
Chairman, Light Electric Vehicle Association
Saturday, May 8, 2021
I have an electric bike in my garage. The battery on that bike contains enough energy to propel my 100 Kilo self, plus the relatively heavy Pedego Stretch Cargo Bike, and whatever cargo I am hauling (often more than 50 kilos of SCUBA gear) at 20 MPH (32 KPH) for …. more than an hour… (I have never actually exhausted that battery.)
The amount of energy needed to push me, my cargo, the bike, at 20 mph for an hour is something any physicist can calculate. I cannot. But I will just say it is a LOT of energy crammed into a battery box about the size of two cigar boxes (or one shoebox, size 13, for those who do not smoke cigars).
And at 20 mph, I am drawing current at a glorious rate (more than 30 Amps) that is heating wires, connectors, circuit boards, and despite a very efficient motor….the motor.
As recently as a couple decades ago, using a lithium manganese battery in such a fashion would have risked of starting a battery fire. Today, on that bike, fire is extremely unlikely.
There are a bunch of reasons why:
- Smart BMS (Battery Management System) and motor controllers. Never mind how hard I push the bike, the microchip-controlled BMS and controller have software that is intended to both prevent damage to the components, and to absolutely prevent any chance of a fire. They accomplish this by monitoring the current being used, limiting, or shutting it off if needed. The BMS has a temperature sensor (often a thermistor, which will allow current to flow when it reaches a particular temperature) which alerts the BMS to a high temperature in the battery package. A BMS can then shut down or limit current using solid-state switches called MOSFETs (metal–oxide–semiconductor field-effect transistor).A BMS will also prevent overcharge, over-discharge, react to short circuits, and monitor the state of charge.The result: A shutdown and wait while things cool down. Or more likely, limits to use that mean nothing I will notice.
- Safety features built into the cells themselves. An ebike battery-package might have some or all of these: a device built into the cell to inhibit high current surges; a safety vent that allows a the release of high-pressure gas created by a malfunction.
- The material of the cell. Magnesium Spinel, the most common metallurgy used in ebike batteries, is inherently pretty stable. All batteries can burn, and it is true that Lithium Iron Phosphate is even more stable than Lithium Magnesium Spinel. But LiMa is the most common choice due to energy density, and cost. These cells are supposed to stay stable up to about 250 C (Really, really hot!)
- Smart chargers. Battery fires seem to happen most often when on charge. So a quality charger that can monitor the state of charge, and shut off when the charge is finished, helps prevent problems. Some chargers even have a way to monitor the temperature of the battery during charging. Some chargers will refuse to work with any bike except the correct one.
- Better designed cells and battery packages that can tolerate high rates of discharge and charge are common now, and represent lessons learned and progress in technology.
A battery fire on an electric bike is pretty unlikely. With about 300 million electric bikes in use worldwide, fires are so rare that they make the news and get on YouTube.
There is always a caveat (Latin word that makes me feel smart for using it: a warning or limitation) however….
The ingredients of the battery packages that are so unlikely to be trouble are Quality cells, well-designed battery packages, well-made battery packages, well-designed and manufactured battery management systems, good BMS software, well designed and manufactured chargers. Good motor controllers, and sensible ebike owner/operators.
A famous brand name bike will have these ingredients (well the bike owner may be sketchy…) and the bike and battery (and charger ) will have been tested extensively (and often show approvals such as CE, UL, BATSO, etc.) And there are quality bikes that are from brands not-yet-famous, that also test carefully and build with only quality components.
There are, however, damaged bikes, home-built bikes, DIY kit bikes, cheap bikes, bikes intended to go ridiculously fast, and mistakes in engineering or production. These are the bikes that tend to have problems.
Problems can start when the product manager (a person who takes care of all details in the production of the bike) realizes that the battery is EXPENSIVE and tries to save money by asking for a lower-priced battery. That can lead to short cuts in design, material, and features. This affects lowest-priced bikes the most.
How to protect yourself from a fire, and what to do if one does occur.
Large format lithium batteries make for wicked fires that are hard to extinguish. The smoke and vapors are toxic, and just taking the flames away does not mean the event is over. The huge amount of energy stored in the battery will continue to create heat, and the battery will re-ignite after being “put out”.
This is called Thermal Runaway.
Prevention: Buy a quality bike from a reputable maker and retailer.
Investigate a bit. Online complaints and bad reviews should be taken seriously. Warranty can be an indicator of the quality and the seller’s confidence in the product. (I suggest buying from a local retailer when possible, but if you are buying over the internet, try calling their customer service line as though you had a problem and see what the experience is.) Read the manual and the warning stickers.
Only use the charger that came with the bike. Follow the instructions for charging.
Even better:
Only charge when someone is present. Do not charge while you are sleeping or away.
Set a timer to limit the time on charge. A simple current timer from Amazon is about 20.00 and can be set to turn off the charger even if you forget.
Charge in a clean dry location that has nothing flammable above, below, or adjacent to the bike or battery. (I know the story of a case where a charging battery caught fire below a shelf that had isopropyl alcohol and acetone stored in gallon containers. It is not a happy story.)
I know a couple of other stories of batteries that overheated and caught fire while charging on wooden workbenches. Those stories involved the bike shops burning down.
Important: Charge the battery or bike in a place where you can eject the bike or battery out of the building if it starts to smell, emit vapor, smoke, etc.
Have a smoke detector above the charging area.
And now…what to do if it is on fire despite your precautions:
First and foremost get everyone out of the building. Call 911 and get the fire department on the way. Then consider if you can do any of the following. If you cannot be safe, don’t try.
- Avoid breathing the fumes and smoke. They are wicked bad for you. Don’t pick the battery up, it is HOT, and it will probably fall apart. And the cells can explode, tossing burning material about.
- Extinguish the flames with a fire extinguisher. Almost any kind will work. Even a water extinguisher or a hose. I have a couple of ABC extinguishers in my workshop.
- Cool the battery package. If we only put out the flames, the battery will re-ignite as the thermal runaway created by the stored energy in the cells continues to create heat. Water in generous amounts works.
- Keep cooling the battery package. I have a hose connected to a tap that I can use. And a bucket as backup. Avoid breathing the fumes.
- Keep cooling the battery package.
- Keep cooling the battery package.
- The fire department has probably arrived by now, and they will probably keep cooling the battery package. When they decide it is inert, it will be time to remove it to a safe spot…like the parking lot or driveway.
- The remains of the battery pack are of interest to the insurance company, the people who made and sold your bike, the people who designed and manufactured the battery package, BMS, and cells. Photograph it, document what happened. Keep the battery remains intact and ready for visiting inspectors.
- If you have a fire, please Click Here send me an email telling me what happened.