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How do lithium-ion battery packs catch fire?

Updated: Jan 11, 2023

Lithium-ion battery packs are the most energy dense form of rechargeable power. They fuel everything from Tesla cars to electric bikes and scooters. As electric vehicles (and lightweight personal transport devices) become more commonplace, so do the news stories of battery fires. In dense urban cities like New York, these fires can wreak havoc on apartment buildings and can often be fatal. New legislation is being proposed to curb the incident rate of fires like this. Both buildings and transit centers are re-thinking when and how to allow devices with high-powered battery packs like these.

The most common causes of lithium battery pack fires:

Mis-matched chargers

Every electric bike or scooter comes with a battery pack defined by a nominal voltage, ranging from 36V (most common on electric scooters), to 72V (often seen on higher powered electric mopeds). A charging box will need to match the voltage of the device you are charging.

If I mistakenly use a 48V e-bike charger on my 36V scooter's battery pack - the charger won't automatically shut off current at 36V, it will continuing feeding power into the battery pack at a levy beyond what is safe. If the battery pack doesn't have its own internal circuitry to stop taking additional charge after it senses its full, this will result in the cells overcharging and overheating, leading to a sharp increase in combustion.

Most high-quality battery packs won't be affected by this, because they use a smart BMS to regulate current input. But with cheaper cell and knock-off battery packs, they are often built without this technology. More on that below.

Physical damage to the battery pack

Every battery pack includes complex circuity and assembly which regulates charge / discharge among cells. A puncture or break inside a battery pack could cause one or more cells to fail. As one failed cell can generate a rapid amount of heat, this can move from cell to cell in an event known as Thermal Runaway. This chain reaction can cause an entire battery pack to catch fire within just a few short seconds.

The contents of the battery are under pressure, so if a metal fragment punctures a partition that keeps the components separate, the lithium reacts with water in the air vigorously, generating high heat and sometimes producing a fire.

Most battery packs are built with protective casing that would prevent this from happening, even in the event of an accident. But DIY built battery packs often lack the structural integrity that prevent incidents like this.

All Levy battery packs are built with fire-safe outer casing built from A380 Aluminum.

Lack of a proper BMS

A BMS (short for Battery Management System), is a circuit built inside a battery pack that regulates charge levels between all individual cells, which for an electric bike or scooter could be up to 80 cells in one pack.

One function of a BMS is to act as another layer of overcharge protection. A BMS will stop taking current from a charger once it sees that all cells in the pack have already reached their maximum voltage. This would prevent failure in the case of the first example, where a charger with a higher voltage is used on a lower voltage battery pack.

On the other end, it performs a similar function when the battery is being used under load and discharging. It limits the amount of current that can exit the cells at once, which regulates heat and safety of the battery pack according to manufacturer specs. These parameters are known as maximum continuous charging and discharging current limits.

A BMS ensures a cell's peak currents aren't exceeded

Another aspect of the BMS is manage the input charge for each cell on an individual basis. Over time through natural charge / discharge cycles, some cells may degrade faster. This means they will discharge faster than others in the pack. In a battery without a BMS, the input charge and discharge during riding would be spread equally over all cells, likely resulting in the degraded cells to overheat with the additional strain placed on them. The role of the BMS is to manage the load on each cell appropriately based on it's current state of charge, which is monitored in real-time.

Example BMS Wiring on a Battery Pack

Extreme Temperatures

Most battery packs can safely operate in temperatures up to 130 degrees fahrenheit, so it's more rare that this will be the cause. However, putting the battery pack under increased loads that it is not designed for can increase the temperature above safe operating limits.

For example. some people may wish to modify their electric scooter or bike to use a higher power motor than is recommended by the manufacturer. This will result in above-spec current discharge, which can overheat the battery pack. A good BMS will include temperature sensors that automatically limits current output when it notices above normal temps, but this isn't the case for many cheaply made battery packs.

What makes Levy batteries a safe option?

1) Our batteries contain built-in protection circuits, called a BMS in the electric vehicle industry. These protection circuits help prevent overcharging, over-discharging, and short-circuiting. All Levy Batteries come with state of the art BMS systems to ensure a battery pack is always operating at safe voltages and heat levels.

2) Levy's chargers automatically prevent voltage above the required range. Combined with our battery's BMS, this a secondary safety measure to ensure voltage never goes above the allowable range in our battery packs, as charging a lithium-ion battery pack outside of its recommended voltage range can cause it to overheat and catch fire.

3) Fire and puncture proof outer casing. Our battery packs are completely enclosed in a metal outer casing. Making them drop proof and puncture proof, as physical damage to a lithium-ion battery pack can cause it to short-circuit in rare occasions.

4) Reputable components. Our chargers and battery packs are produced according to detailed specifications and strict quality control measures, and come with the highest quality UL Lab ratings. Always know how you are buying from before ordering components like this, and always try to order from the original source and don't compromise on quality.



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