Flame Retardants in Battery Enclosures
Flame retardant chemicals are often added to plastic battery enclosures to meet flammability requirements. The flame-retarded plastics are unlikely to slow or stop highly energetic lithium-ion battery fires and may be harmful
The flame retardants can migrate from plastic cases of many products—from earbuds to EVs—into air, dust, water, people, animals, and end up in our food supply. Exposure to these harmful chemicals can contribute to cancer, neurological, reproductive, and immune harm. Young children and pregnant women are the most vulnerable.
When plastics burn, flame retardants can create additional smoke, soot, and toxic gases, which are major causes of fire deaths. They can also produce dioxins and furans, which can contribute to firefighter cancer.
At the end of their useful life, flame-retarded plastics can contaminate the recycling stream and impede the circular economy. Their disposal, either by burning or in landfills, leads to toxic emissions and can be harmful to the global environment and human health, especially in low-wealth countries.
Flame-retarded plastics should not be used in the casings around lithium-ion batteries unless a significant fire safety benefit can be demonstrated.
UL-94 Flammability Test
- A small open flame test commonly used to assess flammability of plastics.
- A gas burner is applied to the plastic for ten seconds and then removed; the process is then repeated. For each rating classes, the plastic must not completely burn and must not burn longer than a prescribed time period.
- V0 and V1 ratings are usually achieved with flame retardants.
- Not adequate to simulate highly energetic thermal runaway fires of lithium-ion batteries.
- Performance on the test may not correlate with flammability behavior of a battery.
Case study: e-bikes and micromobility devices
Industry safety standards are being applied widely to e-mobility devices:
- UL 2849: A US standard for electrical systems for e-bikes, including the drive train, battery, and charger
- UL 2271: A US standard for just the batteries in light mobility devices
- GB 43854-2024: A Chinese safety standard for lithium-ion batteries used in e-bikes
- Both UL standards mandate that enclosures must meet a minimum of V1 flammability rating, and the Chinese standard
requires a V0 rating—either rating is almost certainly met with
the use of flame retardants
Legislation:
To address e-bike fires and fire deaths, new legislation referencing the above UL standards has been quickly developed and adopted by New York City and California and is being considered by the Consumer Products Safety Commission and Congress.
Pros: These standards contain effective safety measures such as requiring battery thermal management systems and testing battery’s ability to withstand real-world use scenarios (e.g. a vibration test, overcharging test, and short circuit test). Safety standard certifications can also help avoid faulty products.
Cons: Flammability requirements for plastic battery enclosures can result in the addition of harmful flame retardants, followed by migration out of products, and health and environmental harm. A fire-safety benefit of flame retardants in plastic enclosures around lithium-ion batteries has not been demonstrated.