Fire safety of batteries in electric vehicles

The use of electric vehicles is expected to increase in the coming years, mainly due to the cost of traditional fuels and rising environmental concerns. Billions of dollars are being invested globally to develop and promote this technology, including almost $3 billion from the 2009 American Reinvestment and Recovery Act.

According to a market analysis*, the estimated number of plug-in and battery electric vehicles that will be on the road over the next decade will range from 1 million to 5 million new vehicles per year. Along with this growth comes the potential for fire, electric shock and other safety hazards.

Electrical batteries may increase the risk of fire and electrical hazards. Therefore, standards to mitigate these potential risks were developed in the last years. One particular issue is the use of large lithium batteries in plug-in and battery electric vehicles.

To this end, a set of requirements for large batteries in electric vehicles has been finalized by Underwriters Laboratories (UL) as UL Subject 2580**. Before becoming a standard, these requirements will be reviewed by a global standard technical panel (STP).

The Subject Standard covers nickel, lithium ion, and lithium ion polymer cells, cell modules, and battery packs. The battery packs are of a secondary (rechargeable) type and are suitable for various electric vehicle (EV), hybrid electric vehicle (HEV), and plug-in hybrid electric vehicle (PHEV) applications. Various battery chemistries are included, such as lithium-ion, nickel-metal hydride, lead acid, etc. These batteries have been investigated for potential electric shock, fire and explosion hazard.

Requirements are made for polymeric materials employed as battery pack enclosures; they shall have a minimum flame rating of V-1 to UL 94 Standard for Tests for Flammability of Plastic Materials for Parts in Devices and Appliances. In addition, the polymeric materials should meet the Relative Thermal Index (RTI) to UL 746B with impact suitable for temperatures encountered in the application, but no less than 80°C. Finally, the pack enclosure materials intended to be exposed to sunlight in the end-use application shall comply with the UV Resistance and the Water Exposure and Immersion tests to UL 746C.