Hybrid cars have been on US roads for many years, starting with Honda InSight (1999) and Toyota Prius(2000) in mass production. Electric vehicles in mass production are more recent with the Chevrolet Volt and the Nissan Leaf introduction in 2010. Tesla and Fisker also produce electric cars in the high-price range and several other manufacturers are now also introducing such vehicles. A recent case of fire in an electric vehicle (Volt), that occurred several days after a crash test, was widely reported, signifying that risks associated with new technologies will receive intense scrutiny, even if these risks are less than the ones in the replaced technology.

As compared to conventional automobiles, hybrid & electric vehicles use high-voltage batteries (known as 'rechargeable energy storage systems' or "RESS") to store energy and then supply it as electric current when needed for operation. These batteries consist of multiple cells connected together inside a rigid enclosure. Electric vehicles (e.g. Volt, Leaf) have larger batteries that can propel the vehicle for significant distances whereas hybrid vehicles have smaller battery units. Current vehicles use either Lithium-ion (Li) cells or Nickel- metal hydride (Ni-Mh) cells.

Although some discussions in the media have been of the relative safety of these RESS-powered vehicles (as compared to their gasoline-powered counterparts), the appropriate question to ask in designing and evaluating these vehicles is whether all applicable safety standards are met and whether all steps have been taken to maximize occupants' safety in foreseeable situations. In this regard, the presence of high-voltage RESS units present several unique challenges. This note briefly discusses some of the issues.

'Functional Safety' and 'Safety in Accidents'

Risks are inherent in all systems that store energy, including conventional automobiles powered by liquid fuels or by compressed gas. Risks are also associated with any moving object because its motion gives it kinetic energy, which is a form of stored energy. For any new technology such as hybrid and electric vehicles, such risks need to be identified early and vehicle design and testing completed to assure maximum overall safety which is composed of

(a) Functional safety, and

(b) Safety of occupants (and others in proximity) in accidents.

Functional safety may be defined as consisting of (i) safety during normal operations, (ii) safety during service and maintenance, and (iii) safe disposal of vehicle at end of life. Safety during normal operation of these vehicles requires that, in addition to other aspects of conventional vehicles, one takes into account factors such as heat & thermal energy management, electrical system integrity, control system reliability, EMI shielding and charging system safety, etc. Test conditions for functional safety are specified by the vehicle manufacturer. Safety in accidents is evaluated by crash tests defined by the NHTSA and by the Insurance Institute for Highway Safety, as well as by additional tests that may be conducted by manufacturers to represent likely accident scenarios.

Some Risk Factors associated with RESS-powered vehicles:

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