The numbers of electric and hybrid cars on US roads have increased significantly over the years, starting with the first introductions of Honda InSight (1999) and Toyota Prius (2000). As with all new technologies, any issues with these vehicles have received intense scrutiny1,2, such as the case of fire in a Chevrolet Volt, which occurred in a vehicle that had been stored for several days after undergoing a crash test.

All hybrid & electric vehicles use high-voltage batteries ('rechargeable energy storage systems' or "RESS") to store energy that is used for propulsion when needed. Current energy storage technologies generally use Lithium-ion (Li) or Nickel- metal hydride (Ni-Mh) cells. A typical RESS consists of multiple cells connected together and the entire assembly surrounded by rigid structure. There are some unique challenges in assuring the safety of high voltage RESS units and these are discussed below.

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'

The overall safety of an automobile can be considered as being composed of (a) Functional safety, and (b) Safety of occupants (and others in proximity) in a crash or in accidents.

Functional safety consists of factors such as (i) safety during normal driving, (ii) safety during storage, (iii) safety while undergoing service and maintenance, and (iv) safe disposal of vehicle at end of life. The functional safety of hybrid and electric vehicles requires consideration of factors such as heat & thermal energy management, electrical system integrity, control system reliability, EMI shielding and charging system safety, etc., in addition to accounting for other factors associated with conventional vehicles. 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. Also, additional tests are conducted by some manufacturers to assess safety in crash scenarios which are likely but not included in the above.

Some Risk Factors associated with RESS-powered vehicles:

1. Electrical Shock & Injury: Thresholds for electrical injuries depend on many factors such as a person's body size, type & amount of current, time duration of contact, etc. The amount of current passing through a body is

. . .Continue to read rest of article (PDF).