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.
‘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.
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
The sketch is (1] an example that a person with a body resistance of 500 Ohms contacting an electrical source of 50 volts will experience 100 milliamperes of current.
For reference, the International Electrotechnical Commission (IEC) uses 1000 Ohms as a typical value for adult human body resistance.The following are some guidelinesfor the effect of direct current (DC), the ranges corresponding to females and males (or different body sizes) respectively.
Generally, values higher than 50 volts DC are considered ‘high voltage’ and require adequate protection. As examples, Chevrolet Volt specifies its RESS of lithium-ion batteries at 360 volts, whereas Tesla batteries are stated as 375 volts. In addition to the batteries, all components connected to the batteries directly or indirectly are also considered to be part of the high-voltage system (shown in the sketches). In such vehicles, it is necessary to assure that those likely to come in contact with the automobile during normal operations or in accidents are adequately protected from all high- voltage parts.
There are no US regulationsthat explicitly address this issue of electrical safety but there are recommended practices (SAE J-1776) that specify that one or more of the following criteria be met after any crash test:
– Voltage at specified locations on electrical bus < 60V DC or <30V AC;
– Electrical energy < 0.2 Joules;
– Isolation between high-voltage bus and conducting structure > 500 Ohms/volt (or > 100 Ohms/volt for DC-only buses not connected to electrical grids).
The above are post-crash criteria and may not govern functional safety for RESS-powered vehicles.
European countries have such regulations as part of ECE R94, etc. for crash safety of RESS-powered vehicles.
2. Post-crash fire: Risks arise if flammable materials come in contact with a vehicle’s hot parts or if sparks resulting from an accident ignite such material. In addition to the ‘usual’ hot parts in a conventional car, the batteries may themselves become hot surfaces during operation. Therefore, such batteries need to be properly isolated during operation. Safe levels of such isolation also need to be maintained during and after accidents as well and it is also necessary to ensure that any flammable electrolytes in the battery cells do not leak by any significant amount.
3. Post-crash Rescue: It is widely regarded that trauma is a ‘time-dependent disease’ and survival probabilities and injury outcome depend greatly on the ‘time elapsed’ from the moment of accident to the moment when proper care is given at an appropriate facility. It is thus highly desirable that this elapsed time be as short as possible and the term ‘golden hour’ is often used as a guideline to be met for victims with serious injuries. One of the components of ‘golden hour’ is the ‘time needed to extricate occupants from vehicle’which depends to a significant extent on several aspects of vehicle design, as well as on the type and the severity of the crash. In cases of serious damage to a vehicle, extrication of its occupants may require emergency responders to cut and remove parts of the automobile. When RESS batteries are present in the vehicle, such cut-and-remove operations may pose unacceptable risks for emergency responders.