CURRENT EVENTS IN VEHICLE BATTERY SAFETY Donald B. Karner, Electric Transportation Applications, Glendale, AZ 85308

As electric vehicles begin to leave the research environment for placement into fleets of electric utilities and government agencies, issues of battery safety become paramount to facilitating electric vehicle commercialization. Through the conduct of electric vehicle competitions, much experience has been gained with battery safety in automotive applications. The race track provides a controlled environment with safety requirements similar to that found in on-the-road applications. While a greater level of risk may be tolerated on the track than on the road, the issues which must be dealt with are the same.

In order to avoid learning of battery safety issues by accident, a project has been undertaken to determine the hazards and risk associated with electric vehicle competitions and develop standards and requirements to ize risk. The Safety of Electric Racing Vehicles (SERV) Project is a joint effort of Arizona Public Service Company, the U.S. Department of Energy, and other electric vehicle industry sponsors. The SERV Project has developed a systematic evaluation of hazards and risk associated with electric vehicle operation. This analysis focuses on the requirements necessary to establish a safe electric vehicle infrastructure.

The SERV evaluation process begins with the identification of hazards associated with electric vehicles. This identification includes electrical, chemical, fire and collision hazards. The risks associated with these hazards are then evaluated, as is the uncertainty with which the risk is known. The methodology used to perform the hazards assessment and riWuncertainty evaluation is an adaptation of a synergistic methodology utilized in process industries to evaluate new chemical processes. Once the risk and uncertainty have been qualitatively evaluated, a ranking is performed to determine priority for development of requirements to reduce both risk and uncertainty to acceptable levels (hence increasing the assurance of safety),

Requirements for reducing risk and uncertainty take the form of existing consensus standards where ever possible. If an existing standard does not exist or is not appropriate to the specific risk, a

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requirement is developed based on engineering judgment. Requirements are then tested for effectiveness and practicahty through pilot application at electric vehicle competition events. Specific feedback in the form of lessons leaned is gathered after each pilot application and appropriate modifications to requirements are incorporated.

Through the SERV Project efforts and actual experience with electric vehicles in a competition environment, several issues involving battery design have emerged as significant to battery safety and electric vehicle commercialization. These include chemical hazards, electrical hazards, and collision hazards.

Chemical Hazards

The chemical hazards associated with a given battery technology are perhaps the most significant of a l l hazards. Issues to consider include the reactivity of the battery constituents, potential interactions between battery constituents and common automotive chemicals, the interaction of battery constituents with chemicals from other batteries, handling issues associated with battery constituents, and storage and disposal of battery constituents . The potential for chemical release is the primary hazard associated with battery systems. This hazard can be reduced or eliminated by using immobilized chemicals. Large volumes of liquid or gaseous chemicals significantly increase the hazard associated with an electric vehicle battery system. Batteries of this type must employ engineered safeguards to reduce the risk associated with the release hazard. These safeguards must be effective against battery tampering by untrained personnel, battery system hardware and software (control system) failures, as well as the results of impact during vehicle collisions. This latter requirement is by far the most difficult to accommodate. With the severe damage possible in a vehicle collision, it is very difficult to provide the protection necessary to prevent chemical releases in all vehicle collision scenarios. Therefore, the consequences of a release

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must be examined and the risk associated with such release evaluated. With the potential for tens to hundreds of human receptors to be in the vicinity of a vehicle collision, the release of even smaU qualities of highly reactive materials may present high consequences and unacceptable risk.

Hazards associated with high temperature batteries are also contributors to risks associated with on the road electric vehicles. Control system failures causing overheating on charge or discharge, insulation failure, and collision damage to insulation or resulting in release of molten materials are a few of the scenarios which can contribute to risks from high temperature batteries. Engineered safeguards must again be employed to mitigate these risks to acceptable levels.

The choice of battery system must consider the hazard involved with battery constituents. If these hazards are ignored, the cost, weight, and p e r f o m c e of the battery system may be unacceptably impacted by the engineered safeguards necessary to reduce chemical release risks to acceptable levels.

Electrical Hazards

The electrical hazards associated with a given battery technology are driven by the voltage level of the battery system and the energy available from a single module. As battery modules are series connected to establish a traction battery, voltages may reach over 400 volts and pack energy may exceed 50 kilowatt hours. The potential for electrical shock from this high voltage, or unrestrained release of battery energy present a hazard to personnel maintaining the vehicle. While it is always desirable to have trained personnel servicing the vehicle, the possibility that inexperienced and untrained personnel may be involved with vehicle maintenance must be considered.

The shock hazard associated with electric vehicle traction batteries is normally reduced by maintaining a floating ground on the battery. This floating ground is only as effective as the insulation level of the battery. Terminal design must evolve to a closed or tightly insulated terminal to provide very high insulation to ground under duty, wet, and salty conditions. The open terminals typically used on contemporary batteries does not provide sufficient track distance to maintain near zero

leakage current to ground under all operating conditions.

Unrestrained energy release fiom battery modules becomes a great concern as battery energy and module size increase. For modules with energy in the range of several kilowatt hours, it is not acceptable to rely on external system fuses to protect against short circuits discharging battery energy. Module handling accidents and collisions provide significant opportunity to short circuit a single module, unprotected by a system fuse. Therefore, integral module fusing must become a consideration in large, high energy modules. The module fuse ampacity may be set substantially above typical system fuse ratings to ensure compatibility with various drive systems, yet provide protection fiom a fault on the battery module.

Collision Hazards

Collision hazards result from chemical release (addressed under chemical hazards) and mechanical forces. The large battery mass in an electric vehicle presents a substantial challenge to the designer of vehicle crash protection features. The efficiency required by electric vehicles demands the vehicle structures be as light as possible and that the vehicle battery mass fraction be as large as possible. This large battery mass contained in a light vehicle structure makes compliance with vehicle crashworhtiness requirements difficult. Batteries typically must be placed beneath the vehicle floor to allow a direct mechanical connection with the frame and battery movement in a frontal crash without intruding in the passenger compartment. Batteries must also be place low and centrally within the vehicle to assure proper handling characteristics. This dictates that a proper electric vehicle battery have a low aspect ratio to allow placement beneath the vehicle, while maintaining appropriate ground clearance. Battery terminals should be placed to avoid top connections.

In a competition environment, accommodations can be made to overcome current battery shortcomings in the areas of chemical, electrical, and collision hazards. These accommodations typically take the form of mitigating measures which increase vehicle weight, reduce vehicle performance, reduce vehicle usefulness, or require specialized personnel training. If electric vehicles are to be extensively

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training. If electric vehicles are to be extensively commercialized, these types of mitigative measures will be unacceptable to both the vehicle manufacturer and the vehicle purchaser. Further, batteries that are not inherently safe may never be installed in mass production vehicles.

The electric vehicle must compete with a technology that is already viewed as safe and in a regulatory environment which mandates safety. The recent r e d order for General Motors CK pickup trucks with side saddle fuel tanks is but one example of the extreme c m which is required in mass production automotive applications. Proper battery design must assist in avoiding the economic and political consequences of such a recall for electric vehicles.

References

1. A r i Z o ~ Public Service Company, Edison Electric Institute, US Department of Energy, SERV Project Race Safety Plans, published by Electric Transportation Applications; September 1993.

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