Safety test methods for EV batteries

The aim of the present paper is to share our experience in battery safety testing and risk analysis from a recent project called BLIXT, which had the objective to speed up the progress of the paradigmatic shift toward electric vehicles (EV). The batteries in question have been three different configurations of lithiumion batteries (LiFePO4). The tests have involved fire, crash tests and sort circuit.


Introduction
When introducing new technologies in vehicles, it is important to obtain at least the same degree of safety as for conventional vehicles, in order not to lose costumers confidence. The safety risks of EVs differ considerable from conventional vehicles and an acceptable level of safety should be maintained during the whole life cycle of the vehicle (assembly, use, service, accident and recycling). This gives rise to a lot of questions concerning electric safety, fire safety, chemical risks and electromagnetic fields. Consequently, the main answer is that numerous new test methods are needed to secure right level of safety.
SP Technical Research Institute of Sweden (SP) was one of the partners in the BLIXT project. The project brought together parties from the Swedish automotive industry with the aims to accelerate and facilitate a Swedish production of EVs. Since the project aimed to speed up the process of a paradigm shift towards EVs, other design solutions were made compared to a project with longer time frames concerning design and development. One example was designing the vehicle with the batteries in the crash zone. This was done to utilize available free space that arose when the ICE (internal combustion engine) and its peripheral equipment were removed. Even though accurate test procedures and standards are being developed [1, 2 and 3], the efforts within the present project have been performed regarding issues related to the unique placing of the battery.
In the SP laboratories the batteries have been fireand crash tested, subjected to short circuit and compression tests. The test have been performed to answer the questions what happens if there is a crash or a fire involving compressed batteries, short circuit or an explosion. Figure 1 shows the most evident outcome of the project, a converted SAAB 9-3 cabriolet.

Management of chemical risks during safety tests of batteries
Batteries for EVs contain chemicals that are potential harmful to persons and/or the environment, or harmful chemicals could be formed by misuse, over-heating or fire. Therefore, already when carrying out electrical, mechanical and thermal testing of EV batteries, it is important to consider the risks from chemicals that might leak out. As an outcome of the project, guidelines were established for handling the chemical risks during testing. Such risk analysis will even be helpful input for risk assessment for batteries in EV.
In the present paper the chemical risks of Li-ion (lithium-ion) batteries with LiFePO 4 cathodes are evaluated in connection to destructive electrical and mechanical testing and during exposure to fire. Batteries made of Fe (iron) based cathode chemistry are more stable than Co (cobalt) based chemistry, which are responsible for some accidents in portable electronic devises that have received major media attention [4 and 5]. The improved stability by using Fe based cathode chemistry is obtained to the cost of reduced energy capacity. Furthermore, the solvent for the electrolyte consist of several flammable organic compounds that will ignite when exposed to external fire.
The chemical compound in the battery that causes most concern is the lithium hexafluorophosphate of the electrolyte. Hexafluorophosphates are toxic and corrosive, and forms hydrofluoric acid by hydrolysis in contact with water [6 and 7]. The rate of formation of hydrofluoric acid by hydrolysis of hexafluorophosphate increases with temperature. Hydrofluoric acid is volatile and vapour is extremely dangerous to inhale and in contact with eyes. Likewise, skin contact should be avoided. Even in diluted solution hydrofluoric acid is harmful and has the ability to penetrate skin and cause injury deeper in the body.
Due to the chemical risks discussed above, both during and after abuse testing, batteries are considered as potential dangerous to humans. Therefore, all tests were carried out in either a fire testing laboratory or in a laboratory for testing explosion protection of products, both with proper ventilation. Furthermore, personal protection equipment was used to avoid any physical contact between the operator and any eventually leaking chemicals. After testing all batteries were treated as toxic waste.

Short circuit test
Safety issues concerning high voltage batteries in EVs have been in focus during this project. A traffic accident could expose both passengers and rescue party to new hazards. One example could be a short circuit in the battery pack caused by a deformation of the chassis. In the beginning of the project the knowledge about the consequences when performing the tests was limited. During the project experience proved that since the results are so unforeseeable it is utterly important to follow adequate safety precautions during the tests. Short circuit tests have been performed on three different configurations of batteries.

Test setup
All short circuit tests were performed outdoors. The test setup consisted the test object (one battery cell) placed inside a 180 liters sheet steel cabinet with a front made of transparent polycarbonate. The short circuit was achieved by a contactor with a current limit of 10 000 A.
During the tests the current, voltage and temperature were registered. The test setup can be seen in Figure 2 and Figure 3.

Test
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Test
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Test
The   The gas analyzes showed high concentrations of HF in the combustion gases from the battery. See Figure 11 - Figure 12. Note that the concentration of HF shown in Figure 10 is in the diluted fire effluents collected by the furniture calorimeter in the test. The concentration in the undiluted smoke from the battery was thus considerably higher and the absolute concentration in a real accident scenario would be dependent on dilution volume and dispersion. Alternative methods to present the production of toxic species from a material in a fire are to use production rates or yields. Both production rates (mean and max) and yields of HF and CO from the battery test are presented in Table  1. Yields are here calculated as the quotient of the total amount of toxic specie produced and the initial weight of the battery. This type of data could be used as input for a risk assessment of toxic emissions in a certain fire scenario.

Future fire test method
The fire tests performed within the project have mainly been performed in order to increase the knowledge of the behavior of Li-ion batteries (LiFePO 4 ) during exposure to fire. However, the principles for the test procedure used within the project could be used as a base for future development of a standardized fire test method. The future work should be focused on defining the testsetup. It is also advisable to define the test object and to distinguish between a battery as a component and a battery as a part of the system, i.e. the vehicle. An external fire in a real situation will affect the battery depending on the installation in the vehicle.

Discussion
In the project, Li-ion batteries with LiFePO 4 cathodes, from three different suppliers, were tested. Even though, the chemistry of the three batteries in principle were the same, results in different tests differs a lot. The general observation is that the overall design of the battery is essential for its safety performance. The battery might have build-in fuse functions that switch off the battery by, e.g. external short-circuit and overheating or vents that relief pressure in a controlled way. In fire situations the casing of the battery cells in to modules could be very important in order to protect the battery cells from an external fire.
It is utterly important to identify the risks involved in testing high voltage batteries. This is a must for the operator who shall perform the tests in the future. In this project it was of great importance to handle the tests in a safe way and perhaps sometimes the safety concerns worked against a rational and effective working procedure.
In conclusion, one cannot rely on test results from other batteries than the actual battery in question, and it is relevant to test the battery on different levels of packaging (cell, module and battery pack).