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Bus Fire Safety: Safer battery systems in electric buses

20 June 2016  •  Author(s): Fredrik Larsson, Johan Anderson, Petra Andersson and Bengt-Erik Mellander

Lithium-ion (Li-ion) batteries offer great energy and power densities accompanied with long battery life time. However, if a mechanical fault occurs or the batteries over-heat, the flammable electrolyte of the Li-ion battery may pose a risk. For Eurotransport, colleagues from the SP Technical Research Institute of Sweden (SP) and Chalmers University of Technology explore further, identifying the risks involved with electric buses.

Safer battery systems in electric vehicles – an electrified bus perspective

Li-ion batteries are widely used in various consumer products and are beginning to be utilised in various types of electrified vehicles (xEV). xEVs have the potential to be safer than conventional combustion engine vehicles, simply because they have less or no flammable gasoline/diesel onboard1,2. Additionally, xEVs can potentially increase safety aspects due to, for example, the increased freedom of design enabled by the small size of the electric motors and the various ways that the batteries can be positioned in the vehicle. However, new technologies may also introduce hitherto unknown risks. It is important to study these risks in order to properly redress them for vehicle design.

Electrified buses include hybrid electric buses, plug-in hybrid electric buses and fully electric buses. Fully electric buses have an electric motor and a large battery pack for propulsion – no additional combustion engine for propulsion is used. In the case of hybrids and plug-ins, both an electric motor and a combustion engine is used in tandem where the battery pack has less energy capacity compared to that of fully electric buses. Fully electrical propulsion of buses provides several benefits: zero tail-pipe emissions; more silent driving and higher efficiency. Different bus applications, such as school buses, city buses and long-haul buses, have different ‘electrification needs’. For example, a long-haul fully electric bus would probably require a battery that is too heavy to be able to carry sufficient load in the present transport systems and would therefore need continuous charging, i.e. electric roads, or other forms of energy storage (e.g. hydrogen and fuel cell) or a conventional combustion engine driveline. City buses, on the other hand, require moderate sized batteries because they typically have a lower average speed and travel a shorter distance per route, with the capability of charging the battery at each end of the route as well as at bus stops.

SP Technical Research Institute of Sweden (SP) is involved in the project ‘Safer battery systems in electrified vehicles – development of knowledge, design and requirements to secure a broad introduction of electrified vehicles’, together with Atlas Copco, Chalmers University of Technology and Elforsk and with financial support from the Swedish Energy Agency. The project includes various abuse (destructive) tests on commercial Li-ion battery cells to study the cell response in terms of variables such as temperature, gas emissions, fire and explosion…

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