Are electric vehicles safer than combustion engine vehicles?

We get to hear electric vehicles are getting burned for 2 weeks in India, recently A video on social media sites went viral of an electric scooter from Pure EV emitting smoke before it catches the fire in Hyderabad. Yesterday also we get to hear a similar incident from Okinawa where the electric scooter had been parked in a garage and being recharged in the night when a blast had occurred. It is the third incident in very few days. As we discussed already the fire risks associated with li-ion batteries & safety recently and today in this article we will be discussing about Are electric vehicles safer than combustion engine vehicles. 

So after seeing all those viral videos of electric vehicles catching fire, many must have thought that electric vehicles are dangerous, we should not buy them or many more thoughts are coming to the people’s minds so let us now look first at Are electric vehicles are safe? are they safer than ICE vehicles? 

Firstly if we look at the conventional vehicles which use internal combustion engines, it is challenging to replace an ICE vehicle with an electric one. New technical solutions, especially those produced for a mass market, may substantially change and possibly increase the risks associated with the products.

Electric vehicles have clear advantages concerning safety as they do not carry conventional fuels onboard such as petrol or diesel, both of which are flammable and toxic. Without a combustion engine onboard, the risks of fire and explosion are thus decreased. Therefore, the introduction of electric vehicles promises a transition to a clean, non-polluting, healthy and safe means of transportation.

Electric vehicles are safer than conventional ones we still need to consider what possible new risks this technological transition will bring, as the risks associated with conventional vehicles are well known to most people and thus easily dealt with in daily life.

The aspects that need to be considered when designing the vehicle 

• What risks are associated with a large onboard chemical energy storage?
• What kind of battery failures may occur and what will the consequences of such failures be?
• Will the hazardous traction voltage of the electrical system pose a danger to the passengers or to rescue personnel in case of an accident?
• How can eventual risks be diminished in traffic, when vehicles are parked and maintained?
• Does the silent electrical drive introduce a risk for pedestrians?

So let us now talk about Lithium-ion (Li-ion) batteries, as it is the most preferred battery pack in current electric vehicles. The safety issues with the li-ion batteries.

Reasons

The risks involved with the use of lithium-ion batteries are closely related to the

• Chemistries used
• The design of cell and system,
• The handling of the battery when in use and
• The quality of the production

Cell chemistry

The choice of cell chemistry is, in turn, determined by the demands regarding energy and power density as well as cost and safety for the specific application. Lithium-ion cells for the automotive industry is, for example, characterized by increased quality, safety and lifetime compared to that of small consumer cells, and generally use more advanced materials with a higher degree of purity. 

BMS

Lithium-ion batteries have many advantages but the window of stability is relatively small (both regarding temperature and voltage). Therefore the cells must be monitored and controlled, by BMS.

Thermal runaway

If there will be overheating a severe malfunction can happen, if the temperature exceeds typically 120-150 °C, exothermal reactions within the cell can start. The exothermal reactions will increase the temperature further, which can trigger additional exothermal reactions and if the overall temperature increases a thermal runaway will be the result.

Which leads to a rapid gas release, electrolyte leakage, fire, rapid disassembling/explosion. The reason for the initial overheating may be an external short circuit, overcharge, over-discharge, deformation of the battery by external forces, external heating or an internal short
circuit.

To get a high level of safety from this, a chain of events must be hindered, or at least, its propagation is delayed. For example, the gas release and electrolyte leakage can also occur in non-thermal runway situations, that is at temperatures lower than those where a thermal runaway is initiated.

The improved battery materials have been produced r to meet the demand of the automotive industry. The selection of the active electrode materials (anode and cathode) strongly affects the thermal runaway and its onset temperature.

Lithium iron phosphate is for example a more stable cathode material than the cobalt-based lithium oxides that are commonly used in consumer Li-ion batteries. Other electrode materials, for example, mixed oxides with cobalt in combination with other metals (e.g. Ni, Mn, Al), have been developed in order to improve safety and other aspects (e.g. lifetime, cost, energy and power densities). 

Fire, Gases and emissions

The higher voltages and currents used in an electric vehicle may be a risk for fires and lithium-ion batteries pose a special risk as the electrolyte is combustible, with properties similar to gasoline or LPG.

Fast charging risks

Fast charging requires high currents and one would therefore intuitively associate this with larger fire risks. The high current would result in a higher risk of overheating or other malfunctions in the charging station or in the vehicle itself and thus careful design is important.

Voltages, currents and electrical hazards

The traction voltage components and cables carrying a hazardous voltage are usually
very well insulated and protected. The risk that a human, e.g. driver, passenger, rescue personnel or firefighter, would be exposed to an electrical shock is in general very low.

Both DC and AC voltage poses a danger for humans in case of an electric shock caused by either a direct touch of the traction voltage/battery poles or indirectly by touching the metal chassis. In order for such exposure to occur, two insulation faults are required (one at each pole of the traction voltage/battery) and the victim must touch metallic parts with different potentials at the same time.

Efforts to be made

The electrical drive train is made safe by several means. The battery cells used are in the general safe judicious choice of Li-ion technology ensures that an unsafe lithium-ion cell would not be chosen.

• The battery safety is ensured by the manufacturer by adding layer by layer of safety.
• Safety issues are of major concern in any introduction of a new product into the market as negative publicity might have a very negative impact on the general public’s perception of a product and the product’s potential to succeed.
• Design safe products and be very open about events that have occurred as ample correct information is the best way to avoid rumours.
• Improved electrode and electrolyte materials in lithium-ion batteries. 

Electric vehicles benefits

Electric vehicles can have several benefits for the overall safety

• The largest fire risk is removed with the absence of gasoline/diesel fuel (or less amount in case of PHEV),
• Lower centre of gravity (with a large battery pack underneath),
• Increased freedom in vehicle design,
• Potentially larger front deformation zone since no space is needed for a large combustion engine and the electric motor is small.

Electric vehicles might however have fewer deformation capabilities for some crash angles, strongly depending on battery size and placement, since the battery has to be protected from deformation. Electric vehicles with li-ion batteries also have other disadvantages, e.g. release of toxic and flammable gas emissions and electrical hazards.

As long as there are no intrinsically safe Li-ion cells, there will always be a small risk of a cell failure, therefore it is essential to lower the probability of a single cell failure and to limit its consequences, i.e. early warning detection, handling of its effects (e.g. gas release) and limited propagation by the design of battery pack and vehicle integration.

Source (Systems Perspectives on Electromobility)

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