Electric cars and fire risks. Questions and answers

The accident a few days ago involving the electric bus produced by the Chinese Youtong company brought the debate on the safety of electric vehicles in the event of an accident back to the center of the news. The bus involved in the tragic event where 21 people lost their lives is the Futon E-12, produced by the Asian company Yutong, a leading company in this type of solution. Technically it is equipped with a 350 kWh battery with Lfp (lithium-iron-phosphate) chemistry capable of ensuring a range of approximately 400 km (here full specifications). Aside from alarmism and populism, to clarify the safety of electric vehicles, in particular the battery, a conversation with the Prof. Claudio Rabissiresearcher at the energy department of the Polytechnic of Milan.

Professor Rabissi, which parts are at greatest risk of overheating and flames?
“Having to guarantee high electrical power, in general, despite the very high efficiency, an electric powertrain has various components that dissipate thermal power during operation, such as the electrical power circuit (busbar and electrical connections, only to a lesser extent the electric motors ), battery chargers, any DC/DC and certainly the battery. However, with the same power “at the wheels”, we are not even remotely talking about the thermal powers dissipated by a traditional engine, in which combustion takes place (therefore at very high temperatures, ed) and which has significantly lower efficiencies than an electrical system. In particular, the risk of battery overheating is first prevented and then managed by protection systems at various levels. A first active level, consisting of the BMS (battery management system, ed), controls the cooling system (whether air or liquid) and the operation of the battery (avoiding discharging or charging at excessive currents that could generate overheating, or cause it to be charged or discharged beyond the permitted voltage limits) . A series of lower level systems then ensures that, in the event of abuse, destructive scenarios are avoided: from the more traditional and at system level such as fuses and extinguishing materials, to the more specific and at the level of individual cells (generally equipped with systems of controlled venting of any offgas that could lead to overpressure and burst or to intrinsically interrupt the cell’s ability to supply current according to its temperature, so-called Ptc and Cid)”.

How do the various parts react in the event of a flame (fire, explosion)?
“I speak for the battery: in the extreme case of extreme overtemperature the most feared risk is that of reaching the so-called thermal drift (Thermal runaway), that is, that point of no return (generally above 100/120°C) at which there is a risk of activating a chain of self-exalting events which can ultimately lead to the battery catching fire and/or exploding. These events in general derive from the instability at high temperatures of the electrolyte and electrodes (with differences from chemistry to chemistry), which lead to unwanted reactions of generation of gaseous products in exothermic reactions (i.e. which in turn contribute to increasing the temperature of the drums)”.

What are the situations that can favor a fire?
“As I was saying, an incorrect operation can lead to thermal abuse and therefore overheating. In addition to this, mechanical abuse certainly constitutes a risk linked to the automotive application (extreme impacts and penetration of foreign bodies which mechanically damage the internal structure of the battery, potentially bringing the positive electrode and negative electrode into contact generating an internal short circuit, are certainly potentially destructive events.) To prevent this, the batteries are typically housed in watertight and extremely robust compartments part of the chassis itself, and the individual cell models to reach the ‘type approval must demonstrate intrinsic safety by successfully passing dozens of destructive abuse tests such as drop tests, nail penetration, crushing and so on”.

How to intervene in case of fire? How much water or agent is needed to put out the fire compared to a thermal vehicle?
“The peculiarity is that a battery potentially contains everything needed for combustion, starting from lithium (an extremely reactive element) and including the oxidizer (the positive electrodes typically contain oxygen in different forms, depending on the chemistry), and a once the actual combustion has started it is possible to try to confine it and slow it down by cooling it (typically using large quantities of water), but it is very difficult to interrupt it. Lately, specific products based on Avd (aqueous dispersion of vermiculite) which has extinguishing and advantageous flame retardants”.

Can an electric car be considered more or less safe in the event of an accident than a combustion engine?
“Let’s start from this evidence: every “thermal” vehicle carries with it a quantity of liquid fuel corresponding to energies greater than those stored in lithium batteries (which, let’s remember, are not a fuel). The battery of today’s high-performance electric vehicle autonomy can store up to 80 kWh of energy, when the full tank of a traditional combustion vehicle can be up to 5-6 times higher, depending on whether it is petrol or diesel. In addition to this, certainly the mass higher than an EV vehicle due to the mass of the batteries must also be considered in the event of an impact”.

In recent years there has been a big step forward regarding the safety of electric cars, especially due to the presence of batteries with safer chemistry and more sophisticated BMS. What will be the next steps forward from this point of view? What strategies can manufacturers implement to keep safety under control?
“The scientific research and technological development of lithium batteries are among the most investigated current topics in the world. In addition to recent chemistries with higher thermal stability, such as Lfp (lithium-iron-phosphate, ed), in particular, the transition from a liquid electrolyte to a solid state battery is increasingly concrete and is among the most anticipated advances, as it would allow significant advantages in terms of chemical and thermal stability of the cells, as well as leading to a potentially non-negligible increase in the energy density of the batteries, which for the same mass and size would guarantee much higher autonomy”.