Main causes of EV fires

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EV fires make for good headlines in the news, but it’s a different story when you look at the numbers. Only 0.3% of battery electric vehicles (BEVs) are likely to catch fire, as per data from the U.S. National Transportation Safety Board (NTSB). In comparison, a much higher 1.5% of petrol/diesel vehicles are at risk of going up in flames, and the percentage is even higher for hybrid vehicles at 3.4%.

The main risk for BEVs catching fire is thermal runaway, where damage to the battery leads to uncontrollable overheating. But that’s a generalization that can be further drilled down to specific reasons, and this, in turn, can help battery engineers come up with safer designs and technologies.

What are the main causes of EV fires?

Some of the primary causes of electric vehicles catching fire are listed below.

Internal cell short circuit: There are multiple cells inside a battery pack. There can be internal cell short circuits which may cause a fire or trigger a fire in an EV.

External short circuit: There could be external short circuits, as the battery pack as a whole is connected through power electronics, through the motor, and through the other accessories as well. And this is a relatively higher voltage battery.

The charge-discharge cycles: There are certain current thresholds, and those thresholds will differ from different operating temperatures or different battery temperatures. But the charging and the discharging involve taking the current out of the battery or putting the current into the battery. This is also going to generate heat that’s going to raise the temperature of the battery cell and, in turn, the battery pack. If it is not managed properly, it can also lead to what we call a thermal runaway.

High operating temperatures: Apart from the battery’s own heat generation, we have higher operating temperatures, which is quite typical in countries like India. So the sink, which is basically the ambient to which the heat has to be dissipated, is at a higher temperature, and the heat dissipation rate goes down.

Avoiding EV fires through design

All the EV design calculations done should take into account worst-case scenarios – what are the ambient temperatures, and what are the highest road temperatures that your customers will face on the roads?

Even if we talk about electric two-wheelers or a 48-volt or 60-volt system, it is much higher than what we have in ICE (which has the 12-volt system in two-wheelers and sometimes 24-volt systems in heavier vehicles and commercial vehicles).

Any battery pack that we see in electric vehicles typically is 48 volts upwards. And then we have a large amount of electric power being carried through the conductors, through the motor, through the power electronics. There can be an amplification of voltage or an amplification of current. All of these need to be protected, or at least there should be a detection and mitigation strategy for short circuits in any of these.

EV battery testing standards as per AIS 156

The Government of India has moved from AIS 048 to AIS 156, with new EV battery testing standards that must be followed. The standards are outlined in detail in this Automotive Industry Standards (AIS) document AIS 156. Section 6.1.2.3 of Amendment III to AIS-156 deals with this same issue that we have discussed.

[BMS of REESS shall be verified for the following safety features during REESS testing as per Annex 8 of this standard. a) Over-charge protection b) Over-discharge protection c) Over-temperature protection: d) Overcurrent protection e) Short circuit protection.]

Clause 6.11 outlines the Thermal Propagation Test that has been added to “evaluate the ability of REESS to withstand thermal propagation which is triggered by an internal short circuit leading to a single cell thermal runaway and subsequent thermal propagation and shall not result in fire and explosion of REESS.”

Avoiding EV fires through proper use and maintenance

Design aspects can mitigate the risk of EV fire as much as possible. But it is prudent to ensure that every EV buyer knows what are the limits of the vehicle based on the specs. More importantly, it is incumbent upon the manufacturer and dealerships to ensure the education of EV buyers in terms of proper use and maintenance.

About the author

Vikrant Vaidya, Co-Founder and President, pManifold evACAD.

Vehicle-powertrain integration enthusiast who has worked in various systems engineering aspects like vehicle dynamics, powertrain matching, drive quality, controls and calibration. Vikrant has over two decades of experience in the engineering teams at Tata Motors, GM and TCS.