Addressing safety concerns in electric vehicles

8 min readOct 24, 2021

Introduction

E-mobility has seen a rapid transformation globally over the past few years. While a lot of research is being undertaken to address consumers concerns on range anxiety and upfront costs, many consumers, OEMs and government bodes continue to have apprehensions regarding the vehicle safety and durably. Given that e-mobility is relatively a newer industry and is still going through multiple technological improvements, it is important to build safety features and research the vehicle failure rates to ensure that various safety concerns are addressed.

An electric vehicle requires bringing together high energy electrical components on the vehicle board and it is this usage of high power components and systems developing high performance and environment friendly transport system. Additionally, the industry is witnessing a lot of research in the battery efficiency, new battery technology, designing procedures to make the vehicles more powerful. These high voltage batteries demand continuous monitoring and controlling of various parameters in each cell is very crucial for a safe operation. Further, the vehicle incorporates protection circuits to safeguard the battery in case of any failures from electrical, mechanical and temperature variations.

For a smooth and safe operation of the vehicle, all the electrical parameters of the battery should be in specified limits while in stationary or moving condition. Therefore, electric vehicles make use of Battery Management System (BMS) to monitor the different parameters of each battery cell and control the parameters within their prescribed levels during charging and discharging procedures. Battery management system continuously observes the current, voltage and temperature parameters in each cell and controls these accordingly for the safe operation of the battery pack. BMS performs the other tasks such as balancing of cells, communication with different parts of the vehicle, maintain the battery parameters so that battery operates within permissible limits and updates the driver about the risk and failure conditions from time to time.

Comparison of safety measures between an EV and an ICE vehicle

The risk in electric vehicles is considered to be low as compared to ICE engines as they do not carry conventional fuels on board. The two main fuels petrol and diesel which are used in ICE vehicles are extremely flammable and toxic. The EVs do not have combustion engines like ICE vehicles therefore they have low risk of fire and explosion. They are mainly powered from a high energy density battery pack. Lithium–ion batteries are becoming the preferred choice of battery manufacturers because of its high energy and power densities. Additionally, these also offer long life time for the vehicle. However, there are a few concerns related to the safe handling and operation of these batteries.

Other than that, these batteries are manufactured using different cell chemistries, high energy density materials and in large sizes that demand additional attention with regard to safety issues.

Various tools and technologies are being developed to safeguard electric vehicles from such issues. Almost all the processes in an electric vehicle system have seamless connectivity on a cloud network. Therefore, all the updates about the battery status, vehicle performance, etc. can be easily monitored and the updates are shared with respective counterparts for any necessary action. Electric vehicles employ different sensors and indicators to ensure safety at cell level, battery pack level, process level and the vehicle level. The system also updates about the initiation and completion of charging process and the specified parameter limits to be taken care of. The BMS in an electric vehicle plays a significant role in controlling the parameters to avoid the thermal runaway situation, so that fire and explosions incidents can be avoided.

ParametersEVICEFlammable fuelsNot presentPresentRisk of fireLowHighSource of energyBatteryFlammable and toxic fuelsTemperature rangeNarrowBroader than EVsCombustion engineNot presentOn boardVehicle drive componentsCharger, battery, controller, motorGas and oil pump, starter, gas tank, engine, exhaust systemLow toleranceShock, vibration, penetrationRobust as compared to EVsSensorsIncorporated at different levels such as component, system and vehicle level to ensure safetyConventional vehicles have minimal usage of sensorsMaintenanceLowHigh

Safety concerns in regards with the usage of lithium-ion batteries in an electric vehicle

Lithium-ion batteries were initially used in consumer products such as electronic gadgets, toys, mobile phones and laptops. These are preferred because of small size, safety and low voltage demands in these applications. Electric vehicles require features such as high efficiency, good rate capability, long life and increased safety. Lithium-ion batteries are capable of satisfying all these conditions but at the same time these have low stability in terms of voltage and temperature. While lithium-ion batteries provide the requisite high energy density, there are concerns with respect to safety and variance of safety with chemistry, operating range and tolerance.

The instability in voltage and temperature conditions can result in electrochemical reactions in lithium-ion battery. The overheating can spread to adjacent cells and impact the overall battery system. This overall increase in the temperature can progress into a condition known as thermal runaway. The energy released during these reactions can cause smoke formation, fire and explosion incidents in the electric vehicles.

Factors that can lead to thermal runaway in batteries:

Thermal — The lithium-ion batteries need to survive in very low as well as very high temperatures, depending on the place where it is used in the electric vehicle. But the environment temperature can cause significant temperature variations within the battery. Hence, temperature swings causes considerable thermal stress within the battery and this thermal stress can be a reason for the occurrence of thermal runaway condition. The temperature in battery cells can be controlled by different cooling and heating systems in the battery.

Electrical — In case the charging and discharging parameters go beyond the specified limits than it results into overheating. This occurs because of the rapid rate of change of battery voltage. The different cases of electrical hazards are overcharging, over-discharging, fast charging etc.

Overcharging — This phenomenon occurs when a cell is charged beyond the maximum charge voltage limits, as specified. Therefore, an excessive current starts flowing into the battery because of continuous charge voltage. This will cause decomposition of the water in the electrolyte. The heat in the battery increase further because of the high overcharge rate. The process of heating up continues and finally leads to thermal runaway, where the battery can catch fire or completely get destroyed.

Over-discharging — This situation occurs when a cell is discharged beyond the lower safe voltage limits, as specified. The over- discharging process allows variation in the volume of lithium material in the battery. A single over-discharge doesn’t result into any major fault in the system. But with continuing over-discharging cycle, the volume variation can cause stress for the battery. This forces electrodes outside their safe potential range and affect the integrity of the cell components resulting into formation of Solid Electrolyte Interface (SEI) layer. This over-discharging in a single cell can propagate the degradation in the adjacent cells and result into permanent damage of battery pack.

Fast charging — Fast charging enables faster adoption of electric vehicles and offers the benefit of charging the vehicle in 20–30 minutes. But at the same time, the temperature and humidity parameters affect the safety and performance of battery cells. The temperature variations during charging or discharging process can result in overheating. The intense rise of temperature may cause the adjacent battery cells to heat up and the pressure can start building inside the battery pack leading the path to thermal runaway. Therefore, thermal management system should be critical in keeping track of electrical heating within the system to avoid these electrochemical reactions.

Short circuiting — The short circuiting issue can take place internal or external to battery. The main reasons and effects of internal and external short circuiting are as follows:

- Internal short circuiting — During an internal short circuit, the local current densities increase because of the interconnection of the two battery electrodes. This may occur due to dendrite growth, manufacturing defect, mechanical deformation etc. This will result in self discharge and increase in temperature. This continuous temperature rise can result into electrochemical reactions, causing thermal runaway condition and permanent damage of the battery.
- External short circuiting — During an external short circuit, a low resistance is connected across the terminals of the fully charged cell. It results into a flow of very high current in the cell causing the increase level of heat in the cell.

Mechanical — Electric vehicle collisions are a real threat and the impacted vehicle must be tolerant enough to survive. In addition, these vehicles face the issues of vibrations and shocks when they come in contact with uneven surfaces or roads.

Fig: Lithium-ion cell hazards

Preventive measures

The electric vehicles require high power density batteries to provide large amount of power for its effective use as a road transport vehicle. As the consumption of power is high and required on daily basis, these batteries must have a rechargeable feature. The energy storage capability in these rechargeable batteries is possible because of the presence of reactive materials in their structure. In case of high temperature, mechanical deformation, vibrations, etc. heat inside the battery cell gets intense and these reactive materials initiate thermal reactions into the overall battery pack resulting into serous fire problem in the battery. Therefore, the safety mechanism and standards should be strictly followed to achieve safe operating environment for the EVs. These measures include:

- Several fuses and short circuit protection circuits employed for the electrical safety of the battery
- Effective thermal management system are essential for maintaining safe temperature range for batteries
- The complete battery structure should be protected using mechanical housing
- An effective and well-engineered cooling system must be incorporated inside the vehicle for controlling the system temperature
- The cell balancing methods should be employed for minimizing the variations between the cell voltages
- EV safety standards should be followed to ensure the safety of the vehicle in different environments
- EV testing standards should be strictly followed and batteries should be certified in accordance with relevant safety standards before commercialization
- Quality of the battery and its counterparts should not be compromised and should be improved for future developments
- Safety standards should be updated and optimized to cope up with the emerging techniques and research initiatives

Fig: Preventive measures

There is a massive competition between the electric vehicle batteries and there is a race to achieve high degree of efficiency and performance from these vehicles. Along with all these initiatives, the safety of electric vehicles cannot be ignored. Therefore, there should be safety standards available for all the electric vehicle segments and these should be strictly followed to assure the safety of these vehicles.

In India, as the number of on-road electric vehicles is still low and most of these vehicles are either 2 wheeler or 3 wheeler, it is difficult to conduct any kind of statistical studies on fire incidents related to electric vehicles. Now, as the preference for electric vehicles is steadily increasing and these become commercially available in market, it creates a possibility to research on the different safety aspects and take any corrective measures. This will foster the development of safer electric vehicles in the emerging e-mobility industry.

Glossary

EV — Electric Vehicle
BMS — Battery Management System
ICE — Internal Combustion Engine
SEI — Solid Electrolyte Interface

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