The coming era of electric energy is changing the energy storage system of the vehicle from fossil fuels to electrochemical energy storage systems. This in turn is changing the propulsion system from the engine to the motor. The change of energy storage and propulsion system is driving a revolution in the automotive industry to develop new energy vehicle with more electrified powertrain systems.
Lithium-ion (Li-ion) batteries have emerged as the most promising energy storage technology in recent years due to their higher energy density, lighter weight, no memory effect, and lower self-discharge rate when compared to other rechargeable batteries types.
These characteristics have brought them in limelight for use in electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (PHEV).
The continual commercialization of EVs in the global market by brands such as Tesla Model S, Nissan Leaf EV, BMW i3, Hyundai Kona Electric as well as Indian market with many big names entering the EV domain such as TATA with NEXON EV, the old school Mahindra REVA along with 2-wheeler domain with ATHER, OLA Electric, SIMPLE, Hero, etc., has further promoted the rapid development of Li-ion battery technology and raised a wide public expectation of its future.
Why is a Thermal Management system needed in EVs?
Currently, most research into Li-ion batteries focuses on the material aspect to improve the specific energy, power, and cycle life, with relatively less attention paid to thermal-related issues. Even the brands are using the terminologies such as range, rapid charging which usually grabs the attention of customers. The end consumer is slowly getting convinced with the range capability for 2 wheeler, but still there exists range anxiety when looked at BEV in 4 wheeler domain.
To extend the total range of an electric car or SUVs, the volumetric energy density, with a unit of Wh·L−1, should be increased. Similarly, the gravimetric energy density also requires improvement for the range extension of the electric buses.
However, the materials with higher energy density may have lower thermal stability, leading to safety problems, e.g., thermal runaway (TR). The TR and TR-induced smoke, fire, and even explosion are the most common features during the accidents of lithium-ion batteries. Smoke, fire, and explosion are serious safety problems that arouse concerns from the public.
The fear of accident hinders the full acceptance of the EVs from the market, therefore many countries require the lithium-ion battery to pass compulsory test standards, e.g., UN 38.3, UN R100, SAE-J2464, IEC-62133, GB/T 31485, etc., before its application in EV.
Abuse Conditions in Accidents and Internal Short-Circuits in Electric Vehicles
There can be two possibilities when considered accidents due to Thermal Runway (TR); one is a self-induced failure and the other is an abuse condition in practical use. In the view of probability, the self-induced failure of the lithium-ion battery exists but at a very low level. The abuse conditions can be categorized as shown.
|Collision and crush||External Short Circuit||Local Overheat ( contact loose of cell connector)|
|Penetration||Over Charge||Overheat By Mechanical and electrical abuse|
The Internal Short Circuit (ISC) is the most common feature of the Thermal Runway (TR). Almost all the abuse conditions are accompanied by ISC.
Following the failure mechanism of the separator, the ISC can be divided into three categories considering the above-mentioned abuse conditions.
The hazard level of the ISC can be evaluated by the self-discharge rate and the exotic heat generation. It can be said that there are three levels of ISC as shown in the table.
To enhance the thermal performance of Li-ion batteries, reducing the amount of heat generated from the battery and boosting the heat dissipation rate are two feasible options:
- From the inside, it is the electrode in a battery cell that generates most of the heat during the charging/discharging processes, thus, an optimal design of electrode is crucial for minimizing the heat generation in the battery. This includes consideration of factors like the shape of the cell (whether prismatic, cylindrical, or pouch), thermophysical properties of electrodes, thermal and current distribution at the cell level, an electrochemical reaction that takes place.
- From the outside, a thermal management system (TMS) implemented in battery packs can help to relieve the rapid temperature rise and improve the stability and safety of Li-ion batteries during charge and discharge procedures. This TMS can be implemented at the module level.
EV Thermal Management Systems
TMS is generally used for battery operating under a high discharge rate, and especially for a large-scale battery pack that requires a long life cycle (>10 years) and is pricy to replace.
For example, all HEVs, PHEVs, and EVs are necessary to be equipped with Thermal Management Systems (TMS) due to the expensive battery repair, high cost of vehicles per SE, and relatively low automotive replacement rate.
In recent years, several techniques for readily removing heat from a battery pack have been tried and tested by various research groups across the globe. They can be classified in various ways, as seen.
The factors that are traditionally considered in the TMS trade-off analysis include energy efficiency, capital costs, ease of operation, maintenance requirements, and reliability. Cold plate and thermoelectric devices are not recommended for use in commercial systems, owing to their high thermal resistance and low coefficient of performance, respectively.
However, Gentherm Incorporation employs thermoelectric devices in their commercial design, whereas cold plates are used in the GMC Volt and Tesla Model S battery packs, signifying that both technologies have improved significantly in recent times.
Temperature range and temperature variation are two critical parameters influencing the battery pack performance. The ambient temperature may vary from -35 to +50 deg. Celsius in different regions, climates, and seasons, whereas the desired temperature range of the battery is about +15 ~ +35 deg. Celsius.
In addition, battery inconsistency and thermal boundary conditions result in temperature differences between cells in the battery pack. These major issues necessitate carefully designed Battery Thermal Management Systems (BTMS).
Conventional Thermal Management systems like fans and cold plates either have low scalability or deliver marginal performance (estimated from combined sets of cooling levels and regeneration rates) under challenging conditions. Other interesting alternatives (such as Thermoacoustic, Magnetic, etc. are available but they have not yet attained full technological maturity.
It is, therefore, concluded that a robust modular Thermal Management system (TMS) would be a hybrid system, designed through the union of at least two different TMSs. Considering factors such as technical risks, ease of integration, cost, and energy efficiency, it can be inferred that PCMs would form an integral part of the modular TMS assembly.
Battery Thermal Runaway in EVs
Thermal runaway begins when the heat generated within a battery exceeds the amount of heat that is dissipated to its surroundings.
What causes the battery to overheat which leads to thermal runaway?
- Ambient Temperature
- Age of the Battery
- Float Charging Voltage
Approaches to Minimize Thermal Runaway
The reduction of the Thermal Runway hazard can be fulfilled in three levels-
- First: improve the intrinsic safety of anti-TR properties by material modification;
- Second: set passive defense against the practical abuse conditions and develop early warning algorithm before TR;
- Third: postpone or inhibit the secondary hazard, such as Thermal Runway propagation, to win sufficient time for the passenger to escape from the EV after the accident.
We believe that the concept of the three-level safety design can significantly help diminish the Thermal Runway hazard during accidents. Researchers should be well equipped with sufficient knowledge of the three-level safety design concept, and prepare for the coming era of lithium-ion batteries with higher energy density.