Thermal Management In Electric Vehicle, Causes, How To Overcome

Hello guys, welcome back to my blog. Here in this article, I will discuss on thermal management in electric vehicle, causes or thermal issues of lithium-ion cells, and short information on battery thermal management system.

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Thermal Management In Electric Vehicle

There are nowadays various blending levels of hybrid EVs and completely pure electric vehicles available on the present automobile market. According to the blending level, different sizes, kinds, and numbers of battery cells are scaled in EVs. Unlike traditional fuel, battery cells as an energy source have more rigid conditions in the working environment. They are extremely susceptible to temperature. To secure proper thermal working conditions, a BTMS or Battery Thermal Management System will usually be integrated with battery cells.

Therefore, understanding the proper working requirements of batteries is essential, and what type of management systems can adequately and efficiently meet these needs. With this cornerstone, the implementation and durability of the lithium-ion battery pack can be maximized in an electric vehicle. Also, the electric range of vehicles is limited due to the limited capacity of the lithium-ion battery.

Thermal issues in lithium-ion battery

Lithium-ion cell performance relies on both the temperature and the working voltage. Lithium-Ion cells perform agreeably when cells work within restricted voltage and temperature. Otherwise, the injury will happen to the lithium-ion cells and will be irreversible. In over-voltage circumstances, the charging voltage surpasses the tolerable cell voltage, resulting in extreme current flows and at that exact time, it generates two issues.

At extreme currents the Lithium-ions are deposited additional rapidly than intercalation to the anode layers, Lithium ions are then deposited on the exterior of the anode as metallic Lithium. This is Lithium plating. It provides rise to the decline in the free Lithium ions and an irreversible (no alter) capacity loss. There are two kinds of metallic lithium plating, i.e. heterogeneous lithium plating and homogeneous lithium plating, but the lithium plating is dendritic in the formation.

Finally, it can result in a shortcircuit between the electrodes. As with over-voltage, under-voltage also gets about issues that provide rise to the analysis of the electrode materials. For the anode, the copper current collector dies down. It generates an expansion in battery discharge rate and battery voltage, yet, the copper ions are poured as metal copper which is irreversible. The problem is difficult for it can cause short-circuit between anode and cathode. For the cathode, the cobalt oxide or manganese oxide will deteriorate after many cycles under low voltage.

Meanwhile, oxygen will be released and the battery sorrows from capacity loss. The battery temperature should be handled carefully. Both surplus heat and lack of heat will fetch about issues. Chemical reaction rates contain a linear relation to temperature. The reduction in the working temperature will reduce the reaction rate and the capacity of having current during charging or discharging. In other words, the battery power capacity is decreased. Moreover, the deduction of reaction rate generates it more challenging to insert lithium ions into intercalation spaces.

The result is the reduction of power and lithium plating pushing the capacity loss. High temperature enhances the reaction rate with more raised power output, yet, it also enhances the heat dissipation and drives even higher temperatures. Unless the heat is dispersed more quickly than heat is developed, the temperature will be better elevated and finally, a thermal runaway will happen. Thermal runaway consists of numerous stages and individual stages will provide rise to more irreversible damage to cells.

First, the SEI layer is liquefied into electrolytes at around 80ºC. The primary overheating may occur from excessive current or high ambient temperature. After the breakdown of the SEI layer, the electrolyte starts to react with the anode. This reaction is exo-thermal which causes the temperature higher. Secondly, the higher temperature drives the organic solvents to break down with the departure of hydrocarbon gases. Usually, this starts at about 110 ºC. The stress inside cells is created by the gas and the temperature is above the flashpoint.

Nevertheless, the gas does not burn due to the shortage of oxygen. A vent is required to release the gas in charge to hold cells under proper pressure and avoid a possible fracture. Then, the separator is dissolved, and short-circuits appear between the anode and cathode at 135 ºC. Eventually, the metal oxide cathode breaks down at 200 ºC and liberates oxygen which permits the electrolyte and hydrogen gas to burn. This reaction is also exo-thermal and causes temperature and pressure still be further.

In addition, the varying temperature distribution is another trouble with batteries. Generally, it is driven by the excessive local temperature, varying current in a cell, the thermal conductivity of the issue, as well as the placement of positive and negative terminals, and so on. It results in local deterioration and even thermal runaway decreasing the battery lifetime.

To overcome these above effects or causes, we need a proper battery thermal management system. Now, let discuss on battery thermal management system.

Battery Thermal Management Systems (BTMS)

The battery pack for safety, interpretation (both power and capacity), and lifespan bases should be held in a controlled surrounding where the temperature is maintained and there is no danger of thermal runaway. According to the foundational investigation, the BTMS should be provided with four fundamental functions to provide the right operation requirements of the battery pack:

01. Cooling: Due to inefficiency, lithium-ion battery cells will not just generate electricity but even heat. This heat should be transferred from the lithium-ion battery pack when the battery temperature matches the optimum temperature or even in advancement. Thus, a cooling function is needed in BTMS.

02. Heating: In cold conditions, battery pack temperature likely falls below the lower temperature limit. Therefore, a heating function, such as a PTC heater, is needed to help the battery pack to get the proper temperature degree in a shorter time.

03. Insulation: In severe cold or hot weather, the temperature contrast between the inside and outside of the battery pack is much larger than that in mild weather. Battery temperature will therefore fall (cold) or rise (hot) sooner out of the appropriate temperature range. To stop this, good insulation can slow down the falling or rise of battery temperature, particularly when the vehicle is parked outdoors.

04. Ventilation: Ventilation is needed to exhaust the dangerous gases within the battery pack. In some methods, such as air systems, this process is combined with cooling and heating functions.

This was about “Thermal Management In Electric Vehicle“. I hope this article may help you all a lot. Thank you for reading.

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Chetan Shidling

Automotive | Technical Blogger | Engineer | Content Creator

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