Electric Vehicle Battery Pack Thermal Management: Analysis And Solutions

The Basic Model of a Lithium-ion Battery

As the time is changing, the technology is also changing. The electric vehicles are becoming more and more advanced. Different types of batteries are employed for the purpose is to optimise everything. The batteries operate by generating energy through electrochemical reactions which are highly temperature dependent. In order to maintain proper working environment of temperature, a battery thermal management system is employed. Therefore the correct knowledge of the system, its application and implementation is very much required. Also note that as the time is changing everything is become compact, and so is the battery but as the batteries are becoming smaller and smaller, its capacity and power requirements are increasing. Therefore the energy consumption is another important aspect in BTMS. Thus increasing the life of the battery.

The goal of this report propose a model for balancing the temperature of a battery so that its performance is increased. For this every aspect of a battery is discussed starting from the basic model of a lithium-ion battery. Many systems for heating and cooling are discussed in detail and at last a combined solution is provided that includes the best systems, ideal for implementing in the electric vehicles today.

This report discusses some acceptable and popular techniques to manage the temperature and heat. The correct balancing between them is discussed in order to increase the performance. At first the basic working of a lithium-ion batteries discussed because the fundamentals play an important role in understanding the whole scenario. The temperature dependency is discussed in a lithium-ion battery along with the operating temperature range required. Next some of the common BTMS solutions are discussed. The mechanism, functions, advantages and disadvantages are discussed. At last a new method is proposed combining the already discussed models in order to increase the performance.

A lithium-ion battery is very popular among the electric vehicles (EVs) as well as the hybrid electric vehicles (HEVs). It constitutes two electrodes, electrolyte and the separator as shown in figure 1. The anode is made up of graphite or carbon, the cathode is made up of lithium metal oxide and electrolyte is an organic solvent in which the thieves also dissolved. During discharging, the goal of the anode is to throw the electrons out to the external circuit. Thus, the anode undergoes oxidation. Whereas, the cathode receives all the electrons from the external circuit that are thrown away by the anode (Techopedia, n.d.). Thus, a cathode undergoes reduction process. An electrolyte is a substance that contains both the electrodes debris inside it and it makes it possible to exchange irons between cathode and anode. The separator helps to act as a boundary between the two electrodes and hence preventing their short circuit. The solid electrolyte interface (SEI) is formed outside the anode during the first. It decreases the rate of the reaction and hence the current (Woodford, 2018). One of the biggest advantage with the lithium-ion batteries is that it can be charged again and again. This means that the electrochemical reactions can be reversed (Poole, n.d.). The lithium ions travel from the negative electrode to the positive electrode during discharging while, it travels from the positive electrode to the negative electrode (Troiano, 2013). The reaction for the lithium-cobalt batt is given as,

BTMS Solutions

There are a variety of lithium batteries available in the market which varies in terms of the material used as the cathode. For example Lithium Cobalt Oxide, Lithium Manganese Oxide, Lithium Iron Phosphate and many more. In the table 1, many types of cathode substances along with its description is mentioned.

Table 1 (Battery University, 2018) 

As per the table 1, three cathode materials are best suited for EVs, li-phosphate, li-manganese and NMC (Evans, n.d.), (Energy Storage Association, n.d.). 

The performance of the lithium ion batteries are dependent on the operating voltage and the temperature. If it doesn’t operate within the range than it can be damaged permanently. First of all we analyse in terms of operating voltage. If the battery is charged at a voltage higher than the acceptable voltage, then the amount of current increases in the circuit. It creates two issues, when the currents are high, more number of lithium ions are deposited at the anode as lithium. This process is called as lithium plating. Due to this process the number of free lithium irons decreases also there is a loss of the capacity of the battery. There are two kinds of lithium plating, heterogeneous and homogeneously lithium plating. It occurs in the form of branch that is the layers of lithium increases subsequently over time and ultimately if the overvoltage situation persists then a time comes when both electrodes gets short-circuited. In the case of under voltage during discharging, the copper current collector of the anode breaks down. The rate of discharge is increased and so the voltage of the battery. The copyrights that are broken down gets deposited as copper metal (Electropaedia, n.d.). This was is not reversible. If the control the situation persists longer, time comes when both electrodes gets short-circuited. It damages the battery permanently as well as it is dangerous. Another plus is also happens. The metallic oxide, either cobalt oxide or manganese oxide, gets reduced by depleting oxygen. This results in further loss of capacity.

The temperature also plays a very important role. The rate of the reaction is linearly dependent upon the temperature. The capacity is directly proportional to the reaction rate. In the case of low temperature, the reaction rate is decreased and therefore the current carrying ions also decreases. In the case of higher operating temperature, the temperature of the battery is increased. The rate of dissipation is less than the heat generated. Hence, the overall temperature is high and hence it will damage the battery in the long run. The resultant in a thermal runaway. Thermal runaway constitutes many stages such that each would make the situation worse.

Around 80° the SEI layer dissolves in the electrolyte. Due to this and exothermic reaction occurs between the anode and the electrolyte. This increases the temperature further. Now, at around 110°C the organic solvents begins to disintegrate in this release is hydrocarbon gases. The pressure increases inside the battery due to the gas but they do not burn because there is insufficient oxygen to enable conversion. At this point the temperature is so much that the separator also gets dissolved and the anode and cathode gets short-circuited. The temperature happens to be around 135°C. Finally around 200°C, the metal oxide also gets disintegrated. This releases oxygen which allows the hydrocarbon gas to burn. This process is also exothermic and increases the temperature as well as the pressure further.

Proposed Method for Increasing Battery Performance

Another problem is the unequal distribution of temperature in the battery. This is due to the huge temperature, fluctuating current, positioning of positive and negative terminals and many more. It results in thermal runaway and hence the life of the battery decreases.

In order to avoid the issues mentioned above, the temperature of the battery should be maintained in the optimum range of operation. This would not only increase the life of the battery but also increase its performance and energy consumption which is highly needed in an EV. The temperature distribution should be even. Thus, the battery thermal management system is very important for a battery.

Figure 4 shows a graph between the operating temperature and the power. When the temperature of the battery is between 20° and 40°, it attains the maximum power. The cycle life of the battery drops below 10°C due to the lithium plating. Also it drops after 60° because of the breakdown of the materials of electrodes as shown in the figure 5. Thus, the operating temperature of the battery should be controlled in such a manner that it lies between 20° and 40° so that the maximum power and performance is achieved. The even destruction of the temperature can be controlled under 5K in order to ensure safety and increase the life of the battery. The ventilation of the battery should be proper so that heat is dissipated properly.

The safety of the battery pack along with its performance in terms of power and capacity has to be ensured. The electric vehicles have a limited power supply therefore it should be stored and controlled in such a way that the thermal runaway is prevented thus increasing its life. The BPMS has to accomplish the following functionality is in order to enhance the battery usability.

Cooling: during the operation of the battery, heat is generated. This heat is in the form of an energy loss which cannot be prevented but it can be minimised by taking proper measures. When the temperature of the battery increases beyond the optimal range, it should be managed to bring it back to the required temperature range. A cooling system can serve this purpose in BTMS.

Heating: the temperature of the battery may become less than the optimum range if it is operating in a cold climate. Therefore, to bring it back to the optimal range, IET mechanism can be employed such as a PTC heater quickly.

Insulation: there occurs a wide range of temperature difference between the outside and inside environment of the battery when it is either kept too hot or too cold conditions. This difference is not like when it is kept under normal conditions and therefore the temperature of the battery falls arises very quickly outside the optimum range. This is a major issue that affects the performance of the battery and to avoid this proper insulation should be ensured.

Ventilation: ventilation is very much important in order to exhaust all those gases which are harmful. These are generated due to the electrochemical reactions of the battery. This function is combined with the cooling and heating functions in case of certain BTMS systems.

Operating Voltage and Temperature: Effects on Battery Performance

Air is used as a medium to transfer the heat around the battery system. The air that is taken in for cooling and heating purpose can be either atmospheric or from a system that could be a

heating or cooling. In the case of cooling it can be an air conditioner system that supplies are conditioned air or a heater. The air passes through the evaporator in case of air conditioning. If there is taken directly it is called as passive while if it is taken directly it is called as active. Active systems can provide more amount of heating or cooling as compared to the present systems by offering about thousands of watts cooling or heating whereas a passive system offering to around hundreds. Both systems are called as forced air systems because the air is supplied with the help of a blower. Figure 6 shows a passive and active systems (Chen, et al., 2017), (Lu, et al., 2016). 

Both systems of full functionalities of heating and cooling as well as ventilation. The ventilation systems in built and there is no need to add an external system. In the case of forced air system with a recovery, the heat can be recovered from the exhaust gases with the help of the heat recovery system such as air to air heat exchanger. This saves the extra energy in the heating purpose as the waste exhausts is used to offer the heat. The system mentioned above is shown in the form of a block diagram in figure 7. The forced air system is highly reliable as well as requires very less maintenance. Although it provides a poor management of heat yet due to economical nature it is used in low ends. If the temperature exceeds the optimal range then it will result in thermal runaway.

When the temperature is above 30°, the inside temperature of the battery surpasses 55° which is greater than the optimum range of operation and hence thermal runaway occurs. The distribution of the heat is on even due to degradation and life cycle. If the optimum range is neglected, a difference of 2°C between the cells occurs with the discharge current rate of 2C and a difference of 4.8 is a great when the discharge current is 6.67C. The fluid affects the conformity of the temperature. With an increase in the flow rate, the measured temperature difference of the cells also increases. It can be as high as 5K. This results in the degradation and the number of battery cycles also. If anyone cell fails, the thermal runaway spreads. The table 2 depicts that the volume of air is much larger than that of water at the same rate while the coefficient of heat in the case of water is larger than that in the case of a. Therefore in the case of air cooling in order to dissipate heat same as what are cooling, it needs larger volume and create. This means that more power and space are required. Hence we can say that air cooling is not efficient.

Thermal Runaway and Unequal Temperature Distribution

Table 2 (Lu, et al., 2016)

Apart from air, water is a great source of heat transfer. There are two types of liquid that are used for thermal management system. The first one is the direct liquid while the second is conducting liquid. The dielectric liquids are in direct contact with the batteries and it includes mineral oils. The conducting liquids has an indirect contact. It includes a mixture of ethylene glycol and water. In the case of direct contact liquids, the battery cells are submerged in that while in the case of conducting liquids a jacket is formed around the batteries through which the conducting liquid flows, where the battery modules are placed on the cooling or the heating plate. Comparing with the two approaches, the indirect systems are an edge because there is a better isolation between the environment and the battery module (Avidtp, n.d.), (Dincer & Rosen, 2018). This gives the system safe (SAE International, 2018). There are many heat sinks used for cooling. Just like the air cooling systems, the liquid crystals can also be categorised into active and passive systems. Radiators is used as a heat sink in passive liquid systems. Figure 8 shows the block diagram of the passive ecosystem (BOYD Corporation, n.d.). The pipe helps to circulate the liquid. This liquid absorbs the heat and the radiator release it from the battery. Fans can help the radiators to dissipate the heat faster and thus increasing the performance. If the difference between the ambient air and the battery temperature is very low or if the battery temperature is lower than the ambient air, the system fails (Hareyan, 2018), (Zhoujian, et al., 2017).

Figure 9 shows an active liquid cooling system. It can be seen that there are two loops, the upper one being the primary while the lower one being the secondary. In the primary loop the fluid circulates through the pipe and exchange the heat, the secondary loop in the air conditioning loop. During cooling operation, the upper heat exchanger works as an evaporator (EVAP) instead of the radiator while during heating, it works as condenser (COND). This is achieved through a 4 way valve. The ambient temperature affects the passive cooling system since the dissipation of the heat depends on the radiator. It dissipates heat by the difference in

temperature between the ambient temperature and the liquid. Normally, there is no issue but when there is high ambient temperature it is unsuitable. Compared to this the active cooling system has a great thermal performance which will keep the optimum temperature intact and the even distribution of the temperature is also maintained because the coolant has a high coefficient of heat. But due to many moving parts they are complicated and thus difficult to maintain enabling leaking out also (Rao, et al., 2017), (Zhao, et al., 2015).

It is similar to the active liquid cooling system as mentioned above but in the place of fluid, direct refrigerant is used for the exchange of heat from the battery. The block diagram of a direct refrigerant system (DRS) is shown in figure 10. DRS is more efficient as compared to the previous systems because the refrigerant is used directly as a coolant instead of cooling the coolant first with the help of the refrigerant and then cooling the system. The only problem with this system is that it is complicated and hence their maintenance is not easy.

Importance of a Battery Thermal Management System

PCM or the phase changing materials stores and releases heat at a fixed point during melting and solidification due to high fusion heat. The figure 11 is a graph plotted between the energy storage and the temperature change. PCM is solid when the temperature is lower than the melting point. As the temperature increases the heat is stored in the form of sensible heat after absorbing. At the melting point the heat is stored as the latent heat after absorbing and reaches a maximum point. Simultaneously the PCM changes its state to liquid. Further increase in temperature makes the PCM becomes liquid and absorbs the heat and stores it in the form of sensible heat back again. At the melting point since it absorbs the heat and reaches its maximum value in the form of latent heat, it delays the temperature rise. Due to this reason it is used as a buffer in BTMS. A PCM system is many times used along with an air cooling and a liquid cooling system to improve the thermal management. When the temperature increases beyond the working temperature of the battery that is from 40° to 50°, the system works well because of high latent heat and thermal conductance, the inside temperature of the cell is below 55°. Table 3 denotes the characteristics of PCM. In the case of hardship situations for example when the current discharge rate is 6.67C with an ambient temperature of 45°, the surpassing above 0.5K is not allowed in the maximum temperature difference of the cell. In the normal conditions this defence is negligible (Global Greenhouse Warming, n.d.). Because the PCM graphite Patrick’s absorbs and distributes heat fast, the prima donna is prevented in the case when one cell fails.

Although PCM is good yet there is a big disadvantage that it should be employed in cool environment as well as spacious. It as ritual of the PCM is lower than that of the battery pack which is released when the PCM melting point is higher than the ambient temperature, to the battery module (Microtek, 2018), (Souayfane, et al., 2016).

Table 3 (Global Greenhouse Warming, n.d.)

Figure 12 shows the comparison of the BTMS systems described above. The energy consumption, caused, reliability, size, weight, safety and the performance and analysed whose numerical values are evaluated in the table 4. Based upon the results, a new and innovative BTMS system can be constructed which consists of a combined liquid cooling system and a PCM system.

The liquid cooling system works on four modes namely: heater working with bypass, heater working without bypass, passive cooling system and active cooling system. One big advantage of using a cooling system is that it has both passive and active systems. Advantage of the passive cooling system is that it is simple and it dissipates heat in the normal situations with very low power consumption whereas during high conditions, the active cooling system comes into play which has a better thermal management in order to keep the temperature of the battery in the required range. The PCM has an advantage of a very good performance in terms of thermal management while the CLS removes the limited range of operation of PCM. The block diagram is shown in the figure 13.

Four figures are discussed that demonstrates the temperature, the consumption of energy, he transferred and the overview. Figure 14 shows the change in temperature over time. The blue line depicts the temperature of the battery while the green depicts the inlet temperature of the coolant before the battery and the red depicts the coolant is outright temperature of the battery. Figure 15 shows the consumption of energy of the different sections. In the case of passive cooling system, the consumption is very less for a radiator only the pump energy is taken into consideration. Figure 16 shows the transfer of the heat between the fluid and the battery. The red curve depicts the heating, cyan depicts the passive cooling and the blue depicts the active cooling. Here, the heating or the cooling power of the battery. Figure 17 gives the overview of the complete operation. It constitutes many important information such as the rate of heat generation, temperature of the battery, BTMS state and the rate of flow of the pump. The PCM employs latent heat in order to deduct the consumption of energy and introduce a delay in the temperature rise

Conclusion

This report highlights the importance of the battery management in EV vehicles. It operates under different weather conditions as well as situations. Because the battery is a source of power so proper techniques has to be evolved so that is increased because the source is limited. The air cooling system dissipates heat much less than a water cooling system. To compensate and to match the latter, it needs to become more complicated and spatial also the energy consumption of increases. Moreover with the increase of the flow rate, the maximum temperature difference between the cells can exceed so much that if any one cell fails, the thermal runaway will spread over the entire battery which will degrade it slowly. Liquid cooling systems have the much better thermal performance that can help the battery to lie in the operating range of temperature but the structure is complicated and hence very difficult to maintain. It may also leak out. The issues mentioned above are solved by direct coolant systems but it is also bulky. PCM on the other hand has the best thermal maintenance capabilities but it lacks in the temperature boundary. Observing the strengths and weaknesses of each system, a combined system of the liquid and PCM is involved that removes the flaws of thermal management through PCM while the maximum range is increased by liquid system. Improvements can be made in the management of the heat. A warning signal could be generated to warn when the system is in danger. Energy saving should be increased. This can be implemented by applying many control strategies. These strategies would increase the charging rate as well as ensure the safety.

References

Avidtp (n.d) What is the Best Electric Vehicle Battery Cooling System? [Online]. Available from: https://avidtp.com/what-is-the-best-cooling-system-for-electric-vehicle-battery-packs/ [Accessed 25 October 2018].

Battery University (2018) Types of Lithium-ion [Online]. Available from: https://batteryuniversity.com/index.php/learn/article/types_of_lithium_ion [Accessed 25 October 2018].

BOYD Corporation (n.d) Air vs Liquid Cooling: Advancements in Thermal Management for Power Electronics [Online]. Available from: https://www.boydcorp.com/resources/technical-papers/air-vs-liquid-cooling-advancements-in-thermal-management.html [Accessed 25 October 2018].

Chen, K., Wang, S., Song, M. & Chen , L. (2017). International Journal of Heat and Mass Transfer. Structure optimization of parallel air-cooled battery thermal management system, Volume 111, pp. 943-952.

Dincer, I. & Rosen, M. A. (2018). Numerical Heat Transfer. Heat transfer modeling of a novel battery thermal management system, 73(5), pp. 277-290.

Electropaedia (n.d) Lithium Battery Failures [Online]. Available from: https://www.mpoweruk.com/lithium_failures.htm [Accessed 25 October 2018].

Energy Storage Association (n.d) Lithium Ion (LI-ION) Batteries [Online]. Available from: https://energystorage.org/energy-storage/technologies/lithium-ion-li-ion-batteries [Accessed 25 October 2018].

Evans, J. L. (n.d). The Good, the Bad and the Ugly How Safe are Lithium-Ion Batteries?. [Online]. Available from: https://www.batterypoweronline.com/markets/testingservices/the-good-the-bad-and-the-ugly-how-safe-are-lithium-ion-batteries/ [Accessed 25 October 2018].

Global Greenhouse Warming (n.d) Phase Change Material [Online]. Available from: https://www.global-greenhouse-warming.com/phase-change-material.html [Accessed 25 October 2018].

Hareyan, A. (2018). Chevy Volt’s Liquid Cooled Battery vs Nissan Leaf’s Air Cooled Battery: 2 Systems. [Online]. Available from: https://www.torquenews.com/1/chevy-volts-liquid-cooled-battery-vs-nissan-leafs-air-cooled-battery-2-systems [Accessed 25 October 2018].

Lu, Z. et al. (2016). Energy Procedia. Thermal Management of Densely-packed EV Battery with Forced Air Cooling Strategies, Volume 88, pp. 682-688.

Microtek (2018) How do PCMs work? [Online]. Available from: https://microteklabs.com/how-do-pcms-work.html [Accessed 25 October 2018].

Poole, I. (n.d). Li-ion Lithium Ion Battery Technology. [Online]. Available from: https://www.radio-electronics.com/info/power-management/battery-technology/li-ion-lithium-ion.php [Accessed 25 October 2018].

Rao, Z., Qian, Z., Kuang, Y. & Li, Y. (2017). Applied Thermal Engineeering. Thermal performance of liquid cooling based thermal management system for cylindrical lithium-ion battery module with variable contact surface, Volume 123, pp. 1514-1522.

SAE International (2018) Integrated Cooling System for Underfloor High Voltage Devices in PHEV [Online]. Available from: https://www.sae.org/publications/technical-papers/content/2018-01-1184/ [Accessed 25 October 2018].

Souayfane, F., Fardoun, F. & Biwole, P. H. (2016) Energy and Buildings. Phase change materials (PCM) for cooling applications in buildings: A review, Volume 129, pp. 396-431.

Techopedia (n.d) Lithium-Ion Battery (LIB). [Online]. Available from: https://www.techopedia.com/definition/2653/lithium-ion-battery-lib [Accessed 25 October 2018].

Troiano, J. (2013). How do Lithium Ion Batteries Work? A Nanotechnology Explainer. [Online]. Available from: https://sustainable-nano.com/2013/10/15/how-do-lithium-ion-batteries-work/ [Accessed 25 October 2018].

Woodford, C. (2018). Lithium-ion batteries. [Online]. Available from: https://www.explainthatstuff.com/how-lithium-ion-batteries-work.html [Accessed 25 October 2018].

Zhao, J., Rao, Z. & Li, Y. (2015). Energy Conversion and Management. Thermal performance of mini-channel liquid cooled cylinder based battery thermal management for cylindrical lithium-ion power battery, Volume 103, pp. 157-165.

Zhoujian, A. et al. (2017) A review on lithium-ion power battery thermal management technologies and thermal safety. A review on lithium-ion power battery thermal management technologies and thermal safety, 26(5), pp. 391-412.

Order an Essay Now & Get These Features For Free:

Turnitin Report

Formatting

Title Page

Citation

Outline

Place an Order