Electrification of private transport will be required and is expected to be mandated by governments to reduce carbon dioxide emissions from this sector. This transformation is expected to significantly increase requirements on electric vehicles to meet customer demands. Battery charging time is one of the top concerns of consumers who consider buying an ev. batteries need to deliver and handle much higher currents to allow dynamic vehicle driving and ultra-fast re-charging as well as meeting highest battery safety standards to reduce fire risk. Very careful thermal management will be required to pre-condition the battery for fast charging and then during fast charging to prevent local cell temperatures exceeding 50°c. consequently, BEV and PHEV battery management systems are surprisingly sophisticated and can monitor individual cell temperatures and regulate charging rates to cap peak temperatures. overall rates of charging are thus influenced by temperature excursions within individual cells such that charging rates and charging times can be slowed considerably. The simplest and cheapest cooling technologies utilize air, but these are becoming obsolete as the demand for increased energy densities rise, more advanced thermal management schemes are appearing that use liquid cooling with water–glycol mixtures. Immersive thermal management designs are under development for next generation of electric vehicles. in these systems the fluid is in direct contact with electric parts in a battery, cells and busbars, which allows to transfer heat directly and thereby control temperature more effectively than indirect systems with a cold plate design. Conventional water/glycol coolants are significantly more conductive for electric currents, these are therefore not suitable for immersive systems. Immersive thermal management systems require fluids with good dielectric properties to ensure electrical insulation and to prevent electric shorts over the lifetime. Shell is currently developing thermal fluid technology aimed at matching these needs through collaborative relationships with leading primary and tier 1 equipment manufacturers. A newly designed battery module test rig equipped with pouch cells and immersive cell tab cooling was installed at the shell technology center Hamburg. Ultra-fast charging with an uplift from 10% to 90% soc was demonstrated with this system in less than 12 minutes. Immersive thermal management with dielectric fluids is also beneficial in meeting the latest safety and abuse requirements. Shell demonstrated “passes” in abuse tests with cylindrical, prismatic and pouch cells preventing thermal runaway propagation in collaboration with several partners using shell thermal fluids in different immersive battery module designs. Experiments without dielectric fluid present in the modules resulted in massive thermal propagation. In conclusion, the thermal fluid can play a key role to enhance battery safety. Thermal fluids can also be considered as an alternative fluid to transmission fluids for direct e-motor cooling when the fluid is not required to lubricate the e-motor bearing. Shell thermal fluids have already been successfully used in direct cooling of e-motors and inverters. A low viscous shell thermal fluid is used inside the Nissan formula e race car to cool both the e-motor and inverter.

Guest Curators: Prof. Chris Brace, Professor of Automotive Propulsion and Deputy Director of the Powertrain Vehicle Research Centre, University of Bath and David Hudson, Head of Vehicle & Powertrain Engineering, Tata Motors European Technical Centre (TMETC)

Topics & speakers:

• Future Energy Scenarios — Impact on Powertrain Systems: Dr. Bernadette Longridge, Engineering Centre Manager, AVL Powertrain UK Ltd
• Electric and Low Carbon Intensity Fuel Scenarios for Light Duty Vehicles: Nick Powell, Principal Technology Strategy, Ricardo Strategic Consulting
• Future Energy Options for a Sustainable Mobility: Dr. Wolfgang Warnecke, Chief Scientist of Mobility, Shell

Originally broadcast: 16 July 2020

Lithium-ion batteries currently used in electric vehicles have strong thermal limitations. Thus, both excessively high and low temperatures lead to an accelerated aging process in form of capacity losses through SEI-layer buildup on the one hand and lithium-plating on the other hand. Additionally, temperature gradients within a cell, as well as temperature differences between individual cells have to be minimized in order to increase the system’s durability. With a properly designed battery-thermal-management (BTM), not only the aging process can be minimized but also extreme cases like thermal runaways can be prevented, which otherwise can cause fatal ecological and economical damage or in the worst-case-scenario exposes and harms vehicle occupants to fire, explosion or intoxication due to a damaged battery cell system. For the application of an efficient BTM, a detailed characterization of the battery’s thermal behavior during the charge and discharge cycle with different loads is essential. This paper deals with high-performance pouch cells, as they have a favorable ratio of heat transfer surface area to battery volume for BTM. However, they also present a particular challenge for the acquisition of the surface temperature distribution, as they do not have a solid shell. An external force must be applied to this cell type to prevent it from inflating, when it is under higher loading conditions, because this volume change is expected to change the cell’s thermal behavior. Therefore, the determination of the surface temperature in the present study was carried out while the cell was fixated and forced to stay in its geometric shape. The mechanical fixation has an influence in the thermal behavior and therefore the temperature distribution should be probed in this configuration but this also makes the optical measurement of temperature more difficult as pressure transmission plates must be made transparent. The measured surface temperature distribution can then be used as an input for a FEM-simulation to provide information on the whole cell’s internal temperature field. To the authors´ knowledge, no thermal investigations in this configuration have been reported. Two different optical temperature measurement methods are applied. The first technique is IR-thermography, which is based on the measurement of heat radiation from the surface, and which is restricted to regions on the surface using small IR-transmitting sapphire windows. The second technique, phosphor thermometry, exploits the temperature dependent luminescence properties of thermographic phosphor materials. The phosphors are applied as a thin coating on the surface of the battery and illuminated by a light source. The resulting luminescence emission in the visible range is detected to infer the temperature of the whole surface, since normal glass can be used. The surface temperature distribution obtained by these two techniques are provided and the advantages or disadvantages of each technique are described in terms of precision, accuracy, sensitivity, temporal resolution, spatial resolution and also implementation expenses. The internal temperature field provided by the heat transfer model for a known surface distribution is also presented and the impact of this integral thermal characterization on the BTM is discussed.