Experimental and simulation analysis of novel cooling approaches for automotive lithium-ion batteries

Previous research has identified that the ageing rate and performance of lithium-ion cells are negatively influenced by unfavourable cell thermal conditions, specifically, high ambient temperatures and large in-cell temperature gradients. Careful consideration must, therefore, be placed on the design of the battery thermal management system (BTMS) contained within electrified vehicles to ensure that thermal constraints are satisfied whilst avoiding the addition of excessive weight and volume which are detrimental to the overall battery system design. Common cooling approaches that define the current state of the art BTMS focus on cooling the exterior of the cells. For cylindrical cells, cooling the radial surface is susceptible to the formation of large temperature gradients through the cell material, which is exacerbated as the cells heat generation rate increases. Conversely, for pouch cells, the shorter heat transfer pathway between the cell centre and surface makes internal temperature gradients less problematic. However, common cooling approaches using indirect liquid plates to contact the pouch body suffer from inherent leakage concerns. Conduction based thermal management methods offer inherent benefits over common indirect liquid cooling solutions as the external cooling location may be positioned further away from the cell, which can reduce the risk of leakage and simplify the overall design. Further, the use of conduction elements that are incorporated directly inside battery cells offer the potential to improve temperature uniformity by increasing the heat transfer to the external surfaces of the battery. To advance the deployment of a conduction based BTMS, this thesis presents and examines the thermal performance of two novel cooling approaches employing advanced conduction methods that tackle the key thermal management issues facing both cylindrical and pouch type lithium ion cells in automotive applications. In the cylindrical cell study, a mathematical model that captures the dominant thermal properties of the cell is created and validated using experimental data. Results from the extensive simulation analysis indicate that the proposed internal cooling strategy can reduce the cell thermal resistance by up to 67.8 ± 1.4% relative to single tab cooling and can emulate the thermal performance of a more complex pack-level double tab cooling approach. In the pouch cell study, a novel graphite-based fin material with an in-plane thermal conductivity 5 times greater than aluminium is presented for advanced battery cooling in a developed novel BTMS design. The thermal performance of the fin is benchmarked against conventional copper and aluminium fins in an experimental programme cycling commercially available 53 Ah pouch cells. Results from the rigorous experimental testing and subsequently validated thermal model indicate that under an aggressive electric vehicle duty-cycle, the new fin can reduce the peak cell surface temperature gradient by up to 55% and volume averaged cell temperature rise by 6.3 ℃ when compared to a comparably sized aluminium fin with the same weight penalty. The innovative thermal management approaches presented in this thesis offer improved thermal efficiency relative to the current state of the art BTMS reported in the literature, providing important contributions to academia and opportunities for the sponsor company to further the advancement of next generation BTMS

Electrical and thermal behaviour of pouch-format lithium ion battery cells under high-performance and standard automotive duty-cycles

Six pouch-format cells comprising a carbon anode and nickel-cobalt-manganese (NCM) cathode are characterized. Their 1C discharge capacity and open circuit voltage are determined. Internal Resistance is investigated via Hybrid Pulse Power Characterization tests and Electrochemical Impedance Spectroscopy. They are subsequently subject to two different electrical loading profiles, one representing high-performance (HP) driving applications, the other representing urban and extra-urban driving scenarios. The cells are instrumented with thermocouples to determine their surface temperature during cycling. The experimental results show that HP scenarios result in higher temperatures and temperature gradients, requiring bespoke thermal management strategies and suggesting increased degradation over prolonged use

Duty-cycle characterisation of large-format automotive lithium ion pouch cells for high performance vehicle applications

The long-term behaviour of lithium ion batteries in high-performance (HP) electric vehicle (EV) applications is not well understood due to a lack of suitable testing cycles and experimental data. As such a generic HP duty cycle (HP-C), representing driving on a race track is validated, and six NMC graphite cells are characterised with respect to cycle-life. To enable a comparison between the HP-EV environment and conventional road driving, two test groups of cells are subject to an experimental evaluation over 200 duty cycles that includes a representative HP-C and a standard duty cycle from the IEC 62660-1 standard (IECC). After testing, both test groups display increased energy capacity, increased pure Ohmic resistance, lower charge transfer resistance an extended OCV operating window. The changes are more pronounced for cells subject to the HP-C. Based on capacity tests, Electrochemical Impedance Spectroscopy (EIS), pseudo-OCV tests, and Pulse Multisine Characterisation, it is concluded that the changes in cell characteristics are most likely caused by cracking of the electrode material caused by high electrical current pulses. With continued cycling, cells cycled with the HP-C are expected to show degradation at an increased rate due to raised temperatures, and more pronounced electrode cracking

A study into different cell-level cooling strategies for cylindrical lithium-ion cells in automotive applications

Previous research has identified that the ageing rate and performance of lithium-ion cells is negatively influenced by unfavourable cell thermal conditions, specifically, high ambient temperatures and large in-cell temperature gradients. In this paper, the effectiveness of different cell cooling strategies on reducing the in-cell temperature gradient within cylindrical cells is analysed through the development of a 2-D transient bulk layer thermal model displaying anisotropic thermal conductivity. The model is validated against experimental temperature measurements in which the peak error of the simulation was found to be 2% and 5% for the experimental test drive cycle and constant 1C discharge respectively. Results indicate that radial cooling with air or singular tab cooling with liquid may be inadequate in limiting cell temperature gradients to below 5 ℃ for HEV type 32113 cells when subject to 4 loops of the US06 drive cycle

A systematic approach for electrochemical-thermal modelling of a large format lithium-ion battery for EV application

A 1D electrochemical-thermal model is developed to characterise the behaviour of a 53 Ah large format pouch cell with LiNixMnyCo1-x-yO2 (NMC) chemistry over a wide range of operating conditions, including: continuous charge (0.5C-2C), continuous discharge (0.5C-5C) and operation of the battery within an electric vehicle (EV) over an urban drive-cycle (WLTP Class 3) and for a high performance EV being driven under track racing conditions. The 1D model of one electrode pair is combined with a 3D thermal model of a cell to capture the temperature distribution at the cell scale. Performance of the model is validated for an ambient temperature range of 5°C–45°C. Results highlight that battery performance is highly dependent on ambient temperature. By decreasing the ambient temperature from 45 °C to 5 °C, the available energy drops by 17.1% and 7.8% under 0.5C and 5C discharge respectively. Moreover, the corresponding power loss is found to be: 5.23% under the race cycle as compared with 7.57% under the WLTP drive cycle. Formulation of the model is supported by a comprehensive set of experiments, for quantifying key parameters and for model validation. The full parameter-set for the model is provided ensuring the model is a valuable resource to underpin further research

An advanced hardware-in-the-loop battery simulation platform for the experimental testing of battery management system

Extensive testing of a battery management system (BMS) on real battery storage system (BSS) requires lots of efforts in setting up and configuring the hardware as well as protecting the system from unpredictable faults during the test. To overcome this complexity, a hardware-in-the-loop (HIL) simulation tool is employed and integrated to the BMS test system. By using this tool, it allows to push the tested system up to the operational limits, where may incur potential faults or accidents, to examine all possible test cases within the simulation environment. In this paper, an advanced HIL-based virtual battery module (VBM), consists of one “live” cell connected in series with fifteen simulated cells, is introduced for the purposes of testing the BMS components. First, the complete cell model is built and validated using real world driving cycle while the HIL-based VBM is then exercised under an Urban Dynamometer Driving Schedule (UDDS) driving cycle to ensure it is fully working and ready for the BMS testing in real-time. Finally, commissioning of the whole system is performed to guarantee the stable operation of the system for the BMS evaluation

Distributed thermal monitoring of lithium ion batteries with optical fibre sensors

Real-time temperature monitoring of li-ion batteries is widely regarded within the both the academic literature and by the industrial community as being a fundamental requirement for the reliable and safe operation of battery systems. This is particularly evident for larger format pouch cells employed in many automotive or grid storage applications. Traditional methods of temperature measurement, such as the inclusion of individual sensors mounted at discrete locations on the surface of the cell may yield incomplete information. In this study, a novel Rayleigh scattering based optical fibre sensing technology is proposed and demonstrated to deliver a distributed, real-time and accurate measure of temperature that is suitable for use with Li-ion pouch cells. The thermal behaviour of an A5-size pouch cell is experimentally investigated over a wide range of ambient temperatures and electrical load currents, during both charge and discharge. A distributed fibre optical sensor (DFOS) is used to measure both the in-plane temperature difference across the cell surface and the movement of the hottest region of the cell during operation, where temperature difference is the difference of temperature amongst different measuring points. Significantly, the DFOS results highlight that the maximum in-plane temperature difference was found to be up to 307% higher than that measured using traditional a thermocouple approach

Thermal analysis of fin cooling large format automotive lithium-ion pouch cells

Conductively cooling the surface of lithium-ion pouch cells may simplify the external cooling mechanism, as heat transfer mediums are not routed across the cell surface. In this paper, the thermal performance of cooling cells with metallic fins is analysed using a developed test rig and thermal model. Results indicate that single edge fin cooling with aluminum sheets is effective in limiting surface temperature gradients to below circa 5℃ for cells subject to realistic EV and mild PHEV duty cycles. For aggressive track racing EV cycles, double edge fin cooling is required to limit surface temperature gradients to below 12℃

Quantifying cell-to-cell variations of a parallel battery module for different pack configurations

Cell-to-cell variations can originate from manufacturing inconsistency or poor design of the battery pack/thermal management system. The potential impact of such variations may limit the energy capacity of the pack, which for electric vehicle applications leads to reduced range, increased degradation along with state of health dispersion within a pack. The latter is known to reduce the accessible energy and the overcharging/discharging of some of the cells within a system, which may cause safety concerns. This study investigates the short-term impact of such effects, which is highly important for designing of an energy storage system. A generic pack model comprising individual cell models is developed in Simscape and validated for a 1s-15p module architecture. The results highlight that a number of cells and interconnection resistance values between the cells are the dominant factors for cell-to-cell variation. A Z shape module architecture show a significant advantage over a ladder configuration due to the reduced impact of interconnection resistance on differential current flow within the module. Current imbalance is significantly higher for a ladder system and its magnitude is not dependent on the module current. Capacity variation does not have a significant impact on the system. By increasing the capacity variation from 9% to 40% the current inhomogeneity increases from 4% to 13%, whilst 25% resistance variation leads to 22% current dispersion. Further, a linear relationship is observed between the current inhomogeneity and thermal gradient