Improvement of consistency, accuracy and interpretation of characterisation test techniques for Li-ion battery cells for automotive application

Equivalent circuit models (ECM) are required to provide an on-board model of battery behaviour by battery management systems (BMS). The performance of ECMs is dependent on characterisation test results. The components of the ECM are commonly parameterised using electrochemical impedance spectroscopy (EIS) results, open circuit voltage (OCV) test results, and capacity test results. Therefore, these three tests are important for ECM parameterisation. Although the test procedures for these characterisation tests exist to test Li-ion cells for a range of applications e.g. portable electronic devices, they fail to provide essential information for automotive application due to the different requirements of vehicles (e.g. high power and energy, wide operating environment, long service life). This thesis reports research to improve consistency, accuracy and interpretation of characterisation test techniques for Li-ion battery cells for automotive application. The capacity of the battery pack is a vital parameter required for an ECM to estimate driving range. Existing techniques for predicting the driving range of an electric vehicle use the capacity value in Amp-hours, measured by existing capacity test techniques. In this thesis, experimental evidence that establishes the advantages of using capacity in Watt-hours instead of the capacity in Amp-hours as per the standard test is presented for the first time. Moreover, it is reported that measured battery capacity can vary by up to 5.0 % depending on the length of intermediate rest period. The OCV is another crucial parameter of ECM. The path dependence of OCV is a distinctive characteristic of Li-ion batteries which is known as OCV hysteresis. OCV test procedures used previously do not consider the initial conditions of the cells and capacity variations that show a change in OCV, leading to an apparent increase in, or erroneous, hysteresis. Using a new methodology which addresses issues mentioned above, OCV and OCV hysteresis has been quantified for different Li-ion cells for the first time. The test results show that a battery’s OCV is directly related to the discharge capacity, not the more commonly used SoC. The maximum hysteresis was found in a LiFePO4 (LFP) cell and lowest in a LTO cell, although still measurable. A dynamic hysteresis model is used to show how better OCV prediction accuracy can be achieved by a BMS when hysteresis voltage is a function of SoC instead of assuming it to be a constant, as traditionally done. EIS is commonly used to parameterise an ECM. For the first time this thesis reports that the time period between the removal of an electrical load and an EIS measurement affects the results. The study of five commercially available cells of varying capacities and electrode chemistries show that, regardless of the cell type, the maximum impedance change takes place within the first 4 hours of the relaxation period. Therefore a standardised relaxation period of minimum 4 hours should be allowed before performing EIS test. In addition to ECM parameterisation, EIS has been considered for online measurement, integrated with a BMS. This thesis concluded that the use of EIS as a fast measurement tool will be unreliable because of the relaxation effect. The flaws with capacity, EIS and OCV tests for automotive applications have been discussed. Through experimental evidence and electrochemical explanation it has been demonstrated that these tests can be made more consistent (e.g. by allowing fixed relaxation period in EIS test), have improved accuracy (e.g. incorporating hysteresis as a function of SoC) and better interpretation of test results (e.g. Watt-hours instead of Amp-hours in capacity test) are possible. Therefore, the overall contributions of this thesis to the scientific community are better consistency, accuracy and interpretation of these three tests. With the use of a case study, it has been shown that this new knowledge will improve performance of ECM, and thus BMS. This is not only for automotive but also more general applications through adopting the proposed new methodologies

Improving accessible capacity tracking at low ambient temperatures for range estimation of battery electric vehicles

Today’s market leading electric vehicles, driven on typical UK motorways, have real-world range estimation inaccuracy of up to 27%, at around 10 °C outside temperature. The inaccuracy worsens for city driving or lower outside temperature. The reliability of range estimation largely depends on the accuracy of the battery’s underlying state estimators, e.g., state-of-charge and state-of-energy. This is affected by accuracy of the models embedded in the battery management system. The performance of these models fundamentally depends on experimentally obtained parameterisation and validation data. These experiments are mostly performed within thermal chambers, which maintain pre-set temperatures using forced air convection. Although these setups claim to maintain isothermal test conditions, they rarely do so. In this paper, we show that this is potentially the root-cause for deterioration of range estimation at low temperatures. This is because, while such setups produce results comparable to isothermal conditions at higher temperatures (25 °C), they fail to achieve isothermal conditions at sub-zero temperatures. Employing an immersed oil-cooled experimental setup, which can create close-to isothermal conditions, we show battery state estimation can be improved by reducing error from 49.3% to 11.7% at −15 °C. These findings provide a way forward towards improving range estimation in cold weather conditions

Modelling and characterisation of ultrasonic joints for Li-ion batteries to evaluate the impact on electrical resistance and temperature raise

In automotive and stationary Li-ion battery packs, a large number of individual cells, typically hundreds to thousands of cells, are electrically connected to achieve pack specification. These large number of interconnections are mainly achieved by welding cell tab to bus-bar using a welding technique of choice. Ultrasonic metal welding (UMW) is one of the common joining technique employed to join pouch cell’s tabs to bus-bar. Although commonly employed, there is little research currently exist in literature reporting the joint characteristics in terms of electrical resistance and temperature raise due to charge-discharge current. Li-ion batteries reaching sub-milliohm internal resistance, risks the temperature raise at the joint could be even higher than the cell itself which raise a serious safety concern and they are to be addressed. This research investigates the electrical and thermal characteristics of ultrasonic joints of 0.3 mm aluminium/nickel coated copper tabs to 1.0 mm copper bus-bar. This article reports the dynamic behaviour of electrical resistance and corresponding temperature increase as a result of current flow. To capture the electrical and thermal behaviour of the joint, a numerical model has been developed and validated with experimental results, which can be employed to analyse battery pack performance

Electric vehicle battery performance investigation based on real world current harmonics

Electric vehicle (EV) powertrains consist of power electronic components as well as electric machines to manage the energy flow between different powertrain subsystems and to deliver the necessary torque and power requirements at the wheels. These power subsystems can generate undesired electrical harmonics on the direct current (DC) bus of the powertrain. This may lead to the on-board battery being subjected to DC current superposed with undesirable high- and low- frequency current oscillations, known as ripples. From real-world measurements, significant current harmonics perturbations within the range of 50 Hz to 4 kHz have been observed on the high voltage DC bus of the EV. In the limited literature, investigations into the impact of these harmonics on the degradation of battery systems have been conducted. In these studies, the battery systems were supplied by superposed current signals i.e., DC superposed by a single frequency alternating current (AC). None of these studies considered applying the entire spectrum of the ripple current measured in the real-world scenario, which is focused on in this research. The preliminary results indicate that there is no difference concerning capacity fade or impedance rise between the cells subjected to just DC current and those subjected additionally to a superposed AC ripple current

Electro-thermo-mechanical behaviours of laser joints for electric vehicle battery interconnects

An automotive battery pack used in electric vehicle (EV) comprises several hundred to a few thousand of individual Lithium-ion (Li-ion) cells when cylindrical cells are used to build the battery pack. These cells are connected in series and/or parallel to deliver the required power and capacity to achieve the designed vehicle driving range. This triggers the need for suitable joining methods capable of providing mechanical strength together with the required electrical and thermal performances. A range of joining techniques are currently employed to connect large numbers of cells, and of these, laser welding is estimated to be the one of most efficient methods. Typically, the cylindrical cell casing is made of electrical grade steel which is electrically connected to copper tabs representing the cylindrical cell terminal to tab interconnect within the battery pack assembly. This study focuses on identifying the effect of laser welding process parameters on the mechanical, electrical and thermal responses of the laser welded joints produced using a 150 W Quasi-CW IR laser. Mechanical strength is assessed by evaluating the lap shear strength of the joint whereas the electrical and thermal responses are captured using voltage sensors and a thermal imaging camera respectively. It was observed that mechanical strength of the joint is highly correlated with electrical resistance and corresponding temperature raise at the joint. Furthermore, the optical micrographs reveal the microstructural characteristics of the joint

Comparison of tab-to-busbar ultrasonic joints for electric vehicle li-ion battery applications

Recent uptake in the use of lithium-ion battery packs within electric vehicles has drawn significant attention to the selection of busbar material and corresponding thickness, which are usually based on mechanical, electrical and thermal characteristics of the welded joints, material availability and cost. To determine joint behaviour corresponding to critical-to-quality criteria, this study uses one of the widely used joining technologies, ultrasonic metal welding (UMW), to produce tab-to-busbar joints using copper and aluminium busbars of varying thicknesses. Joints for electrical and thermal characterisation were selected based on the satisfactory mechanical strength determined from the T-peel tests. Electrical contact resistance and corresponding temperature rise at the joints were compared for different tab-to-busbar joints by passing current through the joints. The average resistance or temperature increase from the 0.3 mm Al tab was 0.6 times higher than the 0.3 mm Cu[Ni] tab, irrespective of busbar selection

The effects of high frequency current ripple on electric vehicle battery performance

The power electronic subsystems within electric vehicle (EV) powertrains are required to manage both the energy flows within the vehicle and the delivery of torque by the electrical machine. Such systems are known to generate undesired electrical noise on the high voltage bus. High frequency current oscillations, or ripple, if unhindered will enter the vehicle’s battery system. Real-world measurements of the current on the high voltage bus of a series hybrid electric vehicle (HEV) show that significant current perturbations ranging from 10 Hz to in excess of 10 kHz are present. Little is reported within the academic literature about the potential impact on battery system performance and the rate of degradation associated with exposing the battery to coupled direct current (DC) and alternating currents (AC). This paper documents an experimental investigation that studies the long-term impact of current ripple on battery performance degradation. Initial results highlight that both capacity fade and impedance rise progressively increase as the frequency of the superimposed AC current increases. A further conclusion is that the spread of degradation for cells cycled with a coupled AC–DC signal is considerably more than for cells exercised with a traditional DC waveform. The underlying causality for this degradation is not yet understood. However, this has important implications for the battery management system (BMS). Increased variations in cell capacity and impedance will cause differential current flows and heat generation within the battery pack that if not properly managed will further reduce battery life and degrade the operation of the vehicle

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

The influence of temperature and charge-discharge rate on open circuit voltage hysteresis of an LFP Li-ion battery

Open circuit voltage (OCV) is a crucial parameter in an equivalent circuit model (ECM). The path dependence of OCV is a distinctive characteristic of a Li-ion battery; this is known as OCV hysteresis. In this manuscript the influence of temperature and charge/discharge rate on OCV hysteresis has been identified. OCV hysteresis was found to be 13mV higher at 0°C while remaining unchanged at 45°C compared to the 25°C result. In general, OCV hysteresis was found to be less dependent on charge/discharge rate than temperature. The potential explanations of these results have been reported