978-1-7281-3153-5/19/$31.00 ©2019 IEEE The Energy Loss Due to Interconnections in Paralleled Cell Configurations of Lithium-Ion Batteries in Electric Vehicles Mohamed Ahmeid School of Engineering Newcastle University Newcastle upon Tyne, UK Mohamed.Ahmeid1@ncl.ac.uk Simon Lambert School of Engineering Newcastle University Newcastle upon Tyne, UK Simon.Lambert@ncl.ac.uk Musbahu Muhammad School of Engineering Newcastle University Newcastle upon Tyne, UK Musbahu.Muhammad@ncl.ac.uk Pierrot Attidekou School of Engineering Newcastle University Newcastle upon Tyne, UK Pierrot.Attidekou@ncl.ac.uk Zoran Milojevic School of Engineering Newcastle University Newcastle upon Tyne, UK Zoran.Milojevic@ncl.ac.uk Abstract— In Electric vehicles, the Li-ion battery reaches its end of life when the capacity is decreased to 80% of the initial rated capacity. However, a battery with only 20% used capacity does not mean the battery cannot be used in a secondary application with less current demand, in a controlled and secure environment. This necessitates a comprehensive understanding of the configuration of the end of life (EoL) batteries in module and pack level, in terms of inconsistencies in capacity and impedance of the cells forming the module and hence the battery pack. Accordingly, a safer and longer second life use can be granted. This paper investigates the impact of parallel connection on the impedance and capacity of four, pouch lithium-ion cells forming a battery module in 2P 2S configuration. The energy storage capacity and the AC impedance of each parallel pair and individual cells are recorded and compared. The results highlight that the capacity loss due to the parallel connection is 6% less than the sum of the individual capacity for each cell. With the help of a developed equivalent circuit model, the ohmic resistance of the pair and cell is estimated to demonstrate the contribution of interconnections in increasing the total impedance and hence the perceived loss of capacity in the parallel connection. Keywords—Li-ion battery, electric vehicles, second life, module, capacity, impedance. I. INTRODUCTION In recent years the demand for using lithium-ion batteries in many applications has grown considerably. Among these applications, the size of the electric vehicle (EV) fleet has substantially increased due to their zero-emission nature which reduces the contribution to climate change and pollution[1]. With a greater number of EVs entering the market, the number of Li-ion batteries entering the recycling and waste streams at their end of life (EoL) is expected to surge. Consequently, the capability to recycle, refurbish, and reuse those batteries is becoming vital to boost the circular economy and reduce ecological damage. To effectively reuse the lithium-ion battery in a second life application, such as an off-highway energy storage system, testing and analysing its performance in terms of available capacity and power is essential. Based on the application requirements, the lithium- ion batteries can be organised in series to increase the voltage level and in a parallel to increase the current and the available capacity or in a combination of both of these (as is commonly done in EVs)[2]. In hybrid and electric vehicles, the battery pack is composed of many single Lithium-ion cells electrically connected together to fulfil the requirements for power and energy. Generally, there are three main types of battery cells employed in automotive applications. For example, the Nissan Leaf 24 kWh battery pack consists of 33 Ah pouch cells, with 2 in parallel and 96 in series. Contrariwise, the Tesla Model S 85 kWh battery pack uses 74 3.1 Ah cylindrical cells to form a parallel unit, and 96 of these units in series [3]. In the BMW i3 42.2 kWh battery, the battery pack consists of 12 prismatic cells in parallel and 8 in series [4]. Because of the physical design and size, the prismatic and pouch cells allow economically feasible second use applications where the packs can be repurposed at the pack or module level [5]. In the module level, the parallel connection can be made from 12 cells such as in BMW i3 or 2 pouch cells in Nissan Leaf. Regardless of the advantages of connecting Li-ion cells in parallel, there are some challenges reported in the literature related to the electrical, mechanical and thermal integration of cells into packs [6]. In [1] the authors investigated different scenarios in parallel connection which can degrade the overall performance of battery pack, such as, manufacturing tolerance, interconnect resistance, and ambient temperature. The results presented highlight that the cell will be aged faster as the length of the parallel connection increases. Michael et al. [7] presented a detailed examination of all occurring critical parameter variations within parallel cells. After characterisation measurements, it was quantified that the maximum inhomogeneity in capacities and resistances of parallel couples within a retired battery pack are 2.23% and 10% respectively. Gregory et al.[8] stated that the difference in current flow across the battery modules occurs mainly due to the interconnection resistances between highly paralleled cells irrespective of the variations in impedance between cells. Therefore, it is of high interest to understand the interaction between these parameters in particular for retired EV batteries in order to forecast their performance in second life applications and to mitigate any safety concerns. The objective of this paper is to demonstrate the hidden contribution of interconnections in the loss of capacity in paralleled cells. This study comprises disassembly of a battery module from a retired Nissan Leaf 24 kWh battery pack. The discharge capacity and the impedance for a module and cell level are measured to show the correlation between the ohmic resistance and capacity in parallel connection. The overall output of this work is expected to highlight the importance of improving welding and interconnections techniques for pouch cells to utilise the most available capacity within the pack.