Optimum battery capacity for electric vehicles with particular focus on battery degradation

Abstract

Electric vehicles (EVs) are seen as a key future trend in the automotive industry. These vehicles rely on rechargeable batteries to store energy on board. The optimum size of this energy store, often referred to as the battery capacity measured in ampere-hours (Ah) or kilowatt-hours (kWh), depends on the specific application, design limitations, costs and the degradation of the particular battery pack. Validated by 'real-world' driving data from the Imperial College Racing Green Endurance (RGE) flagship electric supercar, the SRZero, a software model following a quasi-steady, backward-forward facing and equivalent circuit approach is introduced. This model is also supported by the results of the 2010, 2011 and 2012 RAC Future Car Challenges as well as by battery life testing from a lab environment. Furthermore, travel surveys from the United Kingdom (UK), Germany and the United States (US) have been analysed and then converted into input parameters for this algorithm. The work considers five different electric vehicle classes ranging from mini cars to sport utility vehicles (SUVs). Results show that varying kerb weights combined with differing levels of driving resistances (aerodynamic drag, rolling resistance, climbing resistance, etc.) lead to reference 'driving forces' of 70-290 Wh/km for the five reference vehicle classes. On average, SUVs consume more than four times as much energy per unit distance as mini cars. Also, driving behaviour has a significant impact on energy consumption and thus on the optimum nominal battery capacity. Empirical data has shown that the mean driving force can vary up to 23% between drivers who follow exactly the same route at comparable traffic conditions and driving another vehicle of exactly the same make and model. Daily range requirements of EVs vary between 150-700 km based on the 95th percentile of the number of all daily trips or the cumulative distance of all trips combined for the UK, Germany and the US. Thus, optimum nominal battery capacities range between 11-203 kWh. In addition, it is shown that the optimum actual size of a battery pack for an electric vehicle depends on the battery's degradation as well. Over time and number of cycles the available capacity as well as the available power fades. This is mainly due to the effects of increased internal resistance, polarisation, corrosion and passivation. Therefore, first it is recommended to reduce the depth of discharge (DOD) to 80% when the battery is in use. Second, a spare capacity at the beginning of life of around 20-40% is recommended in order to satisfy range and power requirements also towards the end of the EV’s lifetime. It follows that the optimum actual battery capacity is around 1.25-1.75 times the optimum nominal battery capacity for an EV.