Management of large format liion batteries

Electric vehicles (EVs) are expected to play an important role in sustainable mobility thanks to the efficient energy utilization and zero-emission when in use. The battery has a great impact on the performance of electric vehicles, basically determining the driving range. As a consequence, the choice of the battery technology and its effective utilization is of paramount importance. Li-ion chemistry is very sensitive to overcharge and deep discharge, which may damage the battery, shortening its lifetime, and even causing hazardous situations. This requires the adoption of a proper Battery Management System (BMS) to maintain each cell of the battery within its safe and reliable operating range. In addition to the primary function of battery protection, a BMS should estimate the battery status in order to predict the actual amount of energy that can still be delivered to the load. This is quite a challenging task, as the performance of the battery in terms of usable capacity and internal resistance, varies over time. Another important function of a BMS is to extend the battery life by facing the charge unbalancing issue that may arise in series-connected cells. This reduces the usable capacity of the battery. Due to the strict voltage limits applying to Li-ion batteries, charge unbalancing cannot be self-recovered, but instead worsens with time. A BMS should thus implement a charge equalization technique to periodically restore the balanced condition. The purpose of this paper is to describe the main issues in the design and management of a battery for an electric vehicle. The paper covers aspects BMS requirements and architectures and techniques for charge equalization.