Thermal and State-of-Charge Balancing of Batteries using Multilevel Converters

Licentiate thesis, 2014

Driven by the needs to reduce the dependence of fossil fuels and the emissions of conventional vehicles there has in recent years been an increasing interest in battery-powered electrified vehicles (xEVs). In xEVs, the battery pack, built from many small cells, is one of the most expensive components in the powertrain. As a result, the battery lifetime is an important factor for the success of xEVs. However, the battery pack lifetime is severely affected by the State-of-Charge (SOC) and thermal imbalance among its cells, which is inevitable in large automotive batteries. Therefore, thermal and SOC balancing is quite important to enhance their life-time. In this thesis, the use of a multi-level converter (MLC) as an integrated cell balancer and motor driver is investigated for application in xEVs. The MLC has a special modular structure which distributes a large battery pack into smaller units, enabling an independent cell-level control of a battery system. This extra degree-of-freedom enables the potential non-uniform use of cells, which along with brake regeneration phases in the drive cycle is exploited by MLC to achieve simultaneous thermal and SOC balancing. An MLC-based optimal control policy (OP) has been formulated, assuming dc machine as a load, which uses each cell in a battery submodule according to its SOC and temperature to achieve thermal and SOC balancing by optimally redistributing the power losses among the cells. Results show that OP reduces temperature and SOC deviations significantly compared with the uniform use of all cells. However, in applications involving three-phase ac machine, the MLC, in addition to its great balancing potential, also poses serious issues of extra battery heating and of extra ampere-hour throughput due to dc-link current ripple. These extra effects may accelerate the battery ageing if not compensated. A simple passive compensation method based on dc-link capacitor has been investigated, but it turns out that the size of the required capacitor is too big for automotive applications. Thus, it is concluded that, from battery's health viewpoint, it is unpromising to promote three-phase MLC as an integrated cell balancer and a motor driver in xEVs, unless some other more advanced active compensation technique is used.