Challenges in modeling high power lithium titanate oxide cells in battery management systems

Two of the key requirements in micro-hybrid vehicles are the reduction of CO2 emissions alongside extended fuel savings. These goals are being accomplished by the usage of start-stop-systems, electric boosting while accelerating and recuperation of kinetic energy during braking phases. The latter one is predominantly limited by the charge acceptance of conventional lead-acid starter batteries. Thus, sophisticated power nets are being developed and a new 48 V voltage level has been established in the last years. In comparison to the standard 12 V grid, this new level is characterized by high power capability in order to accept high charging currents during recuperative braking and also provides high discharge currents for boosting. This paper characterizes lithium titanate oxide cells suited for high power applications, such as in automotive 48 V systems, and emphasizes restrictions using state-of-the-art modeling approaches in terms of state of available power and state of charge. Therefore, open circuit behavior is analyzed in dependency on temperature, hysteresis and relaxation. Furthermore, different equivalent circuit models are evaluated regarding their accuracy for different states of charge, temperatures and current rates. It has been found that the HPPC testing procedure is limited in characterizing high current behavior. Furthermore, it is shown that the previous long-term history has a significant impact on voltage responses at certain states of charge and current rates, which originates from an inhomogeneous distribution of charge carriers on the electrode surface.