Extending life of Lithium-ion battery systems by embracing heterogeneities via an optimal control-based active balancing strategy

Abstract

This paper develops a multi-objective fast charging-minimum degradation optimal control problem (OCP) for lithium-ion battery modules, made of series-connected cells, subject to heterogeneity induced by manufacturing defects and non uniform operating conditions, realized via an active balancing circuitry. Each cell is expressed via a coupled nonlinear electrochemical, thermal, and aging model and the direct collocation approach is employed to transcribe the OCP into a nonlinear programming problem (NLP). The proposed OCP is formulated under two different schemes of charging operation: (i) same-charging-time (OCP-SCT) and (ii) differentcharging-time (OCP-DCT). The former assumes simultaneous charging of all cells irrespective of their initial conditions, whereas the latter allows for different charging times of the cells to account for heterogeneous initial conditions. Simulations on an illustrative case study-a battery module with two seriesconnected cells-are carried out in the presence of intrinsic hetereogeneity among the cells in terms of state of charge and state of health. Results show that the OCP-DCT scheme provides more flexibility to deal with heterogeneity, boasting of lower temperature increase, charging current amplitudes, and degradation. Finally, comparison with the common practice of constant current (CC) charging over a long-term cycling operation shows that promising savings, in terms of retained capacity, are attainable under the new control.

Power allocation method of battery energy storage system considering state balance in smoothing photovoltaic power fluctuations

“…The power allocation scheme redistributing the output current into each battery unit according to their SOH state is designed in Goh et al, 2022 to achieve the balance in SOH. A SOH-awareness balancing scheme is devised in Azimi et al, 2022 by reducing the load current of the battery units with lower SOH. A prediction model and SOH balancing scheme are created in Shili et al, 2016 to compensate for the aging difference of the battery units by generating the voltage dispersion and balancing the aging speed of each battery unit.…”