Cell Balancing of a Multi-Cell Battery Storage System for Renewable Energy DC Micro-Grids

The paper analyzes different concepts for active charge balancing for a multi-cell battery energy storage system used as a backbone for a DC micro grid. The storage system is based on a cascaded multi-level converter topology where each (lead-acid) battery is equipped with dedicated low-voltage buck type DC/DC converter stage placed directly at the battery terminals. To minimize the output ripple, the cells are operated in a phase-shifted PWM manner. For active charge balancing of the individual battery cells first a simple P-type control scheme is developed showing good balancing characteristic provided that the batteries show almost equal Ah capacity. In case of significant different capacity Ah ratings (which may appear in practice if some batteries of the storage unit are in a weak life-cycle condition) the P-type balancing is not further applicable because a tremendous rise of the output ripple takes place. The P-type balancing therefore is extended/replaced by a concept where weak cells periodically completely are taken out of operation for specific time intervals. For this, a dynamically reconfigurable PWM generation unit is required which is implemented fully digital using a FPGA device. It is demonstrated that the proposed concept shows proper balancing behaviour, low output ripple and allows maximum battery utilization.