Optimal charge equalisation control for seriesconnected batteries

Batteries play an important role in sustainable energy systems because the energy stored in batteries can be dispatched at any time. Series-connected batteries can furnish a load with higher voltage and consequently reduce the I²R loss during power conversion and transmission. However, if imbalanced voltage occurs, a battery with high state of charge (SOC) would react more drastically than that with lower SOC under both charging and discharging conditions. Imbalanced charge or discharge would give rise to potential damage to the battery and also shorten the battery life cycle. This study presents a modified charge equaliser (CE) along with an optimal charge equalisation algorithm (CEA). The proposed circuit scheme can distribute an imbalanced charge in a more efficient way without increasing additional circuit cost. To further expedite charge equalisation among a battery string, the activated CE duty ratio is fixed at the upper limits throughout the charge equalisation phase. An optimal CEA with a view towards maximising the final battery string voltage is developed to resolve the CE operating sequence and length. The proposed equalisation strategy is realised using a peripheral interface controller that uses a buck-boost converter as the CE. The experimental results confirm the performance of the proposed strategy.