Hierarchical modular battery equalizer with open-loop control and mitigated recovery effect

In this manuscript, an advanced battery equalizer with open-loop control is proposed. This equalizer is based on a two-layer hierarchical modular architecture. The top string-to-module (S2M) layer consists of a half-bridge inverter and a voltage multiplier (VM) rectifier, and the bottom cell-to-cell (C2C) layer is implemented by bidirectional buck-boost units. Without state-of-charge (SOC) estimation, the battery charge can be automatically transferred from high-voltage cell-modules/cells to low-voltage ones. Only a pair of symmetrical pulse width modulation (PWM) driving signals with fixed switching frequency and duty cycle are required. This reduces the control complexity remarkably. Meanwhile, the balancing current of each balancing path naturally attenuates with the convergence of cell-module/cell voltages. This ensures a fast balancing of cell-module/cell with large voltage mismatch. The battery-recovery-effect induced balancing error is also effectively mitigated. Moreover, simple control facilitates a simultaneous module and cell voltage balancing in static, charging, and discharging conditions. The operation principles are analyzed in detail. An experimental platform with eight series-connected batteries is built and tested. The measured results well validate the theoretical analysis. Both cell and module voltages automatically converge with clearly mitigated recovery effect.