A Fast Charging Balancing Circuit for LiFePO4 Battery

In this paper, a fast charging balancing circuit for LiFePO4 battery is proposed to address the voltage imbalanced problem of a lithium battery string. During the lithium battery string charging process, the occurrence of voltage imbalance will activate the fast balancing mechanism. The proposed balancing circuit is composed of a bi-directional converter and the switch network. The purpose of bi-directional is that the energy can be delivered to the lowest voltage cell for charging mode. On the other hand, the energy stored in the magnetizing inductors of the transformer can be charged back to the higher voltage cell in recycling mode. This novel scheme includes the following features: (1) The odd-numbered and even-numbered cells in the string with the maximum differential voltage will be chosen for balancing process directly. In this topology, there is no need to store and deliver the energy through any intermediate or the extra storing components. That is, the energy loss can be saved to improve the efficiency, and the fast balancing technique can be achieved. (2) There is only one converter to complete the energy transfer for voltage balancing process. The concept makes the circuit structure much simpler. (3) The structure has bi-directional power flow and good electrical isolation features. (4) A single chip controller is applied to measure the voltage of each cell to achieve the fast balancing process effectively. At the end of the paper, the practical test of the proposed balancing method on LiFePO4 battery pack (28.8 V/2.5 Ah) is verified and implemented by the experimental results.