A modular balancing bridge for series connected Li-ion batteries

The operation of a non-dissipative cascaded modular balancing bridge (MBB) is described for automatically balancing series connected lithium ion batteries. Automatic voltage balancing is achieved using coupled inductors; modularization and voltage balancing is achieved across N cells using cascaded transformers with coupling windings. The elementary modular bridge consists of four windings placed on a single core with an asymmetrical half-bridge. Two of these windings are used to automatically balance voltages across two battery cells, while the remaining two windings magnetically cascade one bridge with its neighboring bridges to automatically balance the voltage across N battery cells. This configuration allows for the voltage balancing of N battery cells using N identical transformers, and N/2 asymmetric bridges. The voltage balancing action of the resultant magnetically coupled cascaded asymmetric bridges can be broken into two categories: intra-bridge voltage balancing (within a single pair of battery cells), using the switching action of one asymmetrical bridge and inter-bridge voltage balancing (between pairs of battery cells). The parameters of the balancing bridge that affect the average balancing current is described and related to a physical size using commercially available transformers. The practical limitations of the battery cell voltage balancing methods are described, and a four-cell experimental prototype is used to verify simulated results.