A cascaded transformer-based equalisation converter for series connected battery cells

Hybrid electric vehicles (HEVs) and electric vehicles (EVs) have increased in popularity as they use rechargeable battery as energy source instead of petrol, furthermore, help to reduce the release of greenhouse gases. Generally, for applications such as EVs and HEVs, a battery pack may consist of hundreds individual cells batteries in order to meet voltage and energy capacity requirements. However, due to individual chemical characteristics and manufacturing tolerances each cell in a pack will have a slightly different capacity. The capacity of a cell also changes over time as the cell ages. During charging and discharging this leads to unbalanced voltages across the battery pack, and unchecked could result in over-charging/over-discharging for certain cells. This in turn can lead to cell and pack failure or even fire. Therefore, as part of the battery management system a process for voltage equalisation of cells throughout the stack is essential. This paper proposes a new cascaded multi-winding transformer based equalisation circuit topology which, compared to more traditional multi-winding transformer-based topologies is simpler in transformer construction and is therefore far easier to manufacture. The topology concept and operating principals are discussed followed by simulation results for the proposed system. In addition accelerated equalisation speed compared to existing solutions is demonstrated.