Dynamic equalization techniques for series battery stacks

Secondary battery life can be dramatically improved if the charging process can be regulated to prevent gassing and overcharging. This can be achieved by controlling the battery charging rate especially near the end of the charge cycle. Different charging algorithms have been devised to achieve this goal for individual battery modules. However, extending these algorithms to series connected battery stacks is not as straightforward. Normal differences in cell chemistries and temperature gradients along a battery stack can lead to large nonuniformities in cell charge levels. Maintenance of cells at an equalized charge level is critical for enhancing battery life. Different systems and schemes have been proposed to attain this goal. This paper presents, discusses and evaluates the different techniques proposed to achieve individual cell equalization for series battery stacks.