A Comprehensive Review of Battery and Supercapacitor Cells Voltage-Equalizer Circuits

Voltage unbalances of the series-connected battery and supercapacitor (SC) cells are mainly due to their differences in materials, manufacturing technology, internal specifications, temperature, initial charge, etc. This voltage difference can reduce the battery or SC pack capacity, stored energy, efficiency, and lifespan. Various methods have been proposed to overcome this issue to avoid its disadvantages. Generally, voltage-equalizer circuits are divided into active and passive categories. The passive methods consume high-voltage cells energy as heat, and they are widely used in nonsensitive applications, due to their low cost and volume, high reliability, and expandability. However, these equalizer circuits are not used in sensitive applications, due to their power losses, heat generation, and low efficiency. To overcome these problems, the active balancing methods have been proposed, which transfer energy from high-voltage cells to low-voltage cells. However, these methods have complex structures and control circuits. In this article, different types of batteries and SCs and their optimum combinations are investigated, first. Then, the active and passive equalizer circuits are examined and compared, in detail. In addition, their different control methods have been studied to choose the most suitable circuit and its control method for a desired application.