Evaluation of the New Sensorless Approach in Energy Storage Charge Balancing

High-voltage energy storage systems, which employ large series strings of battery cells or ultracapacitors, can be found in many applications including the utility industry, power backup, and electric and hybrid vehicles. As the energy bank is charged and discharged as a single unit, differences in cell capacities, individual temperature, and resistances can cause capacity imbalances in the form of voltage variations, unbalanced voltages can cause overcharging or over-discharging damage, decreasing the total storage capacity and lifetime of the energy storage devices. While balancing the voltages of the batteries will prevent any problems stemming from capacity mismatch, there are many ways to instantiate the solution. Most of the existing charge balancing techniques need the use of a voltage sensing system to determine which units are nearing the outer limits of their operating range. This paper introduces a new flyback-based converter to transfer energy between storage units. By placing a diode in series with the input of the converter, the need for a voltage sensing network is removed at the expense of a negligible drop in efficiency and a change in the operating characteristics near the equalized voltage state. Comprehensive analytical study is presented. Simulation results are used to verify expected operational characteristics.