A series resonant converter for voltage equalization of series connected supercapacitor. ultracapacitor or lithium battery cells

Supercapacitors are energy storage devices with great potential in many industrial applications. Although they are not as energy dense as batteries, they have much higher power density. This unique feature enables them to be used to provide bursts of energy in electric vehicle applications. They can be connected in parallel with batteries to source and sink dynamic energy which increases the lifetime of the expensive lithium batteries. Typically, the maximum voltage of a single supercapacitor unit is low, e.g. 2.5 V. In many applications, manufacturers need much higher voltages, e.g. 400 V, so it is necessary to connect supercapacitors in series. A series connection of supercapacitor cells can result in voltage imbalance between cells, since individual supercapacitors have different tolerances. Voltage imbalance can lead to damage of the individual supercapacitors and even the failure of the total energy storage system. Cell voltage equalization is a strategy to maintain the reliability of the supercapacitor pack. A single series inductor-capacitor (LC) resonant tank is proposed in this thesis for the voltage equalization of series connected energy storage elements. The circuit can be used for lithium battery cells, or supercapacitors, but the focus of the work targets supercapacitors. The circuit includes two levels of source connected MOSFET switches for the connection between resonant tank converter and each supercapacitor cell. A controller arranges supercapacitor voltages in descending order and makes a decision based on whether switches associated with the corresponding supercapacitors should be operated. If the voltage difference is higher than the pre-determined allowable value, the microcontroller sends pulse width modulation signals to gate drivers which control the on-off time of the MOSFET switches. Simulation results are presented using PSIM and demonstrate that voltage differences among supercapacitors can be removed fast. Experimental results show that the prototype of the proposed circuit can reduce a voltage deviation of 527 mV down to 10 mV in 15 minutes. The circuit is small in size, achieves a relatively short voltage equalization time and has minimal loss, therefore largely alleviating the problems inherent to existing voltage equalization converters.Applied Science, Faculty ofEngineering, School of (Okanagan)Graduat