Characterization of asymmetric ultracapacitors as hybrid pulse power devices for efficient energy storage and power delivery applications

The cost of Ni-MH and Li-ion battery packs is cut considerably for applications in HEV and pHEV onboard energy storage. The operational lifetime of these battery packs is still a cost concern for end users and limits long distance applications. The battery storage devices currently dominate onboard energy storage fields via slow electrochemical reactions. The battery pack experiences an extra burden when absorbing bursts of energy or releasing the required energy to facilitate quick acceleration. The dynamic operation under thermal and mechanical stresses especially during severe weather conditions has adverse effects on the battery operating life due to chemical compounds and electrode materials degradation. To overcome these limitations, an ultracapacitor can quickly polarize electrolyte solutions and accumulate energy via rapid electrostatic charges at the electrode-electrolyte interface. The ultracapacitor combines advantages including long cycle life and high power density of a conventional capacitor with enhanced energy storage capability due to high electrode surface area and low internal resistance. A hybrid ultracapacitor can be designed by using both a double-layer capacitor electrode and a battery electrode. This hybrid structure increases energy conservation and capacitance. It is also recognized as an asymmetric ultracapacitor. A power module consisting of 10 asymmetric ultracapacitors with 300 F at an operating window from 4.0 V to 14.5 V was tested for studying of these unique features. These 10 electrochemical capacitors were connected in series and performed electrical characterizations including Ragone plots, self-discharge, features at high current loads, working voltage, and related energy storage performance. The ultracapacitor module was then operated together with a Pb-acid battery as a hybrid power device. Performance of the hybrid power system was characterized by using an adjusted cold cranking and a modified hybrid pulse power profile. The irreplaceable structures on energy storage and power delivery serve as a potential to further reduce the cyclic burden and thermal stress in advanced battery packs for onboard energy storage applications.