Date of Award
Campus Access Thesis
Battery technologies are rapidly obviating the need for battery/ultracapacitor (UC) hybrid circuit topologies (active/passive) in electric vehicles (EV). In this paper, available models in MATLAB/Simulink are used to assess the need for lithium ion battery/ultracapacitor hybrid topologies. The battery parameters are varied to observe the effects of new (A123 LiFePO4) and old (Sony LiCoO2) battery technologies in these topologies. A sixty percent reduction in the peak power delivery from the ultracapacitor in the passive hybrid (no DC/DC Converter) using the newer technology battery model is observed. However, there is no observable difference in each battery's overall transient behavior within the active hybrid topology simulation. Despite the overall battery behavior parity (due to the DC/DC converter), Individual battery cells within the LiFePO4 battery model output higher currents in the active topology due to the smaller sized battery with fewer parallel cell strings. The newer technology battery aging data indicates that the short transient current that the ultracapacitor is intended to absorb are well within the comfortable operating range. The active topology reduces the overall current's variance by 10% over an entire load cycle when compared to the battery alone; however, the converter/ultracapacitor devices add weight, cost, and losses into the system. Battery power density, C-ratings, and capacity fade characteristics have improved to the point that coupling an ultracapacitor is no longer necessary. These improvements allow a designer to remove the weight, cost, and volume reserved for the ultracapacitor/converter from the system altogether or to use that to assemble a larger capacity battery for an EV range improvement.
Kowski, J.(2010). Advances in Lithium Ion Batteries Obviate Need for Ultracapacitors in Electric Vehicles. (Master's thesis). Retrieved from http://scholarcommons.sc.edu/etd/357