As the use of batteries to power vehicles becomes more common, a robust battery management system becomes necessary to monitor and maintain the batteries. Cold weather places a further burden on this system especially in small electric vehicles such as snowmobiles where it is desirable to use every bit of available energy from the battery cells. The problem with current battery management technology is that use of batteries in cold temperatures is often not addressed. The objective of this research was to develop an appropriate model of a lithium polymer cell with a cathode comprised of LiCoO₂ and develop an optimized charge/discharge method taking into account the effects of extreme cold weather and cell state of charge imbalance. A cell model was adapted and tuned that accurately captures the dynamics of a lithium polymer cell when discharged at temperatures below freezing. The model results were verified against cells discharged in an environmental chamber, which allowed accurate control of ambient temperature. Multiple scenarios were explored, looking at the effects of ambient temperature, cell initial temperature, internal heating, battery pack insulation, and how rapidly the cells were discharged. The results of the optimized battery management strategies showed improvements in the energy delivery capability of lithium polymer battery packs for small vehicles operating in extreme cold environments. In addition, this research extended the LiCoO₂ model down to -20 °C using validated data, showed that perceived cell capacity loss at low temperatures is primarily due to increased internal resistance, demonstrated that measured cell terminal voltage can rise under load at low temperatures, and showed that increasing the capacity of a battery pack has a better than linear gain in usable energy versus increased battery capacity. I.e., doubling battery pack capacity will more than double the useable range of the vehicle.
Thesis (M.S.) University of Alaska Fairbanks, 2018
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