Aspects of Thermal Management of Lithium Batteries in PHEVs Using Supercapacitors

In this paper, the use of supercapacitors in the electric driveline of plug-in hybrid vehicles (PHEVs) is analyzed from the design, performance, and economic points-of-view. The supercapacitors are envisioned to be part of the motor and electronics package and thus the same electric drive package can be combined with batteries of different energy storage capacity (kWh) as needed to meet the specific all-electric range the vehicle. In all cases the PHEVs in the all-electric mode had the performance of an EV having attractive acceleration characteristics. A 120kW electric motor is used in all the PHEVs studied. The use of the supercapacitors to load-level the storage battery permits the use of an energy battery rather than a power battery in PHEVs. Energy batteries have higher energy density, longer cycle life, and lower cost than power batteries of the same energy storage capacity (kWh). The weight, volume, and cost of the supercapacitors plus the energy battery are close to that of a power battery for all-electric ranges of 20 miles and are less than that of the power battery for longer all-electric ranges. Simulations of PHEVs indicate that even using the supercapacitors the energy consumption (Wh/mi) of the PHEVs is slightly lower using the power battery than with the energy battery. However, the use of the supercapacitors improves the system efficiencies with the energy battery and most importantly reduces by about a factor of two the peak and average current experienced by the energy battery. In addition, the dynamic character of the battery current/power with the supercapacitors is considerably smoother than with the energy battery alone. Detailed simulation results are presented in the paper to show these effects quantitatively for both the FUDS and US06 driving cycles and for vehicles with all-electric ranges up to 45 miles.