Battery Characterization and Optimization for use in Plug-in Hybrid Electric Vehicles: Hardware-in-the-loop duty cycle testing

Abstract

Plug-in hybrid electric vehicles (PHEV) with all-electric range (AER) combine battery driven electric motors with traditional internal combustion engines in order to reduce emissions emitted to the atmosphere, especially during short, repetitive driving cycles such as commuting to work. A PHEV utilizes grid energy to recharge the electrical energy storage device for use in the AER operation. This study focuses on battery systems as the electrical energy storage device and evaluates commercially available technologies for PHEV through scaled hardware-in-loop (HIL) testing. This project has three main goals: determine the state of technology for PHEV batteries through an extensive literature review, characterize commercially available batteries including simulated HIL response to a real-world PHEV simulation model, and finally, develop a tool to aid in choosing battery types for different vehicle styles (a battery decision matrix). The five different battery types tested are as follows: A123 Lithium Iron Phosphate (LiFePO4) Li-Ion, Genesis Pure Lead-Tin lead acid, generic absorbed glass mat (AGM) valve regulated lead acid (VRLA), SAFT Nickel-Metal Hydride (NiMH) and SAFT Nickel-Cadmium (NiCd). The batteries were characterized in terms of capacity and maximum power as well as tested on an individually scaled real-world duty cycle derived from a model developed by the University of Manitoba and the University of Winnipeg.

When comparing the results of the characterization testing with the literature review and manufacturers’ data it was found that there are discrepancies between the batteries tested and the manufacturers’ specifications for mass and capacity. Furthermore, the response to duty cycle testing shows that it is imperative that the internal resistance of the batteries and their conductors should be considered when designing a vehicle, although the literature suggest that this is not commonly done. The results from testing were incorporated into a simple decision matrix factoring in vehicle design constraints, battery performance and cost. Through the duty cycle testing, the dynamic resistance of each of the batteries was determined by measuring the voltage response of the battery to variations in current draw. This resistance figure is important to include in simulations as it effectively reduces available energy the battery can supply due to increasing current demands, as voltage drops in response to a load.