As energy storage system (ESS) technology advances, vehicle testing in both laboratory and on-road settings is needed to characterize the performance of state-of-the-art technology and also to identify areas for future improvement. Idaho National Laboratory, through its support of the U.S. Department of Energy’s Advanced Vehicle Testing Activity, is collaborating with ECOtality North America and Oak Ridge National Laboratory to conduct on-road testing of advanced ESSs for the Electric Drive Advanced Battery (EDAB) project.
Test setup and interim results to date are addressed in this article and will be the subject of a paper to be presented April 18 in Detroit as part of the SAE 2013 World Congress technical session called Advanced Battery Technologies (Part 4 of 4).
Battery performance is measured under both controlled and real-world conditions, and the project results will inform the research community and automakers on the state of the art of ESSs for plug-in hybrid electric vehicles (PHEVs) and battery electric vehicles (BEVs). The data and findings from the project to date have been made available to support modeling and energy-storage development efforts.
The continuing test project addresses a variety of advanced ESSs that are close to commercialization in a controlled environment that simulates usage within the intended application. The variability of on-road driving is useful in quantifying ESS capabilities, limitations, and performance fade over the life of the ESS.
To accommodate on-road testing of the ESSs of different size, mass, and intended applications, the EDAB testbed was constructed on a midsize pickup truck chassis. A vehicle of that size was selected because it is large enough to carry the ESS and related system components. The truck was converted by AVL California Technology Center to a series PHEV architecture, which enables vehicle operation consistent with any electrified vehicle type.
The conversion involved mating a UQM 145-kW motor/generator to the stock 5.3-L V8 engine to form an auxiliary power unit, removing the stock driveshaft, introducing a second UQM 145-kW motor/generator as the drive unit, and integrating a custom-built, single-speed gearbox and driveshaft assembly. Power electronics—including the motor controllers, dc/dc converters, onboard charger, and ESS cooling fans—were located in the bed of the truck, along with the ESS.
Sophisticated software algorithms were prepared and integrated into the testbed to emulate the physical characteristics and ESS demands of the intended application during on-road operation. Such emulation is vital for proper ESS operation since the testbed is larger and heavier than the typical vehicle type for which ESSs generally are designed (in this case, a small EV).
The testing is being conducted over a range of ambient temperatures and driving route types ranging from stop-and-go city driving to constant-speed highway driving. Battery laboratory cycling with standard test procedures has been conducted throughout all phases of testing to corroborate the on-road data and to accurately measure the ESS degradation.
The first ESS to be tested is EnerDel Inc.’s Type I EV Pack designed for a small EV sedan. It has a rated capacity of 70 A·h (at a C/3 rate). Its lithium-ion chemistry consists of a mixed-oxide cathode and amorphous hard carbon anode. The pack has 384 cells (96 in series, four strings in parallel), and each cell has a maximum voltage (at 100% state of charge [SOC]) of 4.1 V, a minimum voltage of 2.5 V (at 0% SOC), and a rated capacity of 17.5 A·h (at a C/3 rate). The pack has a maximum voltage of 393.6 V, a nominal voltage of 345.6 V, and a rated energy of 23 kW·h.
The ESS enclosure is sealed, meaning that there is no thermal management system (TMS); therefore, cooling or heating of the ESS can only be accomplished by heat conduction through the ESS enclosure assisted by forced air over the enclosure. The ESS uses CAN for communication with the vehicle controls systems.
End-of-test (EOT) criteria are 100,000 mi (161,000 km), three years of operation, or a 23% decrease in battery capacity—whichever occurs first. The researchers note that ESSs are typically oversized, in comparison to vehicle performance design specifications, by 30% at the beginning of life (130% of energy and power requirements) to ensure performance requirements are met at the end of life. Once the performance drops below the requirements, the ESS can be considered to be at “end of life.” The degradation from 130% to 100% in capacity is a 23% decrease.
The ESS is periodically removed from the vehicle for in-lab reference performance testing. Such testing showed a 6.25% decrease in static capacity to 59.21 A·h over the course of the first 115 days of testing, compared to 63.15 A·h at the start of the project on Feb. 2, 2012. The figure for percentage of rated capacity at the start was 90.2%; after 115 days it was 84.6%.
To date, the testbed has accumulated 5327 mi (8573 km) of on-road testing. A total of 9645 A·h throughput (into and out of the pack) has been measured through on-road driving and charging as well as in reference performance testing.
Additional packs using cells from different companies will be evaluated as the project moves forward. Test results and updates to the project are available at http://avt.inel.gov/energystoragetesting.shtml.