Other goals of the project are:

• To demonstrate, via long-term testing of sub-pack assemblies, reduced stress on the Li-ion batteries;

• To develop new state of charge (SOC) control strategies for ultracapacitor bank power blending;

• To identify component costs for net energy storage system hardware and opportunities to explore technologies that will reduce that cost (such as higher frequency DC-DC converters); and, importantly for the development of the PHEV market,

• To demonstrate that limiting peak powered delivered by the Li-ion battery, especially in cold-weather operation, will eliminate the need to oversize the battery pack, thus reducing overall cost.

The ultracapacitor bank actively coupled via the power electronics allows the use of an energy optimized battery by reducing peak loads and minimizing internal battery heating. There are four key elements in the actively coupled ultracapacitor/battery system for PHEVs:

• Energy-optimized Li-ion batteries with thicker electrodes. A thick electrode in Li-ion cells could save up to $285/kWh, Bohn said.

• Reduced-cost EDLCs, through capacitor construction that reduces labor cost (machine assembled with fewer parts). Results of an earlier simulation study of capacitor sizing for the system shows that a 75-100 Whr capacitor bank is appropriate for a Saturn VUE-sized PHEV.

• SOC controller with an algorithm tuned for maximum battery life, aggressively utilizing all available capacitor energy; and

• DC/DC converter, featuring an aggressive cost/mass reduction ($15-$25/kW), high frequency (200 kHz), SiC devices, and powdered magnetics.

The project, which began in mid-2007, has so far completed hardware proof of concept testing. The team currently is working on control tuning and integration, and initiating a long-term effects study. For the future, the researchers will use a lower-power density, higher energy-density battery, complete the implementation of the control software in the power electronics, and run the system with a 300V, 72 Whr ultracapacitor bank in a PHEV and hybrid vehicle.

It will also work with OEM and Tier 1 supplier to identify the production cost and size of a DC/DC converter that meets requirements for such as actively couple ultracapacitor system and energy optimized battery for a Chevy Volt-sized PHEV.

OEM involvement and interest. In terms of OEM interest and contributions, Bohn said that:

• Maxwell Technologies is investing resources in developing a new capacitor form factor, as well as bipolar li-capacitor technology (building an 18-cell evaluation pack). The deep drawn can with square-ish shape is easier to stack, and is machine-assembled with three 3 sonic welds.

• Gold Peak USA is supplying prototype LiMnO₂ polymer batteries.

• Magna Corp will use an ANL ultracapacitor/power converter on a prototype OEM EV targeted to meet the 2012 California ZEV Mandate. This unit is under test Mar-June 2009.

• Continental Automotive and US Hybrids are working on reduced cost, highly aggressive DC/DC converter designs.

• GM has verbally committed to apply this technology on the Chevy Volt energy storage system and to perform lab evaluations.

• Proctor and Gamble interested in developing processed to fabricate lower cost, higher energy density batteries using paper/web handling methods.