Research drives battery health
Imagine waiting in a sea of cars at a traffic light. There are no idling, chugging engines-and best of all, no fumes-because each car's fuel-powered engine has shut itself off and a battery-powered electric motor awaits the command to start it again.
Imagine driving 700 miles on a single standard-size tank of gas.
These technologies aren't just around the corner-they're practically in the driveway. The vehicles, called hybrid-electric vehicles (HEVs), are just one more mile marker on the highway to more fuel-efficient, environmentally friendly automobiles. They still provide all the standard creature comforts and safety of traditional cars, but combine two energy sources: a battery-powered electric motor and an auxiliary source, such as a small internal-combustion engine.
Although Toyota and Honda will release their first versions of these cars in 2000, a new UW-Madison method of monitoring and controlling HEV battery health will be another contribution toward making HEVs a success.
"The ability to avoid the inefficient modes of the battery and to predict its health will increase the reliability and lifetime of the battery," says Herman Wiegman, an electrical and computer engineering PhD student. He developed the system with Mechanical and Electrical and Computer Engineering Professor Robert D. Lorenz and others.
Wiegman says people perceive short HEV battery life as a problem with the vehicles. Batteries used in applications like HEVs have short life spans because of high current and thermal stresses. It's a problem that's not new, says Lorenz. "These techniques will make HEVs a more feasible transportation option by lowering the risks associated with highly stressed batteries in the vehicle environment," says Wiegman.
In other words, drivers will get more mileage and a longer lifetime-perhaps as many as five years-out of their HEV battery.
Wiegman might say a "cow" inspired his research. Although its only bovine features are its black-and-white faux-hide seat covers, the cow in question is UW-Madison's lightweight, fuel-efficient, aluminum-body HEV, christened the Aluminum Cow. Wiegman is a five-year member of the student team that built it and its predecessors.
For the national FutureCar Challenge competition, UW-Madison engineering students modify a popular production-line vehicle to drastically increase its fuel economy, yet retain all the comfort and safety features consumers expect. The competition's goals, as well as those of a government-industry initiative called Partnership for a New Generation of Vehicles, are to increase fuel efficiency, cut the United States' dependence on imported oil and improve the country's competitiveness in the global economy by developing and promoting super-fuel-efficient full-size passenger vehicles.
UW-Madison's HEV is a charge-sustaining parallel-assisted model, the type most popular with automakers. Both the engine and the battery-powered electric motor drive the vehicle, either independently or together when the vehicle needs additional power. The high-voltage HEV battery, unlike the 12-volt battery that powers a traditional car's starting, lighting and ignition (SLI) systems, has a much bigger workload. It not only controls SLI functions, but also delivers energy to and recuperates energy from the car's drive train, essentially recharging itself.
Wiegman's system is unique in that it monitors and regulates the battery while the vehicle is operating. "The traditional approach is to test batteries in a laboratory over a range of operating conditions and then determine the control strategy and hope they live up to the promised performance," he says. "In many applications, very few diagnostics or control adjustments are applied to the battery during normal operation."
In Wiegman's system, a battery control unit takes readings of the voltage, current and temperature about 10 times per second, then analyzes the readings and adjusts the HEV control strategy on the spot. "It is like having a monitor on a marathon runner, which can give accurate predictions of how much reserve the runner has for sprinting or hill climbing," he explains. "These capabilities are dynamically changing as the runner progresses through the race. The same is true of batteries in hybrid electric vehicles."
Because it continually tries to improve battery performance, the system also makes the HEV's performance more consistent. And its possibilities aren't limited just to HEVs, says Lorenz. "This type of system would fit well in recreational-vehicle applications where a generator is used for power but cannot provide peak power for air-conditioning compressor start-up or other peak power problems," he says.
Companies that make devices for the utility industry can also benefit from this technology. "It is useful for solar photovoltaic power systems, which cannot source peak power easily, so battery storage must be optimized," says Lorenz. "It can also be used in power line back-up systems, which are used to reduce flicker problems due to peak power loads."
The system can improve something as everyday as a home computer's battery back-up, too. Wiegman wants it that way. "I'd rather develop generic techniques for the good of the industry," he says.