Simple -- And Cheap -- Process Can Double Life Of Rechargeable Batteries

April 03, 1997

CHAMPAIGN, Ill. - A simple, inexpensive process developed at the University of Illinois could double the life of rechargeable batteries used in laptop computers, portable power tools and electric vehicles.

The new process for battery-charge equalization was developed by a research team headed by Philip Krein, professor of electrical and computer engineering. The researchers have applied for a patent.

"Rechargeable batteries generally consist of a number of individual battery cells strung together in a series," Krein said. "No two cells are exactly alike, and they rarely have the same charge or expend it at the same rate. Ultimately, overall performance of the string is limited by its weakest cell."

At present, battery manufacturers address this problem by recommending that users forcibly overcharge the string to bring the weakest cells up to normal operating levels, Krein said. "After repeated cycles, however, this overcharging damages cells, degrades performance and shortens the string's useful life. We found a way to efficiently equalize the charge in these battery strings without overcharging individual cells."

Krein's technique uses a group of capacitors and transistorized switching networks to shift the charge among adjacent battery cells, similar to the way in which water fills an ice cube tray. As the capacitors are switched back and forth repeatedly, the total electrical charge is spread uniformly through the cells. The cascading effect is independent of string length, and can occur continuously during battery operation.

"By extracting energy from higher-charged cells and delivering it to cells with a lower charge, we can bring the battery-cell voltages in a string to equal values," Krein said. "The cells will remain balanced as long as the switched-capacitor system is operating, whether the battery is being charged or discharged. We expect this to significantly extend the life of the weaker cells."

Because the battery equalization technique is not dependent upon the normal charging process, the cells in partially charged batteries also can be brought into balance.

"That's an important point," Krein said. "In some applications, time constraints prohibit users from fully recharging the string. This process still would ensure optimum battery performance."

The circuitry required for the battery-charge equalizer is both inexpensive and simple to build, Krein said. "We're expecting costs on the order of a couple of dollars per large battery."

The circuitry also lends itself to miniaturization. While current prototypes sit as modules external to the cells, Krein envisions someday packaging the components within the battery itself.

University of Illinois at Urbana-Champaign

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