Sophisticated estimation of lithium ion battery performance allows for greatly improved efficiency

October 5th, 2012 by UCSD Jacobs School of Engineering

Professor Miroslav Krstic and UC President's Postdoctoral Fellow Scott Moura are developing sophisticated estimation algorithms that allow lithium-ion batteries to run more efficiently (click image to expand - image©Erik Jepsen/UC San Diego Publications)

Engineers at the University of California, San Diego, have developed sophisticated estimation algorithms that allow lithium-ion batteries to run more efficiently, potentially reducing their cost by 25 % and allowing the batteries to charge twice as fast as is currently possible. In one instance, electric batteries could be charged in just 15 minutes.

Professor Miroslav Krstic and UC President’s Postdoctoral Fellow Scott Moura in the Department of Mechanical and Aerospace Engineering at the Jacobs School of Engineering at UC San Diego have received a $460,000 share of a $9.6 million grant from ARPA-E, a research agency within the US Department of Energy to further develop the estimation algorithms and the technology they will drive.

Others receiving a portion of the grant are automotive products supplier Bosch and battery manufacturer Cobasys.

“This research is bringing the promise that, with advanced estimation algorithms that are based on mathematical models, batteries can be charged faster and can run more powerful electric motors,” said Professor Krstic.

“This technology is going into products that people will actually use,” said Dr Moura, the co-lead researcher on the project.

Professor Krstic and Dr Moura are taking a unique approach to making lithium-ion batteries more effective.

Instead of monitoring voltage and current, they have designed sophisticated algorithms that can estimate what is physically going on inside the lithium-ion battery.

“We have the unique ability to address the difficulties in estimating the battery’s state of charge heads-on, at the electrochemical level,” said Professor Krstic.

“Manufacturers usually rely on voltage and current to monitor the battery’s behaviour and health. But those are very crude measures,” Professor Krstic said.

Relying on these measures leads to over-designed, over-sized batteries that weigh and cost more. Lithium-ion batteries also take a long time to charge, compared with filling up petrol and diesel-powered vehicles. Toyota recently canceled mass production of its second all-electric car, the eQ, citing concerns over the viability of electric vehicle technology, including the amount of time vehicles take to charge.

Lithium-ion batteries are cylindrical and made of three sheets rolled together, very much like a jelly roll.

One layer is the anode, another layer acts as a separator, and yet another layer is the cathode.

When the battery is fully charged, the lithium ions are stored at the anode. The battery is designed so that the ions want to move from the anode to the cathode, powering the device it’s connected to in the process. To know whether the battery is functioning properly, it’s important to know where the ions are in the anode. But that’s very difficult to measure, even with sophisticated equipment. The ions are usually lodged deep inside irregularly-shaped particles within the anode.

A schematic of a lithium-ion battery (click image to expand - schematic courtesy of Dr Scott Moura, Jacob School of Engineering at UC San Diego)

Trying to estimate the particles’ charge by measuring only the voltage on the battery is similar to having the person that collects tickets at the entrance to a cinema try to estimate which of the seats the patrons are taking by watching the speed at which the queue at the entrance is moving, Dr Moura said. In this analogy, the ions are patrons making their way to seats within each row, which represent the particles.

The algorithms that Professor Krstic and Dr Moura have developed allow researchers to estimate where the particles are. So the movie theater can now be filled to capacity safety and efficiently.

The model can also estimate how the health of the battery evolves over time—the equivalent of which seats are breaking down in the cinema and need to be fixed or replaced.

The grant will allow researchers to refine the algorithms and to test them on actual batteries on testbeds developed by Bosch and Cobasys.

They will estimate the charge distribution within the battery. Then they will estimate its state of health. Finally researchers will devise a strategy to find optimal rates of charging and discharging batteries.

By testing their algorithms on electric vehicle batteries and comparing their performance to an electric battery run with existing technology, Professor Krstic, Dr Moura and colleagues plan to formulate a strategy to charge and use batteries to their maximum potential—safely.

“We monitor these crucial states directly,” said Dr Moura. “It allows us to operate right at the battery’s limits without damaging it.”

 

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