SIEMENS

Sparks can sometimes fly in the Siemens Energy Sector labs in Erlangen and Fürth, both of which are located in southern Germany. When several hundred amps flow through testing systems consisting of large inverters, capacitors, and transformers, technicians have to be extremely careful — in order to protect not only themselves but also the components they're testing.

“We develop stationary direct-current (DC) chargers with an output of between 12 and 100 kW,” says Heike Barlag, who manages the tests. “The devices are designed for traction batteries in electric vehicles.” Barlag's goal is to develop charging units for use at highway rest stops or parking lots that all drivers will be able to use safely and easily as a type of electric-vehicle filling station. “Here, we're using components that Siemens normally manufactures for industrial applications and are adapting them to our requirements,” she explains.

But why DC? Wouldn't a conventional alternating current (AC) socket like those found in households suffice? “No, because charging times would be much too long,” Barlag says, and she offers a simple calculation to demonstrate her point: A normal 230V, 16A European household socket supplies an output of around 3.7 kW. That's enough to charge a 30kWh traction battery in around eight hours — in other words, overnight. Charged in this manner, an average electric car could travel up to 200 km, which is enough for city use but not for longer trips. Obviously, drivers will want to recharge their batteries in just a few minutes.

Automakers around the world are therefore trying to increase the charging power of chargers in electric vehicles — for example, through the use of three-phase, 400-V supply terminals with currents of up to 63 A (44 kW). This would enable a 30 kWh battery to be charged in less than an hour. “Charging with AC from a plug is basically feasible for everyday use,” says Sven Holthusen, a Siemens product manager specializing in electric mobility infrastructures. Automakers have announced that they will begin introducing electric vehicles in large volumes by 2014. When they do so, such technologies will usher in a new age of electric mobility.

Energized Tanks. AC technology also has drawbacks that should not be underestimated. For one thing, the inverters it requires become larger and heavier as output increases, which in turn drives up energy consumption during operation and thus overall operating costs. That's why Siemens is pursuing a different goal, namely that of having vehicles “fill up” directly with DC rather than converting AC inside the vehicle to the DC needed for batteries. Here, the heavy equipment required for AC-DC conversion would be housed in the charging station itself. Holthusen explains the benefits of this approach: “It enables us to achieve very high charging powers of several hundred kilowatts, which means an electric car could be recharged in only a few minutes — just like a vehicle with a combustion engine.”

This puts a great strain on the battery, however, because the higher the charging power, the faster the electrons and ions in the battery move around. Cell power losses then increase, and the cells begin to heat up. Rising temperatures then disrupt the chemical processes in the battery, which is why today's standard batteries with an energy capacity of 30 kWh, for example, are only charged at a rate of 1/3 C per hour. In this case, that means a power of 10 kW, which increases the charging time to three hours.

“That's why, together with our automotive partners, we are trying to determine how we can charge batteries more rapidly in the future,” says Holthusen. “We need batteries that are designed for higher temperatures, exhibit lower power losses or have better cooling properties.” Such developments will take some time to achieve, according to Holthusen. Until the breakthrough comes, Siemens researchers are looking to further optimize the charging process — for example, by participating in a Danish research project known as EDISON (see Pictures of the Future, Spring 2010, Electric Vehicles). The acronym stands for “Electric vehicles in a Distributed and Integrated market using Sustainable energy and Open Networks.”

Other EDISON project partners include the Technical University of Denmark (DTU) and its Risø research center, as well as Denmark's Dong Energy and Østkraft power utilities, the Eurisco research and development company, and IBM. The goal of the partnership is to determine how frequently unused wind energy in Denmark can be temporarily stored in electric car batteries and later returned to the grid. Siemens is responsible here for fast charging technologies, among other things.

Battery Management. The experts who work for Barlag and Holthusen enjoy ideal test conditions at Risø. “We can test all components individually in a closed power grid,” Barlag explains. Siemens developers have already integrated the first 10 kW charger into a testing device, along with lithium-ion batteries and a battery management system. Communication between the battery and charger is controlled by a Risø lab computer program. Plans call for trials to continue with a 90 kW charger in the fall of 2010. Such a unit would act as a 3 C charging device for a 30 kWh battery, which could then be recharged in 20 minutes.

“We want to find out which charging algorithms can be used to optimally charge batteries in various states,” says Barlag. That's because the speed at which a battery can be charged depends on both the charging power and the state of the battery, whereby a completely discharged battery can generally accommodate a higher power than one that is partially charged.

Researchers are therefore testing the most diverse types of charging techniques, one of which is known as pulse charging. Here, a battery is charged at a high current for a short time, after which the heated cells are cooled down and the charging process begins anew. “Our rapid charging tests in Risø will show us if we can save time and transfer a higher output with pulse charging, or whether a continuous charging curve would be better,” says Barlag. “We expect to have initial results by the end of this year, and we're hoping to achieve a charging rate of two to three C.“