DC fast charging not as damaging to EV batteries as expected

Mon, Mar 17 2014

As convenient as DC fast charging is, there have been lots of warnings that repeated dumping of so many electrons into an electric vehicle's battery pack in such a short time would reduce the battery's life. While everyone agrees that DC fast charging does have some effect on battery life, it may not be as bad as previously expected.

Over on SimanaitisSays, Dennis Simanaitis, writes about a recent presentation by Matt Shirk of the Idaho National Laboratory (INL) called DC Fast, Wireless, And Conductive Charging Evaluation Projects (PDF) that describes an ongoing test of four 2012 Nissan Leaf EVs that are being charged in two pairs of two. One pair only recharges from 50-kW DC fast chargers, which the other two sip from 3.3-kW Level 2 chargers exclusively. Otherwise, the cars are operated pretty much the same: climate is automatically set to 72 degrees, are driven on public roads around Phoenix, AZ and have the same set of dedicated drivers is rotated through the four cars.

"Degradation depends more on the miles traveled than on the nature of recharging."

What's most interesting are the charts on page seven of Shirk's presentation (click the image above to enlarge), which show the energy capacity of each of the four vehicles. When they were new, the four batteries were each tested to measure their energy capacity and given a 0 capacity loss baseline. They were then tested at 10,000, 20,000, 30,000 and 40,000 miles, and at each point, the DC-only EVs had roughly the same amount of battery loss as the Level 2 test subjects. The DC cars did lose a bit more at each test, but only around a 25-percent overall loss after 40k, compared to 23 percent for the Level 2 cars. Simanaitis' takeaway is that, "INL data suggest that the amount of degradation depends more on the miles traveled than on the nature of recharging." The tests are part of the INLs' Advanced Vehicle Testing Activity work and a final report is forthcoming.

These initial numbers from IPL do mesh with other research into DC fast charging, though. Mitsubishi said daily fast charging wouldn't really hurt the battery in the i-MiEV and MIT tests of a Fisker Karma battery showed just 10-percent loss over 1,500 rapid charge-discharge cycles.

By Sebastian Blanco