When Ford Motor Co. announced in 2012 that its new hybrids would use lithium-ion batteries instead of nickel-metal hydride, many experts raised an eyebrow.
Lithium-ion, after all, had a reputation for high cost and unknown durability, largely because the technology was still comparatively new. In contrast, approximately 95 percent of full and mild hybrids up to that time had used nickel-metal hydride.
But Ford engineers now say their decision to use lithium-ion was based on accelerated lab tests showing lithium-ion would actually be more durable than nickel-metal hydride over a long lifetime. The tests, combined with mountains of field performance data on nickel-metal hydride, convinced them that they could predict the eight- or 10-year future of a chemistry that didn't even have five years worth of reliable field data.
"We are really confident that our Key Life Tests are mimicking the duty cycle of some of our most stringent and abusive customers," Kevin Layden, Ford's director of electrification programs and engineering, told Design News. "Given that, we feel lithium-ion will be better than nickel-metal hydride. We expect it to be absolutely stellar."
Ford's confidence in lithium-ion is based on so-called Key Life Tests. The tests predict that the working capacity (y-axis) of lithium-ion batteries (green line) will be greater over a high-mileage lifetime (x-axis) than that of nickel-metal hydride (yellow line). Past field data for nickel-metal hydride (blue dots) has shown that the testing results are conservative -- that is, batteries generally do better in the field than they do on tests.
(Source: Ford Motor Co.)
Ford's confidence in the technology is real. The giant automaker had previously used nickel-metal hydride on the Ford Escape hybrid, Fusion hybrid, and even on the old Ford Ranger EV, which it produced in limited quantities in the late 1990s. In contrast, its second generation Fusion hybrid now uses lithium-ion. So does the C-Max hybrid, the Fusion Energi plug-in hybrid, the C-Max Energi plug-in hybrid, and the Focus Electric.
The key to Ford's testing effort is a lab that was part of a $135 million company investment in engineering and electric vehicle battery testing. Using a so-called Key Life Test at the battery test lab, Ford engineers believe they are able to accurately forecast the 10-year, 150,000-mile life performance of lithium-ion. To do so, they cycle batteries at the cell-level, pack-level, and vehicle-level. They cycle them in hot and cold chambers and run them on powertrain dynamometers. Using 10 months of testing, they believe they can understand how the batteries will behave at a wide range of temperatures, under various acceleration conditions, in multiple locations within the vehicle.
"Somehow, we have to take out the unused time when the customer's car is in a garage or parking lot," Anand Sankaran, Ford's chief engineer for energy storage and high-voltage systems, told us. "And then in the test cycle, we have to accelerate all the failure modes."
From prior experience with nickel-metal hydride, Ford engineers knew that their accelerated test results were conservative -- that is, their batteries actually performed better in the field than they did in Key Life Tests. Thanks to the availability of mountains of field data on nickel-metal hydride batteries with hundreds of thousands of miles, they had developed a good feel for the relationship between test and field performance. So when lithium's predicted behavior improved on that of nickel-metal hydride, the engineering team reasoned that it was time to make the switch. "At some point, you get confident and you realize that what you're doing correlates to what the customer is doing," Layden said.
Ford began rolling out several of its lithium-ion-equipped hybrids late in 2012. The company's engineers say the new breed of batteries will offer an improvement over nickel-metal hydride in a number of ways, including size, weight, fuel efficiency, and life performance.
"We know that customers will have questions about these batteries," Sankaran said. " 'Will I have to replace my battery after eight years?' 'How will lithium-ion be different than nickel-metal hydride?' Now we can provide a much more accurate picture of how it will perform."