Of the 42,629,318 battery cells used in hybrid vehicles produced by Ford Motor Co. since 2004, only five cells have failed.
"The odds are approximately 8.5 million to 1—about the same as someone being struck twice by lighting—that a customer would experience a battery cell issue," Chuck Gray, Ford's Chief Engineer of Global Core Engineering for Hybrid and Electric Vehicles, said at a July media briefing in Dearborn, MI.
With a near-perfect record for the nickel metal-hydride battery cells, Bob Taenaka, Hybrid Battery Technical Leader at Ford, credits Sanyo Electric Co., the cell supplier for the Escape Hybrid SUV and the Fusion Hybrid sedan, for providing a quality product.
Taenaka told AEI: "Ford identified process improvements after determining the five cell failures were due to a manufacturing process issue, not a degradation issue. The cells actually developed a high self-discharge rate, becoming out of balance with the rest of the battery."
The fact that nearly 43 million battery cells have survived real-world usage in 190,000 hybrid vehicles underscores the strong correlation predicted by virtual simulations and physical tests.
"I would have expected a slightly larger level of imbalance, but the cells were all perfectly well matched. And that's what we strive for in design and manufacturing—that every cell behave identically," said Taenaka, a scientist who previously helped engineer the batteries for the probe on the unmanned Galileo spacecraft's mission to planet Jupiter.
At the media briefing, journalists were shown a battery pack from a Ford Escape Hybrid that was part of a hybrid-electric fleet of San Francisco taxis that has accumulated 80 million mi (130 million km).
Said Ted Miller, Ford's Senior Manager of Energy Storage, Strategy and Research: "We had confidence in the computer models, but getting the field data back and being able to evaluate the battery cells after actual usage just reinforces what we do."
Various hardware and software tools provide engineers at Ford with a framework for determining the durability and longevity of batteries.
"The electrical testing and calendar life storage of cells—combined with physical analysis of cell components afterward—give us the ability to develop cell life models, and these models form the basis for understanding how the cells will behave in different climates and different usage conditions," explained Taenaka.
During the product development process, Ford engineers integrate a specific cell life model into a battery system model, which includes all the functions that will control the battery. All information is then input into a full vehicle system model so engineers can determine how the battery will behave in real-world conditions. The simulated conditions for a hybrid-electric vehicle include ambient temperatures of different locations around the country using information obtained from the National Weather Service as well as Ford's proprietary drive cycles that replicate San Francisco, Denver, and other U.S. roadways.
"We run 10-year life-cycle tests on any technology. But the details—meaning the states of charge, the currents, and the temperatures—that the cells operate at can be very different, depending on the battery technology and the chemistry. That's why the testing is all tailored to the specific characteristics of the battery technology that's being used," said Taenaka.
The same testing scenario used for NiMH batteries—including the evaluations of significant numbers of aged batteries taken from actual road-driven Escape Hybrid vehicles—applies to lithium-ion batteries.
"We now know that our computer simulations gave us accurate predictions of NiMH battery life in our production hybrid vehicles. By following the same disciplined process of extensive cell testing, physical analyses, cell/battery/vehicle system computer simulation, and battery life testing, we are highly confident that the lithium-ion batteries in Ford's future electrified vehicles will also age as expected," said Taenaka, declining to give specifics about the aging process of Li-ion batteries.
Ford plans to launch five different electrified vehicles in North America by 2012 and in Europe by 2013, including the C-MAX Hybrid that will use a second-generation Li-ion battery pack.