Just like every other car component (and everything else in life), the batteries used in hybrid powertrains degrade and eventually wear out. But a buyer paying a premium for a hybrid wants the best possible information about just how long those components will last. To try and predict how these batteries will fare, Ford developed the Key Life Test, which is actually a batch of tests designed to put 150,000 miles and a decade?s worth of wear and tear on its lithium-ion batteries in just 10 months.
How do you pack a decade of battery abuse into less than a year? Ford engineers first need to create a model of how its different hybrids use their batteries?specifically, how they charge and discharge their batteries under normal driving conditions, known as the cycle life. "The battery use is very much dependent on the vehicle, the hybrid architecture, the control systems, and calibration that you put in place in these cars," says Anand Sankaran, executive technical leader for Ford?s Energy Storage division.
That gives Ford an "average use" charging cycle model. However, not every hybrid lives an average life. To forecast the effects of varying climate on battery lifespan, Ford gathers climate data from three test fleets out in Phoenix, San Francisco, and New York to see what temperatures the batteries must operate in and endure. "In Phoenix, the ambient temperature has a big role to play," Sankaran says.
Kevin Layden, director of Ford?s electrification programs and engineering, says extreme temperatures?which can hit at least 140 F in Phoenix?have a compounding effect on battery life. "If you have a high ambient temperature, and you do a very fast discharge, the battery is going to have a high heat generation as well as a high ambient temperature, and you have to combat that."
By combining the two data sets?cycle life and temperature range?Ford can stress its test batteries in a lab setting by constantly charging and discharging them while exposing them to the temperatures they?d experience inside the vehicle. After 10 months of this torture testing, the company says, the batteries endure as much punishment as they would in real-world Ford cars (which get to rest frequently)over 150,000 miles of driving. The company says it also compares its lab-test batteries to those in Escape and Fusion hybrids from various fleet and taxi drivers to make sure the estimations are realistic.
According to Ford guidelines, the batteries must show minimal signs of degradation after the 150,000-mile, 10-year test in order to pass. And the rigorous testing seems to be working. "We have gotten packs back from fleets with in some cases 200,000 to 260,000 miles, and we?re seeing 10 to 15 percent degradation or less," says Sankaran. So far, he says, just 6 of about 50 million batteries have failed. "The wear mechanisms are something we?re not that concerned about in real-world use."
While they are an important step to ensure battery quality, experiments like Key Life Test are just the last stop in battery development. By the time they get this far, batteries have been in development for six years and the automaker works with the battery maker to find just the right chemistry. "Early on, it?s a pure science research project to make sure we make the right selection," Layden says.
And prolonging a battery?s life doesn?t end there. "It?s not just a matter of putting a D-cell in there and taking what you get," Layden says. Within the vehicle itself, engineers tailor the car?s battery management system to ensure the battery stays within its window of efficiency?not letting the charge drop too low or letting the temperature get too high, either of which could accelerate the battery?s degradation. "It really does show you the complexity of the system design for the battery," Layden says. "If you get the chemistry right, if you get the characteristics of the battery correct, you get to reduce the battery?s size, which affords a more efficient design."
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