Understanding the life of lithium-ion batteries

Scientists have answered a question that worries millions of owners and potential owners of electric and hybrid vehicles using lithium-ion batteries: how long before the battery pack dies, leaving a price-shock bill for a fresh pack or a car ready for the junk heap?

The answer, presented at the 245th National Meeting & Exposition of the American Chemical Society (ACS), may surprise skeptics. "The battery pack could be used during a quite reasonable period of time ranging from 5 to 20 years depending on many factors," said Mikael G Cugnet, PhD, who spoke on the topic. “That’s good news when you consider that some estimates put the average life expectancy of a new car at about eight years.”

Cugnet explained that the life span depends mainly on the battery’s temperature, state of charge and charge protocol. Battery performance begins to suffer as soon as the temperature climbs above 86° Fahrenheit. “The higher the temperature, the lower the battery service life,” he said. “A temperature above 86°F affects the battery pack performance instantly and even permanently if it lasts many months like in Middle East countries.”

Cugnet also recommended that electric vehicle (EV) owners pay attention to how much their battery is charged, another factor in a battery’s longevity. He reported that a fully charged battery is more vulnerable to losing power at temperatures above 86°F.

To test the limits of lithium-ion EV batteries, Cugnet’s team reconstructed the experience of a typical EV battery in the laboratory. Using data gleaned from a real five-mile trip in an EV, they put EV battery packs and cells through simulated lifetimes of driving with cycles of draining and recharging. The researchers considered a battery to be beyond its useful life span when it had lost 20% of its full power.

The question of longevity matters to EV owners and manufacturers alike. The cost of the lithium-ion batteries that power these vehicles remains high, and an EV can cost twice as much as a gas or diesel equivalent. Customers want to make sure they get their money’s worth, and manufacturers are eager to demonstrate that EVs are economical.

One obvious saving is the cost of fuel over a car’s lifetime, but EV makers are also pushing so-called ‘second life’ uses for batteries that could make them valuable even after they’ve lost too much power to be useful in cars. These applications could include backup power for computers and medical equipment, or electrical grid storage, which would go hand-in-hand with renewable power like wind or solar to keep electricity flowing even when environmental conditions aren’t right. Another option is recycling a battery’s components to make new batteries.