The Electric Cooling Battery Test
Later this month, two new electric cars—the GM Volt and the Nissan Leaf—will start appearing in eco-conscious driveways across the United States.
The Nissan Leaf promises 73 miles per electric charge, while the GM Volt gets 35 miles per charge, although it also has a backup gasoline engine for longer trips. But GM and Nissan are taking different approaches to ensuring that the batteries in these cars last and remain safe. The way these batteries perform over the next few years will suggest which approach is better, and could shape the design of future electric cars and plug-in hybrids, which all major automakers have promised. Some critics say that Nissan’s battery-pack design, which uses a relatively simple cooling system, could allow the batteries to overheat, decreasing the life of the battery and posing a safety concern.
Both GM and Nissan use lithium-ion batteries (a technology that’s long been used in laptops and mobile phones) as opposed to nickel-metal hydride batteries, which have proved reliable in gas-electric hybrids such as the Toyota Prius, but which are bulky and heavy.
In choosing lithium-ion over nickel-metal hydride, GM and Nissan are taking a risk because such batteries haven’t yet proved reliable in the demanding role of powering a car. Car batteries must endure temperature extremes, harsh jolts, and continuous vibrations from the road, and have to perform well for about a decade. In a few rare cases, lithium-ion batteries can overheat and catch fire, a problem that has required massive recalls of some laptop batteries. The batteries needed for electric vehicles must also store far more energy—so a fire caused by auto batteries could be particularly dangerous.
Another drawback of using lithium-ion batteries is that they quickly lose their ability to hold charge. After a couple of years of use, it’s not unusual for them to store half as much power as they did when they were new. Automakers want car batteries that will last for the life of a vehicle—about eight to 15 years.
To address these issues, GM and Nissan have made significant changes to the lithium-ion batteries they’re using. Instead of using lithium cobalt oxide —the material preferred in laptop batteries because of its high energy density—as the electrode, they’re using lithium manganese oxide, which stores a relatively large amount of energy, but is more stable, in part due to the arrangement of its atoms. In a manganese-oxide electrode, atoms form a three-dimensional structure that maintains its shape even as lithium ions move in and out of the electrode as the battery is charged and discharged. The less stable structure of conventional battery electrode materials can be damaged as lithium ions move in and out, which shortens the useful lifetime of a battery.
GM and Nissan have also switched from a cylindrical shape for the batteries (the battery cells inside typical laptop packs look like large AA batteries) to a flat rectangular shape that saves space and also allows heat to escape better. Overheating can damage the batteries, decreasing their ability to store charge, and, in some cases, can lead to a phenomenon called thermal runaway, when elevated temperatures result in chemical reactions that lead to yet more heat, eventually resulting in a fire.
But major differences emerge when you look at how GM and Nissan designed their battery packs—the collection of battery cells, electronics, and temperature controls that make up the complete battery. The biggest difference is how the companies choose to control the temperature of the packs. Nissan has opted for a simple design, using a fan to cool its batteries. It says that the flat shape of the battery cells makes additional cooling unnecessary. GM’s design is more complex. A liquid coolant carries fluid past the surface of each cell in the pack and to a small radiator outside of the pack.
Liquid cooling systems can carry heat away from battery cells more quickly than air cooling. Additionally, “liquid cooling is much more compact,” says Bill Wallace, GM’s director of global battery systems. “You can move much more heat, and move it much more uniformly.”
Wallace says liquid cooling was chosen to ensure that all of the cells in a pack are within two °C of one another. Along with preventing overcharging, “temperature control is the most important knob you can turn in terms of improving battery life,” he says.
Liquid cooling is the approach chosen by Tesla Motors, which makes an electric sports car using lithium-ion battery cells that were originally designed for other applications. The cooling method ensures that even if some cells overheat and catch fire, the rest wouldn’t. In a recent earnings call with investors, Elon Musk, the CEO of Tesla Motors, criticized the Nissan design, saying it could cause temperatures to be “all over the place,” which could degrade battery performance.
Because cold weather can limit the amount of charge a battery pack can store, and cause damage to the battery, GM’s pack also has a 1,800-watt resistive heater to keep the battery pack from getting too cold. Nissan recommends a cold weather package that includes a battery heater, but this doesn’t come as standard. And the option is not available for the first Leafs to come off of the assembly line, and it cannot be added to a car later. If the Leaf pack gets too cold, or too hot, it enters a limited power mode, which restricts acceleration and top speed.
The Volt is also engineered to hold a certain amount of charge in reserve when the car is new to help preserve battery life. As the battery ages, some of this reserve capacity will be released, which will help the car maintain its electric range over time. GM expects the capacity of the battery to fade between 10 and 30 percent during the life of the car (about eight to 10 years). Nissan has not said if it will use a similar approach, but it has said that the Leaf’s battery capacity will fade 30 percent in 10 years. It has also noted that exposure to hot temperatures could decrease battery capacity faster.
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