Battery Durability and Longevity
First of all, the lithium ion batteries in plug-in vehicles should not be confused with the earlier battery technology developed for laptops and consumer electronics devices. You know, those laptops that infamously caught on fire in the YouTube video. The "thermal runaway" issue was solved by replacing some of the battery materials (cobalt oxides, if you must know) with safer chemistries.
The generic term "lithium ion battery" is applied to a variety of battery designs (lithium oxide manganese spinel, lithium titanate, lithium iron phosphate) that the majority of us who struggled through high school chemistry don't need or want to understand. The shape of the batteries—which are actually packs of cells wired together—has also gotten a makeover. The cylinders used in electronics have given way to easier-to-configure "large format" rectangular (prismatic) batteries.
If lithium ion batteries were to be fully charged and depleted every day—known as deep cycles—they might only last a few years. To greatly extend the batteries' useful life, plug-in vehicles contain sophisticated software that keeps them from fully draining. Battery management systems are the essential brains of the system, enabling the batteries to be partially charged/discharged up to 300,000 times—known as "shallow cycles.”
Simply put, you can take as many trips as you'd like—minimally discharging and charging the batteries over and over again—and it won't perceptibly reduce their ability to store energy. The battery management system also prevents the battery from overcharging and monitors the flow of electricity down from the grid to similarly protect the batteries. The net result is that the batteries are expected to at least last the lifetime of the vehicle.
Oversized for Extra Measure
It’s true that lithium ion cells lose some of their ability to store energy over time, and are expected to have a useful life of five to seven years. But automakers compensate for this by oversizing the battery cells and the battery pack so that a loss in storage capacity shouldn't be noticeable for eight years or more. Once the battery begins to lose capacity, you can choose to either continuing driving your plug-in vehicle with diminished range, or trade in the batteries for a fresh set—although pricing for that is completely undetermined.
The battery shelf life—for anyone crazy enough to buy a plug-in vehicle and to let it sit in the garage—is around 20 years. The batteries don't require any maintenance, and fiddling with them is highly discouraged as they produce a considerable jolt of more than 3 volts.
Hot and Bothered
The Achilles' heel for lithium ion batteries is temperature. If the battery temperature exceeds 104 degrees Fahrenheit, they can suffer a permanent loss of up to 20 percent of storage capacity. Over time a plug-in owner in the moderate Northwest could see a longer useful life of his or her batteries than someone in Arizona who frequently parks the car outside. Frigid temperatures can also take a short term toll on performance, but won't impact capacity in the long run. For example, if the battery temperature drops below freezing, the chemical reaction that produces the power slows, providing less juice to march up a hill, but as the battery warms, the performance returns.
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