The question of battery longevity does not have a simple solution. Many factors affect cycle life of a battery pack, most notably climate, charging habits, driving style, and battery chemistry, and all the tests in the world cannot exactly replicate how each individual driver will use the battery over the life of the vehicle. How long will your vehicle’s battery hold on before being reassigned to a solar system on someone’s roof?
One of the most important variables in battery longevity is temperature. In general, temperatures above 86 degrees F place great stress on the battery and speed up capacity loss. Vehicles that have a liquid-cooled battery, like the Chevrolet Volt or Tesla Model S, are less vulnerable to high-temperature effects as long as they are not parked in the blazing sun.
However, the Nissan LEAF may be notably susceptible to rapid degradation in extremely hot climates. The results of an Idaho National Laboratory study released in March found that the test LEAFs lost 22-26% of their initial capacity after just 40,000 miles. This is most attributable to two factors: the vehicles were tested in Pheonix, where the hot climate accelerated capacity decline; and the vehicles were discharged to less than 5% capacity twice per day.
The second of those factors should not be overlooked. Charging habits, or depth of discharge, directly affect how long the battery will last. It is a well-established fact (though not widely known among the general public) that lower depth of discharge leads to longer cycle life. This is the reason conventional hybrid vehicle batteries can last for the life of the vehicle despite undergoing tens of thousands of charge and discharge cycles in routine driving over their lifetime.
An electric vehicle battery does not last as many cycles because it is discharged much more with each cycle. While a conventional hybrid battery will maintain a low depth of discharge, perhaps 30% of the battery’s usable capacity and last tens of thousands of cycles, a plug-in electric vehicle battery will allow a much higher depth of discharge window (up to 70-80% of usable capacity) in order to achieve more range. The trade-off is that the battery will have a much shorter cycle life, on the order of hundreds of cycles for a lithium-ion battery depending on the depth of discharge and the particular battery in question.
For this reason, an automaker trying to squeeze every last mile of range out of the battery is knowingly reducing the life of the battery pack in number of cycles. Fortunately, as an EV owner you can use this phenomenon to your advantage. If you avoid fully depleting your vehicle’s battery and charge up more frequently you will be reducing stress on the battery and effectively extending its cycle life.
Per Battery University, cutting the depth of discharge in half can very generally improve longevity from 300-500 cycles to 1,200-1,500 cycles. And don’t worry about partial discharge leading to so-called “memory effects” – they don’t apply to lithium-ion batteries.
Though tests can provide an indication of what affects battery life and to what degree, the best way to validate longevity is through real-world results, though none of the mainstream models have been on the road long enough to obtain legitimate results. Luckily, we do have results from a survey of Tesla Roadster owners and they are encouraging for those following the EV industry. As reported by Jim Motavalli, the Plug In America study found that 126 Roadster owners still retained 80-85% of their original battery capacity after 100,000 miles, driving an average of 16,000 miles annually.
Far exceeding Tesla’s initial expectations of 70% capacity after 50,000 miles, the Roadster results point to strong battery life at least for Tesla vehicles. The company currently offers a battery warranty on Model S for 8 years and 125,000 miles, so it clearly thinks the majority will last at least that long.
To reiterate, though, battery longevity can differ significantly from vehicle to vehicle even within the same model and year. Another question to pose is this: if your battery falls to 80% of its initial capacity and is deemed "dead," would it be worth it to invest in a replacement battery?
In theory, if you bought an EV today by the time the battery expired a replacement would be far less expensive than the original, and could even have more capacity. A new battery would give any electric vehicle new life, as their components are generally less vulnerable to wear and tear than those of conventional vehicles.
Replaceable or not, we won’t really know how long most EV batteries will last until near 2020 when they begin to decline in significant numbers. When that time comes the results will be interesting to compare.