One of the most important questions to the electric vehicle industry does not have a clear-cut answer. However, certain factors such as temperature and charging habits are known to have a great effect and the Tesla Roadster provides intriguing real world data.
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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.
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Nissan offers a capacity loss
Nissan offers a capacity loss rider to their battery warranty. 30% capacity loss in 5 years/60,000 miles. So Nissan seems to feel that their battery is still usable at 70% original capacity. I'm not aware of any other manufacturer offering "Capacity loss" coverage to their battery warranty. No other EV manufacturer seems to Need a capacity loss rider to their batteries. The warranty that is offered by almost everybody usually cover "Manufactured defects" and capacity loss is typically not covered.
The Roadster unfortunately is
The Roadster unfortunately is not a good example of EV battery life. The reason is the Roadster uses a completely different battery chemistry(LCO) then any other on the market. LCO out of all the chemistry options has the lowest battery life. So I can't blame them for being surprised. (Tesla now uses NCA chemistry which is one of the highest battery life). But what the Roadster does show is how important water cooling is. Since the Leaf technically has a better battery chemistry but gets overshadowed by the Roadster despite Roadster having a worse chemistry(in terms of battery life).
Though I will point out using hybrids in the mix is a poor example. Hybrids use completely different battery technology. While Plugins use Lithium Ion batteries, Hybrids use Nichel Metal Hydride batteries. Completely different and works different. So habits of one does not reflect the other.
The article says the Roadster
The article says the Roadster retains 80-85% after 100K miles. There really isn't much scope for improvement on that so there doesn't seem to be much reason top associate LCO with low cycle life. Of course it would be great if it turned out NCA performed even better but like I said, not much scope for that.
LCO is the worst chemistry
LCO is the worst chemistry out of all of them when considering battery life, as mentioned even Tesla was surprised it lasted that long. LCO is the same chemistry that goes into your laptops and cellphones. The Tesla Roadster's long lasting is due to good battery management, water cooling, proper cycling and a large battery.
The chemistry used by others are better than LCO in terms of lifespan. The chemistry with the longest lifespan is NCA and LTO. It has nothing to do with "reason" to top LCO, Tesla went with NCA because it is more energy dense than LCO. It just happens to be much longer lasting as well, hence how Tesla can offer an unlimited mile 8 year warranty.
In the case of LTO, it is also a long lasting chemistry though it is not as energy dense as LCO and NCA. The biggest advantage of LTO is that LTO batteries can be recharged at very fast speeds. Proterra Buses use LTO to recharge the battery from empty to full in 10 minutes.
LTO and NCA should technically allow cars to reach 500,000 miles.
Guess ~2000 cycles would add
Guess ~2000 cycles would add up to 500K miles for Model S/85 which would be tremendous and it's fascinating that mostly aluminum and low wearing powertrain Model S should actually have the longevity to actually be able to use that sort of cycle life. Too bad that at some point calendar life will catch up with the battery or the car would last for decades without major expenses.
"The LEAF has the best
"The LEAF has the best battery chemistry". Well than may be your opinion but not mine. The vast majority of the EV manufactures use NMC ( nickel, manganese cobalt) chemistry. Nissan originally used a straight Manganese chemistry which has shown to have poor durability. Nissan tweaked their chemistry in 2012 and added a tiny fraction of Nickel and cobalt to the mix to improve durability. They soon will be introducing their own version of a NMC chemistry later this year. The battery that Nissan put in the 2010, 2011 and some 2012 car is actually the worst on the market. This is why Nissan had to add a "Capacity loss" rider to their battery warranties. Car good battery bad.
You misread and misquoted
You misread and misquoted what I said. I said the Leaf has a BETTER chemistry than roadster, NOT best. The early Leaf models use the 2nd worst chemistry. Right now the best Chemistry is in the Tesla Model S.(NCA).
Though interestingly enough, the iMiev also has one of the best chemistry in terms of battery life(LTO) and like the early Leaf has air cooling instead of water cooling. That is where a lot of data needs to be looked at.
Most hybrids on the road
Most hybrids on the road today do use NiMH, which as you say differs significantly from li-ion. Some newer hybrids are moving to li-ion chemistry. Either way, the depth of discharge-to-cycle life relationship holds for both types of batteries. It is the reason EVs have a much higher "usable" energy capacity, because they utilize an SOC window of approximately 25%-90% depending on the specific chemistry. The depth of discharge for hybrids is on the order of 15-20% of usable capacity.
Weapon: I should note that
Weapon: I should note that LTO is the anode chemistry whereas NCA/NMC/LCO is the cathode chemistry. Most cells use some type of graphite anode in combination with the cathodes that I mentioned above. Cells that are made with LTO anodes are typically good for low temperature charging and should result in less capacity fade (depending on what the cathode pairing is) but also have much lower energy densities than their graphite based counterparts (because those cells have a lower cell voltage than graphite based cells).