The Truth About Tesla Battery Degradation – and Other EVs

In my last video I talked about how I drive a 2002 Subaru WRX with 300,000 miles. Having always driven old, high-mileage cars, I've been very interested in how an electric vehicle will perform as it ages. I think many of you will be interested in this information, as I know that many people have to purchase an older vehicle, either out of necessity or because they don't want to have a monthly payment. When you look at older EVs, my number one question. has always been "how healthy is the

battery

?" We all understand this question because the batteries in our phones and laptops need to be replaced every 3 to 5 years.

In electric cars this is a much more serious issue because we are talking about the most expensive part of the car. In the new Tesla Model 3 and Model Y, the cost of the

battery

is estimated to be between 21 and 24% of the base price of the car. This percentage only increases as the car depreciates. For example, the first generation Nissan Leaf. I did a quick survey on cars.com and found that these cars have an average price of $8000, which is very affordable. However, it has been reported that currently, in 2020, Nissan charges around $5,500 for battery replacement. That doesn't include labor.

This is more than 68% of the average value of these cars! Therefore, if you are purchasing a used electric vehicle, it is very important to know how long you can expect the battery to last. In this video, I'll cover this. I'll start by covering how lithium ion batteries work. From here we will move on to the factors that cause a lithium battery to degrade and how to keep it in good condition. And finally, I'll look at some real-world

degradation

data from some popular electric vehicles. This is very long, so I included timestamps in the description and you can easily skip a section if you're not interested.

To understand how batteries work, let's start with an electric motor like that found in electric vehicles. All engines run on a flow of negatively charged electrons. The electrons move through the motor and are not destroyed in the motor, but do useful work before leaving the motor. I like to think of this flow of electrons as a current with a waterwheel. As water flows through the wheel, it does useful work on the wheel before continuing downstream again. At first glance we might be tempted to assume that the water has not changed before and after the wheel. But the laws of thermodynamics would disagree.

As we know, water only flows downhill. And we see that as the water flows over the wheel, a height equal to the height of the wheel has fallen. Because of this, the water at the top of the wheel has a greater gravitational potential energy and as it passes over the wheel, it gives this energy to the wheel, which makes it spin. Virtually the same process occurs in our electron stream and in the motor. Except, instead of gravitational potential energy, the electrons are driven through the motor by an electrical potential. There are different methods to produce an electrical potential, but in this video we are considering a specific type of electrical potential produced by a battery.

In this case, it is called reduction potential or standard potential and is measured in volts. To understand this, let's look at an atom. Electrons are the negatively charged particles that surround the nucleus of the atom and are actually the most important atomic particle when it comes to interactions with

other

atoms. I won't go into all the physics here, but some elements and molecules are naturally in a higher energy state and that is due to the number and configuration of the electrons around the nucleus. When an atom with a high electrical potential loses one or more electrons, it passes into a lower energy state.

Since all things in the universe like to go from high energy to low energy, these elements lose electrons easily. Other materials naturally have very low energy, which is also due to the number and configuration of their electrons. These materials tend to accept electrons easily. Materials that lose electrons easily are defined as those that have a more negative standard potential, while those that accept electrons more easily have a more positive potential. The magic with these two materials happens when they are connected by a conductor or cable. Electrons flow “downhill” from the material with more negative potential to the material with more positive potential.

And so, we have a flow of electrons that can be used to do useful work. These material potentials and their associated electron flows form the basic operating principle of a battery. For this example, I will consider a lithium cobalt oxide battery. Different manufacturers use slightly different chemistries, but the battery fundamentals are similar to what I present here. There are two halves to each battery, one half is called the cathode and the

other

half the anode. In our simplified lithium-ion battery, the cathode is made of lithium cobalt oxide and the anode is made of graphite. When the battery is charged, an external power source drives a flow of electrons from the cathode to the anode.

The electrons leaving the cathode are supplied by lithium atoms, which remain positively charged after losing their electron. As electrons from the cathode enter the anode, this end of the battery becomes more negatively charged, while the cathode becomes more positively charged. Since opposite charges attract, the two halves of the battery are electrically insulated to prevent electrons from the anode from returning to the cathode. There is an electrolyte solution and a semi-permeable solid barrier that provides this electrical isolation between the cathode and anode. The electrolyte is usually a lithium salt solution and actually contributes to the total amount of lithium in the battery.

While this barrier prevents electrons from returning to the cathode, it does not prevent positively charged lithium ions from moving toward the anode. And as more lithium ions cross the barrier, they balance the charge on the anode. Once all the lithium is transferred from the cathode to the anode, the battery will be fully charged. If we look at this anode and cathode, we see that lithium has a very negative potential while cobalt oxide has a positive potential. In other words, lithium loses electrons easily and cobalt oxide readily accepts them. A charged lithium cell will have a voltage potential of up to 4.2 volts between the anode and cathode.

When the battery is discharged, the anode and cathode are connected by a cable and in our case it passes through a motor. Electrons separate from lithium ions at the anode and flow along the wire to the cathode doing useful work on the motor along the way. As electrons enter the cathode, it becomes more negatively charged, which attracts positively charged lithium ions toward the separator, balancing the charge. Once all the electrons and lithium ions have left the anode and moved to the cathode, the battery is said to be completely discharged. Now, saying that a battery is completely discharged does not mean that the battery no longer has voltage potential.

For lithium-ion batteries, full discharge usually occurs when the cell voltage is around 3.2 volts. You could discharge the battery further, but doing so would permanently damage the cell. I've had this cell unused for a few years, so its voltage is extremely low and I would never do what I'm about to do if it weren't. With the battery cut in half, it was really nice to see the anode and cathode coiled inside. And after removing the metal rod in the center, I was able to simply pull the coil out of the shell. Unwinding the coil was very easy.

It was almost as easy as unrolling a precious roll of toilet paper. Once I unwrapped it, I found two sheets inside the coil. One was a copper sheet that has the anode material of graphite. And the other was an aluminum sheet that had the cathode material. Between these sheets there was a plastic separator. One of these types contains approximately enough energy to drive a Tesla Model S 250 feet. However, after Tesla combines more than 8,200 of these cells into its Model S battery, you can drive up to approximately 391 miles! As we know, these numbers are for pristine cells fresh from the Gigafactory.

Surely after several years and thousands of kilometers they will have lost some capacity and the car will have lost some autonomy. There are two large groups of battery aging mechanisms. On the one hand there is cyclical aging, which I'm sure we are all familiar with. The more times we charge and discharge a battery, the more it degrades. And so there is a specific amount of

degradation

tied to each cycle. Modern batteries can perform between 300 and several thousand cycles before they lose significant capacity. When we look at electric vehicles, mileage is an indicator of cycling. More miles equal more cycles.

Although this is not always the case when we compare one car model with another. For example, for the same mileage, a Nissan Leaf with a range of 73 miles will typically have completed many more cycles than, say, a Model S with a range of 265 miles. The second group of aging mechanisms are those that lead to calendar aging. Calendar aging is a degradation that occurs over time, regardless of the number of cycles. You may have an old lithium battery that has barely been cycled, but will still have lost capacity over time. This is a very important consideration when looking at older electric vehicles.

Even if the car has few kilometers, it could have a lot of degradation. There's a reason why Nissan, Chevy, and Tesla warranty their batteries for both a mileage number and age, whichever comes first. So what exactly is happening to lithium-ion batteries as they age? Is there anything that speeds up or slows down the aging process? When it comes to cyclic aging, a paper by Keil et al suggests that there are two main mechanisms responsible: cathode or anode damage and lithium plating. If we analyze calendar aging there are also two mechanisms: passivation layers and electrolyte oxidation. These are the four most common.

Featured mechanisms. I say more prominent because the inner workings and degradation mechanisms of a lithium-ion cell are very complex, much more complex than I currently understand (or have time to address today). Before we dive into the degradation, let's go back to our idealized battery. As the battery charges and discharges, lithium ions go back and forth between the cathode and anode. Battery capacity is directly related to the amount of lithium available to make this repeated round trip. If any of the lithium is trapped or blocked from entering the cathode or anode, the battery will lose capacity. Lithium can also become trapped in other reactions within the battery, and this can lead to deposits or plating of lithium compounds.

As the battery cycles, the material or structure of the cathode can also become damaged. This damage is due to the movement of lithium ions in and out of the cathode. I like to think of the cathode as two sheets of metal and the lithium as marbles that are repeatedly inserted between the sheets. Over time, this repeated mechanical stress can cause the sheets to crack. And once the cathode begins to crack, some regions become electrically isolated. And this prevents electrons and lithium ions from the insulated region from migrating back to the anode during charging. The second mechanism responsible for cyclic aging is lithium plating.

Lithium plating mainly occurs on the anode during charging. The higher the charge rate, the greater the number of electrons that will be conducted from the cathode to the anode. As the flow of electrons increases, the flow of lithium ions through the separator also increases. These lithium ions must be received into the graphite matrix to balance their charge with the electrons and the problem here is that graphite has a maximum rate at which it can accept lithium ions. If this rate is exceeded, lithium ions begin to accumulate on the anode. And ideally, when charging stops, this lithium will break away from the plate and make its way to the anode.

However, there may also be reactions with the electrolyte, and this may form some permanent surface films on the anode. The first calendar aging mechanism is the formation of inert layers on the anode surface. The most important of these layers is called the Solid Electrolyte Interface or SEI. This layer is formed by reactions between the electrolyte and the anode and grows larger during the first cycles. The formation of the SEI layer consumes lithium, but it is actually beneficial and the manufacturers want it to be that way. See that the SEI layer is electrically insulating and helpsprevent electrons at the anode from tunneling through the separator and electrolyte to the cathode.

Although the SEI layer blocks electrons, lithium ions can still pass through it and this allows the battery to function normally. Ideally, once the anode surface is completely covered by the SEI layer, it stops growing. However, during cycling, the SEI layer can break down, and this will expose new anode material to the electrolyte, and this will allow more SEI to form, depleting the lithium in the electrolyte. When the battery is charged and the cathode has a high positive charge, electrons can be extracted from the lithium-based electrolyte in a process called electrolyte oxidation. The positively charged lithium ions formed during this process are extracted from the electrolyte and balance the charge on the cathode.

Over time, this reaction depletes the amount of lithium in the electrolyte and, according to Jeff Dahn's group, can eventually kill the battery. While some battery degradation is inevitable, the important thing to know is that it is significantly influenced by the way the battery is operated and stored. Temperature is the most important here, especially high ones. Unfortunately, the battery doesn't even need to be in active use for heat to affect degradation. I think you can see the problem here with electric cars that will be parked all day in a sunny parking lot. Cold weather can also decrease battery capacity, but only temporarily until warm temperatures return.

The main problem with cold is during charging. If the battery is charged when it is too cold, the lithium coating can irreversibly damage the battery. The relationship between high temperatures and capacity loss is well documented. In these findings here, you can see that at 25°C, the cells lost about 7% of their original capacity after 100 cycles. However, when the operating temperature was increased to 45°C, more than 12% of the original capacity was lost during the same number of cycles. As we all know, battery degradation is directly related to the number of charge and discharge cycles. But it is less known that a partial cycle does not count the same as a complete cycle.

What do I mean? Well, a battery that discharges from 100% to 0% will experience more degradation per cycle than one that discharges from 80% to 20%. The percentage of capacity that is used during a cycle is called the depth of discharge. and researchers have studied this discharge depth effect for years. In this data set, we see that when a battery was depleted from 100 to 25%, the capacity fell below 80% of its original value in 4000 cycles. A discharge depth between 85 and 25% had better performance, but the best performance was for a discharge depth between 75 and 65%. After 8000 cycles, this battery has maintained more than 90% of its original capacity, which is quite remarkable!

When it comes to electric vehicles, manufacturers know this and typically do not allow the driver to access the full capacity of the battery. This means that the battery has a built-in restriction on the depth of discharge and that helps it age better. Anyone who owns an electric car wants to be able to charge it as quickly as possible, because then they can get back on the road sooner. Unfortunately, it is often suggested that high charging rates can reduce battery life. High charging rates are thought to lead to lithium plating and can also potentially damage the SEI layer.

Higher charge rates can also heat up the battery, which we've already talked about. The Idaho National Laboratory cycled batteries at charge rates typical of DC and Level 2 fast chargers. Level 2 chargers are typically used for charging when you are at home or at a destination, and charging can take several hours. DC fast chargers are used while traveling on the road to recharge your car in less than an hour. The researchers found that DC fast charging degraded the battery a little faster, but the difference was very small. Interestingly, when they charged the cells at 30°C instead of 20°C, the increase in temperature had a much greater impact on battery capacity than fast charging.

From this it would seem that if an electric vehicle can adequately cool its batteries during fast charging, there should be little impact on degradation. So if you want your EV to last a long time, maybe you should move to Alaska and minimize that discharge depth. You only need to drive 10 miles a day, right? But hey, before we go to those extremes, let's take a look at how electric vehicle batteries behave in the real world. You know, under normal conditions. Not in Alaska, and driving normal distances. For this we need some long-term real-world data. For this we need to look at some long-term real-world data.

And fortunately for us, electric vehicles like the Tesla Model S and Nissan Leaf have been in production for almost a decade. Until the end of 2019, the Nissan Leaf was the most popular electric vehicle in the world. Between December 2010 and December 2019, around 450,000 Leafs were sold worldwide. Could they be leaves? The plus... Nissan came very early to the electric vehicle game. To give a little context, in 2010, Tesla only had its original roadster on the road. The Tesla Model S had been presented the previous year, but it would be another two years until 2012, when it went into production. This gives us almost a decade of data on the Leaf's battery.

And contrary to what some headlines say, the Leaf's battery degradation is a problem. In 2012, Leaf owners in Arizona began reporting that their cars were losing bars of battery capacity. For some context, in a first generation Leaf, when you lose that first bar of capacity, you've already lost 15% of the battery capacity. And a 15% reduction in capacity in a car with a 73-mile range is huge. Now, these preliminary cases in the hot American Southwest have shed light on the Leaf's biggest problem. Unlike virtually all other electric vehicles, the Leafs do not have an active battery thermal management system.

They simply rely on passive air cooling to maintain battery temperature. There is not even a fan to cool the battery. And this is a very strange decision given the strong influence of temperature on battery longevity. It's especially worrying because vehicles can't always be parked indoors and I'm sure you know that indoor temperatures can easily exceed 40 or 50°C in hot, sunny places. So let's look at the data for the first generation Leafs that are now over 8 years old. New Zealand electric vehicle promotion organization Flip the Fleet has collected more than 2,000 measurements of battery capacity from older Leafs.

On average, this data shows that after 8 years, the battery capacity can be expected to reduce to 70% of its original capacity. Plug in America has collected similar data for more than 360 Leafs. The Plug in America data is similar to the Flip the Fleet data, although it shows a slightly larger capacity loss for a similar age. Obviously, this data is concerning if you want to buy a used Leaf. The car might cost as little as $5,000, but if its usable range drops below 50 miles on a good day, how good a deal is that deal really? Now I know many of you are probably already commenting that 50 miles is enough to get around town, which is usually true.

However, it's surprisingly short for even moderate-range trips to the next city, especially if you can't or don't have time to charge at your return destination. Furthermore, if you live in a place with winter, like me, you should keep in mind that during the cold months the autonomy will be reduced considerably more. But wait, before you call it a day and walk away from used electric vehicles, let's take a look at the market leader: Tesla. For simplicity, I'm only considering the Model S, which was released in late 2012. Today, there are many 2012 or 2013 Model S for sale at prices as low as $22,000.

Assuming you're okay with buying a car that has over 100,000 miles on it. There is a Dutch-Belgian Tesla owners forum that maintains an excellent battery degradation spreadsheet for many years. As of Spring 2020, it contains over 1,300 Model S entries. And if we plot remaining battery capacity against battery age, we see that a polynomial regression trend line trends toward 90% of original capacity. after 6 to 8 years. Some of these cars have driven over 170,000 miles with battery capacities still over 90%! There is a second database provided by Plug in America, which compares very well with data from the Netherlands and Belgium. There are a handful of outliers with battery capacities at or below 80%, but this data clearly shows that Tesla's battery packs last a long time, unlike the Leaf.

So what is the big difference between the cars? Well, for one thing, Tesla batteries have active liquid thermal management which is far superior to the Leaf's complete lack of thermal management. If the Tesla is charging, accelerating, or parked in a hot parking lot, the cooling system can kick in and keep the cells at their optimal temperature. Another advantage is Tesla's large battery. With a usable range that easily exceeds 200 miles, Teslas not only require less frequent charging. cycling, but most riders will use a smaller percentage of the battery between charges. And this lower depth of discharge will keep the battery in good condition for longer.

There are also some newer EVs with large liquid-cooled batteries that show good initial degradation numbers. A good example is the Chevy Bolt, which came out in 2017. Eric Way of the Youtube channel News Coulomb reported that after 3 years and 100,000 miles, his 2017 Bolt has lost about 8% of its original capacity. I have collected data from several other Bolt battery databases and surveys. And in total, this data only represents 15 vehicles, which is not very statistically relevant. But it's a good estimate of where the Bolt's battery performance is headed. And so far, downgrading appears to be a Model S-like trend, not a Nissan Leaf-like one.

All of this leaves me a little unsure as to the answer to my original question, which was: "how long will used EV batteries last?" It all depends on the vehicle and to a lesser extent on its origin and the use to which it was given. In the case of early Nissan Leafs, I would be very skeptical about the health of their battery, especially if they come from warm climates. There are some tools, such as the Leaf Spy app, that can help buyers check the health of a battery before making a purchase. Still, this isn't perfect and, in my opinion, Leaf buyers should be prepared for noticeable battery degradation.

Used Teslas, on the other hand, seem to hold up very well over time. This, combined with their much greater initial range, means that even downgraded Teslas will still offer very usable range. Personally, I wouldn't hesitate to buy a Tesla with over 100,000 miles assuming the price was right. The interesting thing about the battery data I've presented is that it relates to the first mass market electric vehicles and battery technology has improved a lot in the last 10 years and will continue to do so. At the time of this recording, Tesla is about to unveil some new battery technology at its Battery Day event.

Elon Musk has said that this event will be one of the “most exciting” days in Tesla history. And last year in 2019, Elon promised that a million mile battery would be coming soon and the battery day event is expected to discuss the details of this battery. So, there are a lot of battery improvements coming and I wouldn't be surprised if within 10 to 20 years battery degradation is no longer an issue. Well, there is a lot more to battery technology than I can discuss here. So I've put a playlist here of other great videos I've found. And alternatively, you can watch another of my videos here.

I'm Josh, this is Nikola Garage. I'll see you in the next one.