In my last video I talked about how I drive
a 2002 Subaru WRX with 300,000 miles on it. Since I’ve always driven old, high mileage
cars, I’ve become very interested in how an EV will hold up as it ages. I believe a lot of you will be interested
in this information as I know many people have to purchase an older vehicle either by necessity or because they don’t want to have a monthly car payment When you look at old EVs, my number one question has always been “how healthy is the battery?” See we all understand this question because our batteries in our phones and laptops they need to be replaced every 3 to 5 years. In electric cars this is a much more serious question because you’re talking about the most expensive part of the car. In new, Tesla Model 3’s and Model Y’s,
the battery cost is estimated to be somewhere between 21 and 24% of the car’s base price. This percentage only gets higher as the car depreciates. For example, first generation Nissan Leafs. I did a quick survey of cars.com and found
these cars have a median price point of $8000, which is very affordable. However, it’s been reported, that currently
in 2020, Nissan charges around $5,500 USD for a battery replacement. That doesn't include labor. This is over 68% of these cars’ median value! So, if you’re going to be purchasing a used
electric vehicle, it’s really important to know how long you can expect the battery to last. In this video, I'm gonna be covering this. I’ll start by covering how lithium ion batteries work. From here we’ll move onto the factors that
cause a lithium battery to degrade and how to keep it healthy. And then finally, I’m gonna look at some real-world degradation data for a few popular EVs. This is a really long one, so I’ve included time
stamps 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 the one in electric vehicles. All motors are powered by a flow of negatively
charged electrons. The electrons move through the motor, and they are not destroyed in the motor, but they do useful work before flowing out of the motor. I like to think of this electron flow as a
stream with a water wheel in it. As water flows across 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 is unchanged before and after the wheel. But, the laws of thermodynamics, they would beg to differ. As we know, water only flows downhill. And we see that while the water flows over the wheel, it has fallen by a height equal to the height of the wheel Because of this, the water at the top of the
wheel has a higher gravitational potential energy and as it passes over the wheel, it
gives this energy to the wheel, which makes it turn. Virtually the same process is happening in
our electron stream and the motor. Except, instead of gravitational potential
energy, the electrons are driven through the motor by an electric potential. There are different methods to produce an
electric potential, but in this video we’re considering a specific type of electrical
potential that's produced by a battery. In this case, it's called a reduction potential or a standard potential and it’s measured in Volts. To understand this, let’s look at an atom. The electrons are the negatively charged particles that surround the nucleus of the atom and they really are the most important atomic particle when it comes to interactions with other atoms. I won’t get into all of the physics here,
but some elements and molecules are naturally at a higher energy state and that's due to the number and the configuration of the electrons around the nucleus. When an atom with a high electrical potential
loses one or more electrons, it moves to a lower energy state. Since all things in the universe like to go
from high energy to low energy, these elements easily loose electrons. Other materials are naturally at a very low
energy, which is also due to the number and configuration of their electrons. These materials tend to easily accept electrons. Materials which easily loose electrons
are defined as having a more negative standard potential while those that more easily accept
electrons have a more positive potential. The magic with these two materials happens when they are connected by a conductor, or a wire. The electrons flow “downhill” from the material with more negative potential to the material with the more positive potential. And just like that, we have an electron flow
that can be used to do useful work. These material potentials and their associated
electron flows form the basic working principle of a battery. For this example, I’ll be considering a
Lithium Cobalt Oxide battery. Different manufacturers use slightly different
chemistries, but the battery fundamentals are similar to what I’m presenting here. There are two halves in every battery, the
one half is called the cathode and the other half the anode. In our simplified lithium ion battery, the
cathode is made from Lithium Cobalt Oxide and the anode from graphite. When the battery is charged, an external power source drives a flow of electrons from the cathode to the anode. Electrons leaving the cathode are supplied
by lithium atoms, which become positively charged after they lose their electron. As the electrons from the cathode enter the anode, this end of the battery becomes more negatively charged, while the cathode
becomes more positively charged. Since the opposite charges attract, the two halves of the battery are electrically isolated to prevent electrons in the anode from moving
back to the cathode. There’s an electrolyte solution and a solid
semi-permeable barrier that provides this electrical isolation between the cathode and the anode. The electrolyte is typically a lithium salt
solution, and it actually contributes to the total amount of lithium in the battery. While this barrier prevents electrons from
flowing back to the cathode, it does not stop the positively charged lithium ions from moving
to the anode. And as more lithium ions move across the barrier, they balance the charge in the anode. Once all of the lithium is transferred from
the cathode to the anode, the battery is fully charged. If we look at this anode and cathode, we see that lithium has a highly negative potential while cobalt oxide has a positive potential. In other words, the lithium easily loses electrons, and the cobalt oxide readily accepts them. A charged lithium cell will have a voltage potential of up to 4.2 volts between the anode and the cathode. When the battery is discharged, the anode
and cathode are connected by a wire and in our case it runs through a motor. The electrons separate from the lithium ions in the anode, and flow along the wire to the cathode doing useful work in the motor along
the way. As the electrons enter the cathode, it becomes more negatively charged which pulls the positively-charged lithium ions back across the separator balancing the charge. Once all of the electrons and lithium ions
have left the anode, and moved to the cathode the battery is said to be fully discharged. Now, saying a battery is fully discharged
does not mean that the battery no longer has a voltage potential. For lithium ion batteries, full discharge
generally 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 sitting around for a
few years, so its voltage is extremely low, and I would never do what I’m about to do
if it wasn’t. With the battery cut in half, it was really
neat to see the coiled anode and cathode inside. And after I removed the metal rod in the center, I could simply pull the coil out of the shell. Unrolling the coil was very simple. It was pretty much as easy as unrolling a precious roll of toilet paper. Once I got it unwrapped, I found two foils
within the coil. The one was a copper foil that has the graphite
anode material on it. And the other one was an aluminum foil that
had the cathode material on it. In between these foils was a plastic separator. One of these guys contains roughly enough
energy to drive a Tesla Model S 250 feet. However, after Tesla combines over 8200 of these cells into the battery of their Model S you can drive up to an estimated 391 miles! As we know, these numbers are for pristine
cells straight out of the Gigafactory. Surely after several years and thousands of
miles they will have lost some capacity and the car will have lost some range. There are two broad groupings of battery aging mechanisms. On one hand is cyclic aging, which I’m
sure we’re all familiar with. The more times we charge and discharge a battery, the more it degrades. And so there’s a specific amount of degradation tied up in each cycle. Modern batteries can be cycled
somewhere between 300 and several thousand cycles before they lose significant capacity. When we look at electric vehicles, mileage
is an indicator of cycling. Higher miles equals more cycles. Although, this isn’t always the case when
we compare one model of car to another. For instance, for the same mileage, a Nissan
Leaf with a 73 mile range will typically have completed many more cycles than say a
Model S with a 265 mile range. The second group of aging mechanisms are those that lead to calendar aging. Calendar aging is degradation that occurs over time, regardless of the number of cycles. You may have an old lithium battery that has
hardly been cycled, but it will still have lost capacity over time. This is a very important consideration when looking at old EVs. Even if the car has low miles, it could have a lot of degradation. There's a reason that Nissan, Chevy, and Tesla all warranty their batteries to both a mileage number, and an age, whichever comes first. So, what exactly is happening to lithium ion
batteries as they age and is there anything that will either accelerate or slow down the aging process? When it comes to cyclic aging, a paper by Keil and others suggests that there are two main mechanism that are responsible: Cathode or anode damage and Lithium plating Looking at calendar aging there's also two mechanisms: Passivation layers and electrolyte oxidation These are the four most prominent mechanisms. I say most prominent because the internal workings and degradation mechanisms of a lithium ion cell are very complex much more complex than I currently understand – or have time to get into today. Before we take a dive into degradation, let’s
go back to our idealized battery. As the battery is charged and discharged, the lithium ions shuffle back and forth between the cathode and the anode. The capacity of the battery is directly tied to the amount of lithium that's available to make this repeated journey back and forth. If any of the lithium becomes trapped in or
blocked from entering either the cathode or the anode, the battery will lose capacity. Lithium can also get tied up in other reactions
inside the battery, and this can lead to deposits or plating of lithium compounds. As the battery is cycled, the cathode material or structure can also be damaged. This damage is due to the motion of lithium
ions entering and leaving the cathode. I like to think of the cathode as two sheets
of metal with the lithium being like marbles that are repeatedly forced between the sheets. Over time, this repeated mechanical stress
can lead to cracking of the sheets. And once the cathode begins to crack, some regions becoming electrically isolated. And this prevents electrons and lithium ions
in the isolated region from migrating back to the anode during charging. The second mechanism responsible for cyclic aging is lithium plating. Lithium plating primarily occurs at the anode
during charging. The higher the charge rate, the greater the
number of electrons that are being driven from the cathode to anode. As electron flow increases, so does the flow
of lithium ions across the separator. These lithium ions have to be received into the graphite matrix to balance their charge with the electrons and the problem here is that the graphite has a maximum rate at which is can accept lithium ions. If this rate is exceeded, the lithium ions
begin to build up on the anode. And ideally, when charging stops, this lithium
will de-plate and work its way into the anode. However, there can also be reactions with the electrolyte, and this can form some permanent surface films on the anode. The first calendar aging mechanism is the formation of inert layers on the surface of the anode. The most important of these layers is called
the Solid Electrolyte Interphase or the SEI. This layer is formed by reactions between
the electrolyte and the anode and it grows the most during the first few cycles. Forming the SEI layer does consume lithium, but it’s actually beneficial and manufacturers want it to happen. See the SEI layer is electrically isolating
and it helps prevent electrons in the anode from tunneling across the separator and the 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 entirely
covered by the SEI layer, it stops growing. However, during cycling the SEI layer can
be broken up, and this will expose fresh anode material to the electrolyte, and this allows more SEI to form depleting lithium in the electrolyte. When the battery is charged and the cathode has a high positive charge electrons can be stripped from the lithium-based electrolyte in a process called electrolyte oxidation. The positively-charged lithium ions formed
during this process are drawn out of the electrolyte and balance the charge at 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 how the battery
is operated and stored. Temperature is the big one here, particularly
high temperatures. Unfortunately, the battery does not even need
to be in active use for heat to impact the degradation. I think you can see the issue here for electric cars that are gonna be parked all day in a sunny parking lot. Cold weather can also decrease the battery capacity, but only temporarily until warm temperatures return. The main issue with the cold is during charging. If the battery is charged while it's too cold, it can be irreversibly damaged by lithium plating. 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 around 7% of their original capacity after 100 cycles. However, when the operating temperature was increased to 45°C over 12% of the original capacity was lost over the same number of cycles. As we all know, the battery degradation is directly linked to the number of charge and discharge cycles. But, it’s less-well known that a partial
cycle does not count the same as a full cycle. What do I mean? Well, a battery that is discharged from 100% to 0% will experience more degradation per cycle than one that is discharged from say 80% to 20%. The percentage of capacity that is used
during a cycle is called the depth of discharge. and researchers have studied this depth of discharge effect for years. In this dataset here, we see that when a battery
was drained from 100 to 25%, the capacity fell below 80% of its original value by 4,000
cycles. A depth of discharge between 85 and 25%, performed
better, but the best performance was for a depth of discharge between 75 and 65%. After 8,000 cycles, this battery has maintained more than 90% of its original capacity which is quite remarkable! When it comes to EV’s, manufacturers know
this, and normally they do not allow the driver to access the battery’s entire capacity. This means the battery has a built-in restriction
on the depth of discharge, and that helps the battery age better. Anyone with an electric car wants to be able
to Supercharge it as quickly as possible, because it allows them to get back on the
road sooner. Unfortunately, it’s often suggested that
high rates of charge can reduce battery life. High rates of charge are thought to lead to
lithium plating and it can also potentially damage the SEI layer. Higher charge rates can also heat up the battery,
which we’ve talked about already. The Idaho National Laboratory cycled batteries at charge rates typical of Level 2 and DC fast chargers. Level 2 chargers are typically used to charge
when you’re at home or at a destination, and charging can take several hours. The DC fast chargers are used while you're on road trips
to top up your car in less than an hour. The researchers found that DC fast charging
did degrade the battery slightly faster but the difference was very small. Interestingly, when they charged the cells
at 30°C instead 20°C the increase in temperature had a much greater impact on battery capacity than the fast charging did. From this it would appear that if an EV can
properly cool its batteries during a fast charge there should be little impact on degradation. So if you want your EV to last a really long time maybe you should move to Alaska and minimize that depth of discharge. You only need to drive 10 miles a day right? But ok, before we go to those extremes, let’s just take a look at how real-world EV batteries are holding up. You know, under normal conditions. Not in Alaska, and driving normal distances
For this we need some long-term real-world 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 the Nissan leaf have been in production for almost a decade. Through the end of 2019, the Nissan Leaf was
the most popular EV in the world. Between December 2010 and December 2019, around
450,000 Leafs were sold globally. Would it be leaves? The plu... Nissan was super early to the EV game. For bit of context, back in 2010, Tesla only
had their original roadster on the road. The Tesla Model S had been unveiled the year
before, but it would be another two years until 2012 when that entered production. This gives us nearly a decade of Leaf battery data. And contrary to what some headline say, Leaf battery degradation is an issue. By 2012, Leaf owners in Arizona were beginning to report that their cars were losing battery capacity bars. For some context, in a first-generation Leaf, when you lose that first capacity bar, 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 shone a spotlight straight on the Leaf's biggest issue. Unlike virtually every other EV, Leafs do not have an active battery thermal management system. They simply rely on passive air cooling to
maintain the battery temperatures. There isn’t 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 particularly concerning as vehicles
cannot always be parked indoors and I'm sure you know that interior temperatures can easily exceed 40 or 50°C in hot, sunny locations. So, let’s look at the data for first generation
Leafs that are now over 8 years old? The New Zealand EV promotion organization, Flip the Fleet, has collected over 2,000 battery capacity measurements for the oldest Leafs. On average, this data shows that after 8 years,
you can expect battery capacity to be reduced to 70% of its original capacity. Similar data has been collected by Plug in
America for more than 360 Leafs. The Plug in America data is similar to the
Flip the Fleet data, although it does show a slightly greater capacity loss for a similar
age. Obviously, this data is troubling if you’re
looking to purchase a used Leaf. The car could be as cheap as $5000,
but if its usable range as dropped below 50 miles on a good day – how good of a
deal is that really? Now I know many of you are probably already commenting away that 50 miles is plenty for around town, which is normally true. However, it’s shockingly short for even
moderate-range trips to the next town over, especially if you can’t, or don’t have
time to charge at your turn-around destination. Moreover, if you live in a place with winter,
like I do, you need to consider that during the cold months the range will be reduced
quite a bit further. But hold on, before you call it a day and
walk away from used EVs, 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 a lot of 2012 or 2013 Model S’s for sale at prices as low as $22,000. Assuming you're ok with buying a car that has well in excess of 100,000 miles. There's a Dutch-Belgium Tesla owner forum that has been maintaining an excellent battery degradation spreadsheet for many years. As of the spring of 2020, it contains over
1,300 Model S entries. And if we plot the remaining battery capacity vs battery age, we see that a polynomial regression trendline is trending towards 90% of the original
capacity after 6 to 8 years. Some of these cars have logged over 170,000 miles with battery capacities still in excess of 90%! There’s second database provided by Plug
in America, that lays nicely over the Dutch-Belgium data. There are a handful of outliers with battery
capacities at or below 80% but this data clearly shows that Tesla battery packs are lasting a very long time, unlike the Leaf. So why the big difference between the cars? Well, for one thing, Tesla batteries have
active liquid thermal management which is vastly superior to the Leaf’s, well, total 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 very large battery. With usable ranges easily in excess of 200 miles, Tesla’s not only require less frequent charging aka. cycling but most drivers will use a smaller percentage of the battery between charges. And this shallower depth of discharge will keep the battery healthy longer. There are also some newer EVs with large, liquid cooled batteries are showing good initial degradation numbers. A good case in point is the Chevy Bolt, which came out in 2017. Eric Way of the News Coulomb Youtube channel,
reported that after 3 years and 100,000 miles his 2017 Bolt has lost about 8% of its original capacity. I've compiled data from several other Bolt battery
databases and surveys. And in total, this data only represents 15 vehicles which isn’t terribly relevant statistically. But, it's a good estimate of where the Bolt battery performance is going. And so far, degradation appears to be trending like the Model S, not like the Nissan Leaf. This all leaves me feeling a bit uncertain
as to the answer of my original question, which was, “how long will used EV batteries
last?” It all depends on the vehicle and to a lesser
extent where it's from, and how it was used. In the case of the first Nissan Leafs, I’d
be highly skeptical of their battery health especially if they come from hot climates. There are some tools out there such as the Leaf Spy app, which can help buyers check the status of a battery before they make the purchase. Still, this isn’t perfect and in my opinion Leaf buyers should be prepared for noticeable battery degradation. Used Tesla’s on the other hand, appear to
be holding up very well with age. This combined with their much higher initial range means that even degraded Teslas will continue to provide highly usable range. I personally would not hesitate to buy a Tesla
with over 100,000 miles on it assuming the price was right. The neat thing about the battery data I’ve presented is that it is for the very first mass-market EVs and battery technology has improved a lot in the past 10 years and it's going to continue to do so. As of this recording, Tesla is about to unveil
some new battery tech at their Battery Day event. Elon Musk has said that this event will be one of
the “most exciting” days in Tesla’s history. And last year, in 2019, Elon promised that a million
mile battery would be coming soon and it’s widely expected that the battery day event will discuss the details of this battery. So, there are a lot of battery improvements
coming, and I wouldn’t be surprised that in 10 to 20 years battery degradation may
not even be a question anymore. Well, there's a lot more to battery technology
than I can discuss here. So I’ve put a playlist here of some other great videos I've come across. And alternatively, you can check out another one of my videos here. I’m Josh, this is Nikola Garage. I’ll catch you in the next one.
Tl;dw?
This was an incredibly informative video. Definitely making me feel even better about my SR+ order. With a charger at work and an 8 km (5 mi) commute, I feel that 65%-75% DOD he spoke about will be relatively easy to maintain.
I love you for this video
The best video on this I have seen, you should do one for everything, even marital difficulties...
The graphs are so misleading.
It’s saying approximately 80% remaining when draining the battery from 100%-25% after 4000 cycles. That means it’s used 300k % of the battery capacity.
In the 75%-65% example it says 95% is remaining after 8000 cycles. The issue is that it’s only used 80k% of the battery capacity. If you extrapolate this(assuming it’s linear) to using 300% of its battery capacity, 300k/80k x 5% = 18.75% degradation.
So it’s more like a 3% difference when looking at actual energy usage? I know charging to 100% and discharging to 0% is bad but this graph isn’t comparing apples to apples.