Transcription
Does fast charging your electric car ruin the battery? It's obvious that for convenience and for those that can't charge at home, the ability to fast charge a battery is critical for electric cars to be widely viable. But studies have repeatedly shown that the faster your charging rate, the more degradation a battery has, which naturally raises the following questions that we'll break down.
First, does a faster charging rate degrade a battery faster? Second, why? Because, spoil alert, it does. Third, what practical solutions exist to prevent battery degradation while fast charging? And finally, does it actually matter? In other words, do you need to actually think about it, or is it fine to regularly use fast chargers?
All right, so to start off, do higher charging rates result in faster battery degradation? I'm going to paraphrase a few studies to give you the idea very quickly. Fast charge results in accelerated degradation. Frequent utilization of fast charging can accelerate battery aging. Increasing the current rate will increase the cell degradation rate. Charging a battery at a high current rate decreases the life cycle of the battery drastically.
All right, so first, let's look at a study that shows you the relationship between how fast you charge and how much degradation you have. So here we have different charge rates, and we're looking at how many cycles we can have of that battery before we've degraded it down to 80% remaining capacity. And so here on the right, you can see 1.2 to 5C, meaning a slower fast charge, or 10C here on the left, a very fast fast charge.
So to understand what is C rate, basically, this is allowing us to compare any different size battery, right? It's just relative to the battery size itself. So one over the C rate tells you how many hours it takes to charge a battery. So, in other words, if you're charging at a 1C rate, it takes 1 hour to charge the battery. If you're charging at a 2C rate, it takes 1/2 hour to fully charge the battery. If you're charging at a crazy fast 10C rate, it means it only takes 1/10th of an hour, or 6 minutes, to fully charge that battery.
So if you were to do that with a Tesla Model S, which has a 100 kWh battery pack, well, that means you'd be charging it with 1,000 kW of power. Crazy fast. So just for context, Tesla's going to be charging somewhere in that 1 to 1.5C rate over here on this right side. But what we learn from this is the faster that you charge, the more degradation you have. And it's actually a pretty dramatic change, right? Between 1C and 10C, that's quite a different cycle life.
Now, we're getting a little bit ahead of ourselves, but looking at this 1.25C here, it's getting nearly 3,000 cycles. So this could be many, many hundreds of thousands of miles driven before getting down to 80% capacity remaining. In other words, really, really good as far as the longevity of that battery. Okay.
But that still begs the question, why does a faster charging rate mean faster degradation? Now, if you haven't yet, it's certainly worth checking out my video on how lithium-ion batteries work. And if you're wondering, is this video going to apply only to specific chemistries? Well, we'll get into the specifics later. However, this video is really focusing on the negative electrode, which is primarily going to be made out of graphite, regardless of what the cathode is made of. Meaning, even if it's an NMC or an LFP battery, this video will apply to both.
Okay, so why does fast charging result in faster degradation? Well, one of the big reasons is plating. So how does this work? Well, when you're charging a battery, you need to move all of the lithium from one side, all these little red dots here, from the positive side. We need to move them to the negative side. So you're going to apply that current to the battery, you're going to be charging it up, and as you move these lithium particles across, they're going to go find their way inside of these graphite particles and intercalate within them. So they diffuse within these particles.
Now, if the rate that you're charging is too fast, that means you're throwing all kinds of these lithium ions over to the other side, and they can't find a parking spot fast enough within these graphite particles. So instead of diffusing within the particles, they build up on the outside of the particle if your rate is too high. Now, why is that a problem? Well, first of all, this consumes lithium inventory, which means you have less lithium to deal with, which means your battery has degradation. That also creates a barrier on these graphite particles, meaning it's harder for those lithium ions to find themselves a parking spot and diffuse within. So the problem just compounds. And finally, worst case, you can have these dendrites form where that lithium metal starts to create these little spikes. Eventually, that might pierce your separator, you're then connecting your negative to your positive, and suddenly you've got a short, and you can have a thermal runaway situation.
All right, so what are the different variables that impact whether or not lithium plating occurs? All right, so the three variables we're going to look at: battery state of charge, charge rate, and battery temperature. And keep in mind, our goal here, we want to make sure our diffusion rate of these lithium ions into these graphite particles is faster than our charging rate. In other words, we want to make sure there's plenty of time for all of these guys to find a parking space rather than just throwing a ton of cars at that parking lot all at once.
So, for example, looking at battery state of charge, if we're at a low state of charge, well, that means there's a ton of available parking spaces to put these lithium ions. So it's very easy to use a high charge rate because there's plenty of places to put those lithium ions. Versus, if they're at a high state of charge, well, now you've got all of these parking spaces taken up, and you're trying to throw all of these lithium ions into it, and they can't find a space. So they start building up on the exterior, on the outside of that graphite particle, and you have that lithium plating occur.
All right, next, let's look at charge rate. So if you're only throwing one car at a time at a parking lot, it's very easy for that one car to find itself a spot. If you're throwing a ton of cars at that parking lot all at once, well, it's going to get all jumbled up, right? And so that's what happens. They build up on the outside of the particle rather than finding themselves a nice parking spot within that graphite particle.
All right, and finally, let's look at battery temperature. So if you have a hotter battery, well, there's less internal resistance. So these lithium ions are moving around within that battery much faster. It's like having a bunch of pro drivers that are driving really fast, and they find those parking spots very quickly. Versus a cold battery, all of these lithium ions are now moving much slower through the battery, right? So it takes them more time to find that parking spot. If you got a bunch of old grandpas driving cars into the parking lot, they're not going to find a spot as quickly. So if it's really cold, those lithium ions start to build up on the outside of the particle rather than finding that parking spot if your charge rate is too high.
Okay, so what solutions exist to prevent lithium plating while fast charging? All right, so let's look at battery state of charge and charge rate first as solutions and variables that we can manipulate. So a study looked at charge rate versus battery state of charge to find out where was it likely and where was it unlikely that you would have lithium plating occur. And as you can see from this graph, when you're at a lower battery state of charge, you can get away with a really fast charging rate up to like 8C without lithium plating occurring. But as you raise in a higher state of charge, well, now you know you can maybe only be a little over 1C to prevent that lithium plating. Versus, if say you're at 80% state of charge and you're charging at a 7C rate, well, it's very likely then that you're going to have lithium plating occur. So what you need to do is decrease your charge rate as you're charging that battery to ensure that you don't have any lithium plating. And this is exactly what electric vehicles do today when you pull up to a fast charger and plug it in.
All right, so what can we do with our next variable, temperature, in order to improve fast charging? All right, so a study looked at the effects of temperature on lithium plating. So here we're looking at how much capacity has been lost from the battery just from lithium plating. And on the bottom, we're looking at how many equivalent full cycles do we have. And you can see that the lower the temperature that we're charging at, it has a huge impact on lithium plating capacity loss. Versus, if you charge at warmer temperatures, like 20°C, well, it's very low. We're getting past 3,000 equivalent full cycles here. And they found above about 25°C, they didn't see any lithium plating causing any capacity loss. So the simple solution: charge at a hotter temperature. Right? And that's exactly what EVs do. They bring up the battery pack temperature when you're arriving to that supercharging station, to that fast charging station, so that you can charge at a faster rate without having lithium plating occur.
All right, but then why not just charge at really hot temperatures? Well, as you increase the battery's temperature, you start to run into another failure mode of lithium-ion batteries that we haven't yet discussed, which is SEI growth. So the hotter the battery, well, then you start to have a reaction occur between these graphite particles and the electrolyte around them, and it builds up this layer on the outside of these particles that consumes lithium inventory. So as you consume that lithium inventory, of course, you have less capacity in the battery overall. So a study looked at, okay, how much capacity loss will we have from SEI as a result of operating at different temperatures? And as you can see, at 50°C versus 5°C, it's a huge difference in how quickly you have capacity loss at higher temperatures. So while raising the battery's temperature is great for eliminating lithium plating, it means you're going to have significantly more SEI growth.
All right, so what's the solution? Well, you need to find a happy middle ground. So for any specific charging rate, there's going to be a curve where if you go too cold in temperature, you're going to have too much lithium plating, and if you go too hot in temperature, you're going to have too much SEI formation. So there's going to be this bottom point, this ideal temperature for any given charge rate. And so in the ideal world, while your vehicle is fast charging, it's maintaining the ideal temperature for the battery pack in order to minimize degradation.
All right, so finally, does any of this matter? Do you actually have to worry about fast charging? All right, so obviously, the engineers that design these electric vehicles know all of this, right? So they take it into consideration. When you're charging, you have faster charge rates at lower states of charge, and only if battery temperatures allow for it. Otherwise, you simply heat the battery until you can provide those higher charging rates. So if I had to give a yes/no answer to, should I worry about it? I'd say no, don't stress over it and fast charge when you need to. But it's really not that simple because fast charging certainly does have an impact.
For additional context, I have three real-world examples plus one interesting study. Recurrent Auto looked at real-world driving data from 13,000 Teslas, and they found that there was "no statistically significant difference in range degradation between fast charging more than 70% of the time and fast charging less than 30% of the time." A U.S. Department of Energy study found, while testing a 2012 Nissan Leaf, that after 50,000 miles of driving, the Leaf that was exclusively slow-charged was at 75% of the original battery capacity, while the Leaf that was exclusively fast-charged was at 70% capacity remaining. Now, 5% sounds like a lot, and it is. But keep in mind, this was a Nissan Leaf that was exclusively fast-charged. It was done in Phoenix, Arizona, where you have really high temperatures, and this is a 2012 battery pack that was air-cooled. So it's a chemistry that is notorious for faster degradation, as well as the cooling method is notorious for degradation. So real-world, today's modern chemistries are much better than this. Finally, a company, Geotab, claims to have data from 10,000 electric vehicles, and they claimed that battery degradation appears to be strongly correlated with DC fast charging for high-use vehicles in hot climates, with about 90% capacity remaining after 4 years versus a little over 80% after 4 years with frequent fast charging. Now, this also sounds pretty concerning, but it also seems like this data was based on a single electric vehicle make, which is known for having a really high charging rate. The same company also mentions from their latest analysis from 2024, EV batteries only degrade 1.8% per year and could last 20 years or more.
All right, so here's a study that suggests charging rate is not the most important factor. So here on the bottom, we're looking at charging cycles, and on the left, we're looking at capacity loss. And they tried three different charging methodologies, and here, the fastest charging methodology actually had the best longevity. So the tests were as follows: you were charging at 1.5C, that was the fast charging from 2.5% to 82.5% state of charge. That was good for 1189 cycles before reaching 15% capacity loss. Versus charging at 0.5C, so two examples doing slow charging, one going from 25% to 93%, and one going from 2.5% to 100%. And both of those had significantly fewer cycles before reaching 15% capacity loss. So again, other factors can be more important than simply fast charging.
Okay, and to follow up on battery chemistry, the two dominant chemistries today in electric vehicles are LFP and NMC. While it is a generalization and not always true, LFP batteries tend to have greater cycle life, including when fast charging. So if you do plan on very frequent supercharging, you may want to choose an EV with LFP batteries. This reinforces my belief that if you cannot charge at home, LFP is the chemistry to go for. From a degradation standpoint, it doesn't mind as much when you fast charge, it doesn't mind as much when you charge up to 100%, and it doesn't mind as much when you have a higher depth of discharge. If you can charge at home and mostly slow charge, either chemistry is absolutely fine. And if you want to know what are the best charging practices, whether you have an LFP electric vehicle or an NMC electric vehicle, I have a video on each explaining best practices for longevity. If you have any questions or comments, feel free to leave them below. Thanks for watching.