What is battery health? 

Battery health is quite a hard concept to pin down. Intuitively, it means how well a battery can perform as compared to when it was new, but the exact way that is quantified depends on who you ask. In battery science literature, it is commonly referred to as “state of health,” or SoH.

One of the easier ways to talk about battery health is by comparing the original battery capacity, in kilowatt-hours (kWh), to the current battery capacity. The change between the original and the current capacity indicates some degree of capacity fade, or loss in total available lithium that the battery can store.

Since lithium is what powers batteries, any loss effectively means less energy. Battery capacity can be tested in a variety of ways, including extrapolations from “energy in” and “energy out” of packs. It is important to note that a battery's measured capacity will change with temperature, how fast it’s charged or discharged, whether the battery is being used or filled, and the state of charge (SoC) of the battery. Each time you charge your battery from 0 to 100%, you might not read the same number of kWh.

Each time you charge your battery from 0 to 100%, you might not read the same number of kWh.

A more subtle way to look at health is to look at power fade, or the internal resistance that may be degrading the battery performance. This happens when secondary, harmful chemical reactions occur in the battery and impede the free flow of energy. Power fade tends to accompany capacity fade but is harder to diagnose and often requires laboratory and diagnostic testing. 

Power fade tends to accompany capacity fade but is harder to diagnose and often requires laboratory and diagnostic testing. 

True battery health is likely a combination of capacity and power fade, but for the driver of an EV, it translates to whether or not the car can accelerate as fast, and go as far, as when new. 

How do EV batteries degrade?

First, it’s important to know how batteries work at a very high level. Batteries store and transfer energy using both physical and chemical processes. In lithium batteries, lithium is the active material, and charged lithium particles, called ions, move back and forth between two nodes, the anode and the cathode, to generate electricity.

The anode is typically made of graphite, similar to that found in mechanical pencils. At the anode, lithium moves between sheets of carbon in the graphite. This process is called intercalation and it is a physical process.

During charging, lithium is stored in between these sheets of carbon in the anode. During a discharge, the lithium leaves the anode, and enters the electrolyte as an ion. The electrolyte is typically a liquid buffer between the anode and cathode. Here the lithium ion is dissolved into a sort of ion soup, that helps move them to the cathode. While the lithium ion goes to the cathode, an electron travels through a device, like a phone or motor, giving it electric power. The electron and ion meet up again at the cathode, which is typically some sort of composite material of oxygen, metal, and lithium atoms. Instead of intercalating like it does at the anode, the lithium reacts chemically with this material.

The general process is depicted below.

EV battery mechanics

Lastly, a very important aspect of any lithium ion battery is the development of a thin film on the anode and cathode called the solid electrolyte interface, or SEI. This solid film forms when a battery is first made. Some of the lithium in the battery reacts with the electrolyte where it meets the nodes. At first, the formation of the layer consumes lithium, resulting in some initial capacity loss. However, once formed, a healthy SEI layer serves to slow secondary reactions and is important for battery longevity.

Batteries degrade in a few ways - all are normal and unavoidable, but some are accelerated due to the way batteries are used and stored. For instance, the baseline degradation that every battery experiences is due to age and is called “calendar aging.” It doesn’t matter whether or not the battery is even used - it will degrade with time even while sitting on a shelf.

Batteries degrade in a few ways - all are normal and unavoidable, but some are accelerated due to the way batteries are used and stored.

How does this happen? 

In addition to the chemical reactions that cause a battery to work, there are also inevitable secondary reactions that take place within the battery. These are normal, but you can take steps to minimize them. 

While any battery will experience some degree of both capacity and power fade, there are certain things that can accelerate them. The chart below show Tesla Model 3 predicted range plotted as a function of their odometer. It uses real-world, observed data from over 2,000 vehicles in the Recurrent community to show how as cars age, their estimated range falls. Range loss is directly related to capacity fade.

Tesla battery degradation
Observed range degradation in Tesla Model 3 as a function of odometer

What causes battery degradation? 

Since battery degradation is a normal part of the battery life cycle, it’s happening all the time. From the second the battery is made, calendar aging kicks in. Unfortunately, no one can stop the passing of time, but there are some things you can do to slow additional degradation.

The main things that accelerate battery degradation are things that cause the physical and chemical reactions to happen faster. The main culprits that we can control are heat, extreme state of charge, use in lower temperatures, and fast charging. We should note that in most newer EVs, the battery management system helps mitigate a lot of these, but it’s good to understand them all the same. 

1. Heat

Heat is a form of energy, and when you add more energy to a system, chemical and physical reactions happen faster. In the case of a battery, heat causes secondary reactions to happen faster. For example, heat causes the metals in the cathode to dissolve into the electrolyte and become useless more quickly. Heat also can cause the arrangement of atoms in the cathode to change, potentially resulting in lost capacity, higher resistance, or both. Heat can also create wear and tear on other components of the battery such as the binders that hold the components together. These binders can ooze into places they shouldn’t and block lithium or damage the anode and cathode. While EV batteries have complex computer systems and thermal control to help protect from high temperatures, heat is always a concern.

2. Extreme State of Charge

Batteries work because they turn chemical potential energy into electricity. It's quite similar to the potential energy before you drop a ball and it falls to the ground. The ball has the most gravitational potential energy the higher up it is. Similarly, the battery has the most potential energy when all the lithium is charged and in the anode. In this state, the lithium is highly reactive. However, that also means that the battery is inherently less stable when it’s fully charged. The high chemical potential energy may encourage loss of lithium to side reactions. 

Battery state of charge to extreme levels

The side reaction of most concern is lithium plating. This is when lithium deposits on top of the anode, rather than moving between the carbon layers. If the idea of plating is new to you, then a common example of it can be found in the jewelry industry. It's more expensive to buy a 100% golden ring than it is to buy a ring made of a less expensive metal that has a thin gold coating on the top of it. That gold is “plated” onto the less expensive metal.

Every time the battery is charged and discharged, this pure lithium layer can expand. It blocks the ability of other lithium ions to enter the anode or cathode. Therefore not only does it reduce the capacity of the battery by consuming lithium, but it makes it harder for the lithium to move around and increases the resistance. This is the root of power fade. In the most extreme of circumstances, the lithium plating may build up enough to build spines, called dendrites, that reach the opposite node and cause a short circuit, ruining the battery.

However, that isn’t the only thing that may grow at high states of charge. The SEI layer grows more when the chemical potential energy is highest. As it grows, more lithium is consumed and is unavailable as energy. Whatever active lithium is leftover will have a harder time getting through a thicker SEI layer. 

Similarly, when the battery is fully discharged, it can also be unstable and have higher resistance. This increased resistance is similar to how a line to get into a parking garage might increase as it fills up and drivers have a harder time finding spots. The resistance of a battery is dependent on the state of charge. 

It is best to avoid a very high or very low state of charge, particularly if you will not be using the battery right away. Storing a lithium ion battery with a high or low charge can mean damage down the line.

3. Use in Very Low Temperatures

A battery’s performance does decrease in very low temperatures, too. Temperature is a measure of energy, so when it is cold out, there is less energy overall, so less available power. Additionally, at lower temperatures, the reactions and movement of lithium inside the anode, cathode, and electrolyte are slower. It is more difficult for the lithium to squeeze into the anode and there is less energy available to support lithium reacting with the cathode. Finally, because the anode and cathode are having a more difficult time accepting lithium, the lithium is more likely to plate, leading to decreased battery performance.

It is prudent to note that most modern EVs have battery thermal management systems which will kick in to warm up a battery pack so that this low temperature degradation is minimized. Unless the vehicle is plugged in, though, the energy used to warm the battery will come from the battery itself, resulting in more aging due to use. Either way, lower temperatures will lead to accelerated aging.

4. Fast Charging

Fast charging is done by applying a very high voltage to the battery. The higher the voltage, the more force is applied to electrons and lithium ions that are shuttling from the cathode to the anode. When the lithium ions arrive at the anode at a quicker rate than can enter the anode, lithium plating can occur. This is especially true at lower temperatures when it is even more difficult for lithium to enter the anode.

EV battery fast charging

High voltage charging is a physically taxing process that can create a lot of heat in a battery, which can lead to the heat based degradation mentioned above. While modern EVs are built to handle occasional high voltage charging, it is best to use lower voltage charging for every day.

5. Usage and Mechanical Stress

While the other factors on this list contribute much more to premature battery degradation, it would be dishonest not to mention that actually using the battery also contributes to degradation. Moving lithium ions from one side of the battery to the other is a physically taxing process, and is an opportunity for secondary reactions to occur. Studies have shown that the deeper a battery is discharged, the faster degradation will occur. The shuttling of the lithium in and out of the anode and cathode can cause mechanical stresses due to expansion and contraction. The cracking of the SEI layer can lead to additional lithium loss. The cracking of the anode and cathode can lead to disconnections in the battery mechanics, lost material, or higher resistance. The more aggressively and longer a battery is used, the more these stresses build.

However, that does not mean you shouldn’t use and enjoy your EV. EV batteries are built to handle normal usage, including daily use and long drives. A modern battery pack that is only built to last 500 full charges and discharges can net a driver 150,000 miles. For comparison, it is common for a cell phone battery to degrade enough within two years for folks to consider getting a new one. Cell phone batteries are often fully charged and discharged once each day. Over 2 years, that would be approximately 730 cycles. On average, EV batteries are built to last over 1000 cycles.

Do car batteries degrade like my mobile phone?

Car batteries are the same sort of battery as those found in your phone or laptop, but they are designed, built, and maintained so that they last. We generally see 1-2% range degradation per year, with slightly faster degradation over the first 50,000 miles as the car settles into its long term state.

We generally see 1-2% range degradation per year...

Some of this comes down to design choices meant to optimize lifetime in EV batteries, and some of it is due to the battery management system that cars have to protect from heat, high voltage, and extreme state of charge. Read all about it!

EV batteries degrade differently than mobile phones

Does battery health degrade in a straight line? 

Battery health does not degrade in a straight line. When a new lithium ion battery is first used, there can be a short but dramatic decline in health right out of the gate. However, this is totally normal. This happens because some of the lithium salts in the battery react with the other materials to form the protective SEI layer. In our observations, this first phase of degradation only lasts for the first 10-20,000 miles. 

Battery health does not degrade in a straight line.

After this initial protective layer is built up, battery degradation levels off and decreases slowly and in a straight line. This is what EV owners and drivers will see for most of the car’s use. In a well functioning battery, degradation is limited and will be hard to detect over short periods of time. It can only be noticed over years. Additionally, the manufacturer may allow more of the battery to be used as it ages, masking some degradation effects in the short term.

Finally, as the battery nears the end of its life, it will have a sharp decline in capacity and performance. Most EV drivers will never see this, since the high voltage battery is considered “dead” when it is only around 70% empty! And, as a reminder, most EV drivers have not seen a battery degrade that far, since that is when most cars are eligible for a battery replacement. Very few modern EVs have been on the road long enough to reach this failure point.

battery health and degradation follows an S-curve

How long do EV batteries last?

If you’ve been paying attention, it can seem like EV batteries are a ticking time bomb. However, they are actually quite robust and last longer than you’d expect - modern EV batteries are expected to outlast the lifetime of the car itself.

Modern EV batteries are expected to outlast the lifetime of the car itself.

At a bare minimum, the federal government requires a warranty for at least 8 years or 100,000 miles for all EV batteries. Manufacturers don’t want to pay to replace such expensive components, so they work hard to meet that minimum. 

EV battery replacements by model year

We don’t know exactly how long the EV batteries on the road today will last because we haven’t seen many hit the end of life stage, yet. However, from observing the oldest models we can - Tesla Model S and Nissan LEAF - we see a range of outcomes. Early Nissan LEAFs had passive thermal management systems and problematic chemistries for hot climates, so the first generation batteries often degraded enough to warrant a replacement. The lesson was that high heat absolutely impacts battery health. On the Tesla Model S, we see that many older models still get 80% of their original range, which is an indication that the batteries have held up quite well over the decade since they were released. 

Can EV batteries be replaced? 

EV batteries can absolutely be replaced. If there is something wrong with the battery construction or management system, it may need to be replaced while it is still under warranty. If it is not under warranty, it is good to understand the costs of a replacement (in the next section).

This replacement will be performed by a repair shop that works with the car manufacturer or through a dealer. Some battery designers even make packs in which only faulty cells can be replaced, rather than the whole pack. 

However, most healthy batteries will only need to be replaced after 15+ years of life. There are body shops who specialize in this work since special equipment and training is required. 

How much is a battery replacement? 

There are a lot of rumors about the prohibitive cost of battery replacement. While we have seen price quotes as high as $20,000, the majority of replacements happen under warranty. Depending on the car and the reason you need to have the battery replaced, the cost to replace may be more or less than this. If your battery is going to fail, the ideal is to catch it early and get it fixed under warranty. 

Are there apps or tools to monitor EV battery health?

There are many ways you can monitor your battery health, but Recurrent offers one of the least invasive solutions. While other options include hooking up a diagnostic tool to your car, or removing the battery for chemical testing, we use your vehicle’s telematic data to build machine learning models that predict your battery’s health today, as well as a prediction of what the range will be in three years.

Whether you’re an EV owner, shopper, or a dealer, we can share insights about a vehicle’s range. 

Comparing EV batteries over time