EV Battery Health after 250 Million Electric Car Miles

EV Battery Health after 250 Million Electric Car Miles

250 million is a lot. That’s how many miles Recurrent vehicles have driven since they connected to our services to monitor their EV performance and health. There are some optimistic, high-level takeaways in all this data.

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.

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.

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, which is typically a liquid buffer between the anode and cathode.

The general process is depicted below.

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.

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.

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.

2. Extreme State of Charge

Batteries work because they turn chemical potential energy into electricity. When fully charged, the battery is less stable, encouraging loss of lithium to side reactions.

3. Use in Very Low Temperatures

A battery’s performance does decrease in very low temperatures. Temperature is a measure of energy, so less available power signifies less energy overall.

4. Fast Charging

Fast charging can be physically taxing for a battery, creating a lot of heat which can lead to heat-based degradation.

5. Usage and Mechanical Stress

Using the battery also contributes to degradation. Moving lithium ions from one side of the battery to the other is taxing and can create further wear.

Do car batteries degrade like my mobile phone?

Car batteries are designed for longevity. 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.

Does battery health degrade in a straight line?

Battery health does not degrade in a straight line. There can be a short but dramatic decline in health right at the start, but this is normal as the protective SEI layer forms during initial use.

How long do EV batteries last?

Modern EV batteries are expected to outlast the lifetime of the car itself. At a bare minimum, federal law requires a warranty for at least 8 years or 100,000 miles for all EV batteries.

Can EV batteries be replaced?

EV batteries can absolutely be replaced. This process will be performed by a repair shop associated with the car manufacturer.

How much is a battery replacement?

While quotes can go as high as $20,000, most replacements happen under warranty. Catching a battery failure early can minimize costs.

Are there apps or tools to monitor EV battery health?

Recurrent offers a solution to monitor battery health using telematic data, providing insights about a vehicle’s range.