Battery Degradation

Battery degradation is the gradual loss of battery capacity and performance over time. In an EV, this usually shows up as reduced usable energy, slower charging in some conditions, higher internal resistance, or lower peak power.

All lithium-ion batteries degrade, but the rate depends heavily on how the battery is designed and how it is used. Temperature, state of charge, charging speed, depth of discharge, and time all play a role.

In practice, battery degradation is usually driven by two main processes:

• Calendar aging: degradation that happens over time, even when the battery is not heavily used
• Cycle aging: degradation caused by charging and discharging the battery

Both matter, and both are influenced by temperature and battery management.

Calendar aging

Calendar aging is battery degradation that occurs with time, even when the vehicle is parked or only lightly used.

Calendar aging is strongly influenced by:

• High temperature
• High state of charge
• Long periods spent at high voltage
• Time

This is one reason EV batteries should not be stored for long periods at 100% charge, especially in warm conditions. Heat and high SOC together create some of the most stressful conditions for lithium-ion cells.

During calendar aging, the battery slowly loses capacity and internal resistance increases. This can reduce range, reduce efficiency, and make the battery less able to accept or deliver high current.

Cycle aging

Cycle aging is the degradation caused by charging and discharging the battery.

Each cycle creates some stress inside the cell. Over time, the active materials in the electrodes change, side reactions consume lithium, and internal resistance tends to increase. This gradually reduces how much energy the battery can store and how efficiently it can deliver power.

Cycle aging depends on several factors, including:

• Depth of discharge
• Average SOC
• Charging speed
• Discharge power
• Cell temperature
• Cell chemistry

A battery that is repeatedly pushed to high charge and discharge rates under poor thermal conditions will usually degrade faster than one operating in a more moderate range.

Calendar aging vs cycle aging

For many EVs, calendar aging is just as important as cycle aging, and in some cases more important.

That is because a vehicle spends far more time parked than it does charging or driving. A battery that is regularly left at high SOC in hot conditions may degrade faster than a battery that is driven frequently but managed carefully.

This is also why battery protection strategies such as charge limits, buffers, and thermal management matter so much.

Effect of state of charge

Battery degradation is not equal across the full SOC window.

Very high SOC usually increases stress because the cells spend more time at higher voltage. Very low SOC can also increase stress, especially if the battery is left empty for long periods.

The middle part of the SOC range is usually gentler on the battery than the extremes.

Cycle aging based on SOC area

The diagram below shows how the state of charge area used during charging and discharging affects degradation.

This is why many manufacturers recommend charging to 80% or 90% for daily use, while reserving 100% charging for longer trips when the full range is needed.

Fast charging and degradation

Fast charging increases stress on the battery, but the effect depends strongly on temperature, SOC, and battery design.

The greatest risk usually appears when a battery is charged very quickly at:

• Low temperature
• High SOC
• High current
• Poor thermal control

One important degradation mechanism is lithium plating, where lithium deposits as metallic lithium on the anode instead of being stored normally within the anode structure. This can reduce available lithium, accelerate capacity loss, and in severe cases affect safety.

Fast charging does not automatically damage a battery in normal use. Modern EVs are designed to manage charging power through the Battery Management System and thermal system. But repeated aggressive fast charging under unfavorable conditions can contribute to faster ageing than slower AC charging or more moderate DC charging.

Temperature and degradation

Temperature is one of the most important factors in battery ageing.

High temperature accelerates unwanted chemical reactions inside the battery and can increase both calendar aging and cycle aging. Low temperature does not usually cause the same long-term damage on its own, but it can make charging more stressful if high current is applied before the battery is warm enough.

This is why EVs with strong thermal management and good preconditioning usually protect their batteries better during repeated fast charging.

Real-world degradation statistics

Real-world battery degradation is often lower than many buyers expect.

Geotab's 2024 analysis of approximately 5,000 EVs found an average battery degradation rate of about 1.8% per year, an improvement from its earlier 2019 analysis. The company concluded that most EV batteries are likely to last longer than the practical life of the vehicle.

Tesla's 2024 Impact Report also shows relatively modest battery-capacity loss over high mileage in its Model S and Model X fleet data.

These results should not be treated as universal for every EV, since battery chemistry, thermal management, charging behavior, climate, and usage patterns all matter. But they do show that modern EV batteries are generally more durable than many early assumptions suggested.

What affects degradation most in the real world

In practical EV use, the biggest factors are usually:

• Time
• Heat
• Spending long periods at very high SOC
• Repeated high-power charging in unfavorable conditions
• Poor thermal management
• Very deep cycling over many years

By contrast, occasional fast charging, occasional charging to 100%, or normal daily use are usually much less important than the overall pattern of temperature and time spent at stressful SOC levels.

How to preserve battery health

Battery degradation cannot be avoided completely, but it can be slowed.

Good practice usually includes:

• Avoid leaving the battery at 100% for long periods unless needed for a trip
• Avoid leaving the battery at very low SOC for long periods
• Use AC charging when convenient
• Use DC fast charging when needed, but avoid repeated high-power charging on a cold battery
• Let the battery precondition before fast charging in cold weather
• Park in moderate temperatures when possible
• Follow the manufacturer's charging recommendations

For more practical advice, read our guide on how to preserve your battery.

Summary

EV battery degradation is a normal result of time, temperature, and use.

The two main forms are calendar aging and cycle aging. Calendar aging is strongly affected by time, heat, and high state of charge. Cycle aging is influenced by how often and how hard the battery is charged and discharged.

Modern EV batteries are generally proving more durable than many people expected, especially when supported by strong thermal management, conservative buffers, and well-tuned battery-management software.