Battery Degradation

Battery degradation is the permanent loss of energy-storage capacity or power capability caused by time, temperature, and use.

In an EV, degradation can appear as:

Less usable energy and driving range
Higher internal resistance
More heat generated under load
Reduced charging or regenerative-braking power
Lower peak or sustained drive power
One weak cell group limiting the rest of the pack

All rechargeable EV batteries degrade at rates shaped by chemistry, electrode design, battery temperature, state of charge, charging power, pack cooling, software limits, and vehicle use; this permanent loss should not be confused with temporary winter range loss or a battery-system fault.

What Battery Degradation Changes

Battery health is often reduced to remaining capacity. That is important, but it is not the complete picture.

Two batteries with the same measured capacity retention can behave differently if one has developed substantially higher internal resistance.

Capacity Fade

Capacity fade means that the battery stores less usable energy than when it was new.

For example, if a battery originally delivered 80 kWh across its usable operating window and later delivers 72 kWh under comparable test conditions, its measured usable capacity has fallen by 10%.

Capacity retention =
Current measured capacity ÷ Reference capacity when new

Capacity retention =
72 kWh ÷ 80 kWh
= 90%

Less usable energy can reduce driving range, although the actual range change also depends on vehicle efficiency, weather, tyres, speed, and route.

Resistance Growth and Power Fade

Internal resistance normally increases as a battery ages.

Higher resistance causes:

A larger voltage drop during acceleration
A larger voltage rise during charging
More heat generation
Earlier arrival at cell-voltage limits
Lower charging power
Less regenerative-braking capability
Reduced peak or sustained drive power

The relationship between current, resistance, and resistive heat is approximately:

Resistive heat ∝ Current² × Resistance

A battery can therefore retain much of its original energy capacity while becoming less capable of accepting or delivering high power.

Manufacturers often include sufficient power margin that moderate resistance growth is not immediately noticeable. Capacity loss may become visible through reduced range before the driver notices power fade.

Charge-Power Fade

A degraded battery may need to reduce charging current earlier because increased resistance causes cell voltage and temperature to rise more quickly.

Possible effects include:

Lower peak DC charging power
Earlier charging taper
Longer charging time
More active cooling
Greater sensitivity to cold temperature
Less repeatable charging over several stops

A slower charging session does not prove that the battery has degraded. Charger capability, battery temperature, arrival SOC, power sharing, and preconditioning must also be considered.

Cell Imbalance

Battery packs are limited by the first monitored cell group to reach its upper or lower voltage limit.

If one group has lost more capacity or developed higher resistance than the others, it may:

Reach full voltage early during charging
Reach minimum voltage early during discharge
Reduce usable pack energy
Cause charging to taper sooner
Limit drive power near empty

Cell balancing can correct differences in charge level, but it cannot restore capacity that a degraded group has permanently lost.

Temporary Range Loss Is Not Degradation

Driving range can change substantially without any permanent battery damage.

Temporary range loss can be caused by:

Low battery temperature
Cabin heating
Wet, snowy, or cold roads
Winter tyres
Strong wind
Higher motorway speed
Roof racks or roof boxes
Towing
Changed driving style
An inaccurate range estimate
A recent BMS recalibration

A cold battery has higher resistance and may reach its lower voltage limit earlier under load. Some of the stored energy can become more accessible after the cells warm or after the battery rests.

Permanent degradation remains after temperature, load, and SOC-estimation effects have been accounted for.

This is why the range displayed after charging is not a reliable battery-health measurement. The range estimator may be reacting to recent consumption rather than measuring battery capacity.

Calendar Ageing

Calendar ageing occurs with time, including while the vehicle is parked.

The battery does not need to complete charging cycles for chemical ageing reactions to continue.

Calendar ageing is strongly influenced by:

Time
Cell temperature
State of Charge
Cell voltage
Battery chemistry
Electrode and electrolyte design

High temperature increases the speed of unwanted chemical reactions. High SOC places many lithium-ion chemistries at a more stressful cell voltage. The combination of heat and prolonged high SOC is therefore particularly demanding.

This does not mean that occasionally charging to 100% is necessarily harmful. The more important distinction is between briefly reaching a high SOC before a journey and leaving the vehicle parked near full for days or weeks.

Battery storage stress at different temperatures and states of charge

Calendar ageing can result in:

Loss of usable lithium
Electrolyte decomposition
Growth of surface layers
Increased internal resistance
Gas generation
Reduced energy capacity
Lower charge and discharge power

Calendar and cycle ageing are useful classifications, but they are not completely separate. Many of the same chemical reactions occur during both, and cycling can create conditions that continue to cause ageing after the current has stopped.

Cycle Ageing

Cycle ageing is degradation associated with charging and discharging.

Every time energy moves through the battery, the electrodes expand and contract, ions move through the cell, and some energy is lost as heat. Small unwanted reactions also occur.

The effect depends on more than the number of times the charging cable is connected.

Important factors include:

Total energy throughput
Depth of Discharge
Average SOC
Charging C-rate
Discharge power
Battery temperature
Cell chemistry
Time spent resting between cycles

A shallow cycle under moderate conditions is not equivalent to a deep, high-power cycle at an unfavourable temperature.

Equivalent Full Cycles

One charging session does not equal one battery cycle.

An Equivalent Full Cycle, or EFC, represents cumulative energy throughput equal to 100% of the battery’s usable capacity.

Examples:

Two 50-percentage-point cycles equal approximately one EFC
Four 25-percentage-point cycles equal approximately one EFC
Five cycles using 20% of capacity equal approximately one EFC

Regenerative braking also contributes to charge throughput.

Geotab uses the same cumulative concept in its fleet analysis: repeatedly using and replacing 25% of capacity four times represents one complete equivalent cycle. (Geotab)

This makes EFC more useful than simply counting charging sessions.

Depth of Discharge and Average SOC

Depth of Discharge and average SOC describe different kinds of stress.

Depth of Discharge

Depth of Discharge describes how much of the battery window is used during a cycle.

Examples:

70% to 50% is a 20-percentage-point cycle
80% to 30% is a 50-percentage-point cycle
100% to 10% is a 90-percentage-point cycle

Repeated deep cycling can create more mechanical and electrochemical stress than shallow cycling, although the exact relationship depends on the chemistry and operating window.

Average SOC

Two cycles can have the same depth but take place at different average charge levels:

20% to 70%
50% to 100%

Both use 50 percentage points, but the second cycle spends more time at high cell voltage.

The higher average SOC can increase calendar stress even though the depth is identical.

Battery cycle stress at different states of charge

Do EVs Need to Stay Between 20% and 80%?

The widely repeated 20–80% rule is too simplistic.

Geotab’s updated fleet analysis found little difference between vehicles with low and moderate exposure to SOC below 20% or above 80%. A clearer increase appeared only in the group that spent more than 80% of its total time at those extremes. That high-exposure group averaged 2.0% annual degradation, compared with 1.4–1.5% in the lower-exposure groups. (Geotab)

This does not mean that SOC has no effect. It suggests that prolonged time near full or empty matters more than occasionally using those parts of the battery.

For most owners:

Using 100% before a long journey is normal
Arriving home or at a charger with low SOC is normal
Leaving the vehicle near full for an extended period is less desirable
Leaving it nearly empty for an extended period should be avoided
Manufacturer-specific charging guidance takes priority over generic rules

Some LFP vehicles are also instructed to charge fully at defined intervals so the BMS can calibrate SOC accurately.

What Changes Inside the Cell

Battery degradation is not one single reaction. Several mechanisms can occur simultaneously.

Loss of Lithium Inventory

Some lithium becomes trapped in side-reaction products and is no longer available for normal movement between the electrodes.

A major contributor is growth of the Solid Electrolyte Interphase, or SEI, on the negative electrode.

The SEI is necessary because it protects the electrode from continued electrolyte decomposition while still allowing lithium ions to pass. However, forming and growing the layer consumes active lithium and electrolyte.

Laboratory and diagnostic studies identify loss of lithium inventory through SEI growth as a major source of early capacity fade, followed in some cells by increasing loss of active material. (US Department of Energy)

As the SEI becomes thicker or damaged, it can also increase internal resistance.

Loss of Active Material

Parts of an electrode can stop participating in the battery reaction.

Possible causes include:

Particle cracking
Structural phase changes
Electrode delamination
Loss of electrical contact
Binder deterioration
Dissolution of active material
Blocked ion pathways

The active material can still be physically present inside the cell but no longer connected well enough to store and release ions.

Research into battery ageing identifies both loss of lithium inventory and loss of active material as important contributors to capacity and power fade. (US Department of Energy)

Electrolyte Degradation

The electrolyte is not perfectly stable across the complete cell-voltage and temperature range.

It can react with the electrodes, particularly at:

High cell voltage
Elevated temperature
Damaged electrode surfaces
High charging current
Contaminated or defective areas

Electrolyte decomposition can contribute to:

Gas formation
Resistance growth
Loss of lithium
Surface-film growth
Reduced ion conductivity
Dry areas within the electrode structure

The electrolyte and electrode interfaces are closely connected. Damage to one component can accelerate degradation elsewhere in the cell.

Lithium Plating

During charging, lithium ions should move into the structure of the negative electrode.

If lithium reaches a graphite-based anode faster than it can be inserted, metallic lithium can deposit on the surface.

Lithium plating is more likely when:

The cell is cold
Charging current is high
SOC is high
The negative electrode is thick
Internal resistance has increased
Ion pathways are uneven
The cell has manufacturing defects

DOE research identifies high charge rate, low temperature, and high electrode loading as important plating conditions. Deposited lithium can cause capacity loss, increased resistance, and potentially an internal short circuit. (US Department of Energy)

Some plated lithium may be reversible, but part of it can lose electrical contact and become inactive dead lithium. A normal discharge does not necessarily restore it.

This is why a cold EV may sharply restrict DC charging and regenerative braking until the battery has warmed.

Mechanical Damage and Silicon Expansion

Electrode materials change volume as ions enter and leave them.

Graphite handles this movement relatively well. Silicon can store much more lithium but expands far more during charging.

Repeated expansion can:

Crack silicon particles
Damage the electrode coating
Break electrical connections
Disrupt the SEI
Expose fresh surfaces
Consume additional electrolyte and lithium
Increase cell swelling

Silicon-graphite and silicon-dominant anodes therefore require carefully engineered particles, binders, electrolyte additives, porosity, and pressure control.

Calendar and Cycle Ageing Interact

Calendar ageing continues during driving and charging, while cycling changes the conditions under which calendar-type reactions occur.

For example:

Fast charging raises temperature
High SOC after charging increases voltage-related stress
Driving can warm the pack before it is parked
Cracked particles expose new surfaces that react while the vehicle rests
SEI growth raises resistance, creating more heat during later cycles

It is therefore not possible to assign every percentage point of degradation neatly to either time or cycling.

The distinction remains useful because it explains why both low-mileage older EVs and high-mileage newer EVs can show measurable degradation for different reasons.

Temperature and Degradation

Temperature affects nearly every ageing mechanism.

High Temperature

Heat accelerates chemical reactions inside the cell.

Prolonged high temperature can increase:

SEI growth
Electrolyte decomposition
Gas generation
Transition-metal dissolution
Calendar ageing
Resistance growth

Geotab’s latest fleet data found vehicles in its hot-climate group degraded an average of 0.4 percentage points faster per year than vehicles in milder conditions. The study defined the groups according to the share of days above 25°C and could not isolate consistently cold climates in the same way. (Geotab)

Active cooling reduces the effect, but it cannot remove every temperature difference between climates and usage patterns.

Low Temperature

Cold storage is generally not equivalent to hot storage. Low temperature slows many ageing reactions.

The main concern is charging a cold graphite-based cell too aggressively.

At low temperature:

Ion movement slows
Internal resistance increases
Graphite accepts lithium more slowly
Cell voltage rises more quickly during charging
Lithium-plating risk increases

The BMS should restrict charging current or heat the battery before permitting high power.

Cold-weather charging stress therefore depends heavily on preconditioning, thermal management, and charging software.

Fast Charging and Degradation

DC fast charging is a normal design use of a modern EV. The battery and BMS are built to accept it within defined limits.

The degradation effect depends on much more than the number printed on the charger:

Actual battery power
Battery capacity
Charging C-rate
Battery temperature
Arrival SOC
Charging curve
Cell design
Cooling performance
How often high power is used

A 150 kW charge represents approximately:

1C for a 150 kWh battery
2C for a 75 kWh battery
3C for a 50 kWh battery

The same charger power therefore does not create the same cell-level stress in every vehicle.

Real-World Fast-Charging Data

Geotab divided more than 22,700 EVs into charging-use cohorts. Vehicles where DC charging represented less than 12% of charging sessions averaged 1.5% annual degradation. Vehicles using DC charging more frequently averaged 2.5%. (Geotab)

Among the frequent-DC group, charging power also mattered:

Charging-use group	Definition	Average annual degradation
Low DC frequency	DC charging below 12% of all charging sessions	1.5%
High-frequency, lower-power DC	DC charging above 12%; fewer than 40% of DC sessions above 100 kW	2.2%
High-frequency, high-power DC	DC charging above 12%; more than 40% of DC sessions above 100 kW	3.0%

These are aggregated cohort averages, not predictions for a particular vehicle. Geotab anonymised the models and did not isolate chemistry, pack size, cooling design, climate, and charging curve into controlled laboratory variables. The results nevertheless show a clear association between frequent high-power charging and faster capacity loss across the dataset. (Geotab)

The practical conclusion is not that owners should avoid DC charging. Fast charging exists to make long journeys and intensive use possible.

Use it when it provides practical value. Slower charging is preferable when the car is already parked for several hours and rapid charging offers no benefit.

High-Power Driving and Regeneration

High discharge power also creates heat and mechanical stress, but its effect depends on duration and thermal control.

Short acceleration bursts are usually less demanding than:

Sustained high-speed driving
Long mountain climbs
Towing
Track use
Repeated full-power acceleration

Regenerative braking adds charging throughput. The BMS limits regeneration when temperature, SOC, voltage, or power capability make high charging current unsuitable.

For most private owners, normal acceleration and regenerative braking are unlikely to dominate battery ageing. Time, temperature, SOC exposure, and charging pattern are usually more significant.

Differences Between Cell Chemistries

Named chemistry does not determine degradation by itself, but it changes the baseline trade-offs.

NMC, NCA, and NCMA

Nickel-based cells offer high energy density and strong power capability.

Their ageing can be sensitive to:

High cell voltage
High temperature
High-nickel cathode stability
Particle cracking
Electrolyte oxidation
Transition-metal dissolution

Manufacturers manage these stresses through coatings, electrolyte additives, voltage limits, cooling, and BMS calibration.

LFP

LFP generally offers:

Long cycle life
Strong thermal stability
Good tolerance of frequent cycling
Lower cathode-material cost

It is not immune to degradation.

LFP cells still experience:

SEI growth
Loss of active lithium
Electrolyte ageing
Resistance increase
Cell imbalance
Cold-charging limitations

Long-term storage at high SOC and high temperature can still create avoidable stress.

LFP’s flat voltage curve also makes SOC estimation more difficult, which can make apparent range or percentage changes look like degradation when the BMS is recalibrating.

Silicon-Containing Anodes

A higher silicon content can increase energy density, but it also creates more electrode expansion and repeated surface-film damage.

The degradation result depends heavily on:

Silicon content
Particle structure
Binder
Electrolyte additives
Cell pressure
Charging strategy

A battery advertised as using silicon does not automatically have poor durability. The implementation matters.

Sodium-Ion

Sodium-ion cells use different electrode materials and have different ageing mechanisms from lithium-ion cells.

Potential advantages in cost and temperature behaviour do not automatically translate into longer life. Long-term automotive field data remains limited compared with established lithium-ion chemistries.

Claims about sodium-ion durability should therefore be evaluated for the specific cell design rather than applied to the complete technology family.

Degradation Is Usually Not Linear

A battery does not necessarily lose the same percentage of capacity every year.

A typical curve may include:

An initial settling period with relatively visible early loss
A long period of gradual degradation
A possible accelerated late-life region, sometimes called the knee

Not every EV battery develops a pronounced knee within the life of the vehicle.

Geotab observed that many newer vehicles in its data showed a sharper decline during the first year or two before stabilising. Eight of the 11 established models carried over from its earlier dataset averaged 1.4% annual degradation after their results had stabilised. (Geotab)

This is why multiplying an average annual percentage by the vehicle’s age is unreliable:

2.3% × 10 years ≠ a reliable prediction of 23% loss

The 2.3% figure is a cohort average, not a universal linear ageing law.

Degradation Is Not the Same as Battery Failure

A battery can degrade gradually without failing.

A battery-system failure can instead be caused by:

A defective cell
An internal cell short circuit
Excessive self-discharge
A failed contactor
An isolation fault
A coolant leak
A faulty voltage or temperature sensor
A damaged busbar
A communication failure
Collision or underbody damage
A failed Battery Management System component

A pack can retain 95% of its energy capacity and still require repair because one electrical component has failed.

Another pack can operate normally with 80% remaining capacity but provide less driving range.

This distinction also matters for warranties. A defect warranty and a capacity-retention warranty do not necessarily cover the same conditions.

How Battery Health Is Measured

Measuring battery health accurately is more difficult than reading the dashboard range.

Common methods include:

BMS-reported State of Health
Controlled energy-discharge testing
Manufacturer diagnostic procedures
Workshop capacity tests
Measuring energy across a known SOC interval
Analysing cell-voltage spread and resistance
Long-term telematics analysis

BMS-Reported State of Health

The BMS estimates battery health from data such as:

Energy throughput
Current integration
Voltage behaviour
Internal resistance
Cell imbalance
Temperature history
Charging history
Battery age

A displayed SOH percentage may represent capacity retention, power capability, or a proprietary combination.

SOH figures from different manufacturers are not necessarily comparable because they may use different:

Reference capacities
Buffers
Temperature corrections
Test methods
Estimation models
Definitions of “new”

Controlled Energy Test

A controlled capacity test measures energy delivered across a defined SOC window under known conditions.

For a useful comparison, the test should account for:

Battery temperature
Starting and ending SOC
Vehicle auxiliary consumption
Charging and discharge losses
BMS calibration
Whether the reference is gross or usable capacity
Battery buffers

A workshop or built-in manufacturer test is usually more reliable than trying to infer degradation from one charging invoice.

Tesla Battery Health Test

Supported Tesla vehicles include a built-in Battery Health Test.

Tesla states that the vehicle must be connected to an AC charger and that the process can take up to 24 hours. The test may discharge the battery to 0%, then reports energy retention compared with when the battery was new. Tesla also notes that the displayed range may be recalibrated afterward. (Tesla)

Tesla recommends running the test only when there is a concern about battery energy retention.

This type of controlled process is fundamentally different from comparing the displayed range at 100% on two separate dates.

Why Displayed Range Is a Poor Health Test

Displayed full-charge range can change because of:

Recent energy consumption
Temperature
Software updates
BMS calibration
Wheel and tyre configuration
Driving mode
Certification-value changes
Battery degradation

Some vehicles show a fixed rated range derived from SOC, while others use a prediction based on recent driving.

A lower dashboard number is therefore not enough to diagnose battery degradation.

Real-World Battery Degradation Data

Large fleet datasets provide a useful view of real-world ageing, but they need careful interpretation.

Geotab 2026 Publication

Geotab’s updated 2026 publication is based on an analysis of more than 22,700 EVs across 21 anonymised models.

It reports:

Average across all analysed vehicles: 2.3% per year
Light passenger cars: 2.0% per year
Multipurpose vehicles, including vans: 2.7% per year
Stable carry-over models: 1.4% per year on average
Hot-climate penalty: approximately 0.4 percentage points per year
High-frequency, high-power DC cohort: 3.0% per year
Low-DC-frequency cohort: 1.5% per year (Geotab)

Geotab’s previous analysis reported 1.8% per year. The increase to 2.3% does not prove that newer batteries are worse. Geotab attributes it to differences in vehicle mix, higher use, more powerful charging, and a greater number of newer vehicles still in their initial decline phase. (Geotab)

The limitations are important:

Vehicle models are anonymised
The data is strongly influenced by fleet use
Battery chemistry is not isolated
Thermal design and battery size differ
Annual rates are cohort averages
The figures should not be extrapolated as straight lines

The study is most useful for identifying trends, not predicting the exact health of one car.

Tesla Fleet Data

Tesla’s 2023 Impact Report states that Model 3 and Model Y batteries had lost about 15% of their original capacity on average after 200,000 miles.

This is manufacturer-reported fleet data rather than an independent controlled study. Tesla also notes that mileage is only one factor and that battery age affects retention. (Tesla)

The data is still useful because it covers high-mileage production vehicles. It should not be assumed to represent every battery chemistry, factory, model year, climate, or use pattern.

What Degradation Means for a Used-EV Buyer

Battery health is one of the most important specifications on an older EV, but it should not be judged from age or odometer reading alone.

Useful information includes:

Remaining usable capacity
Manufacturer or independent SOH test
Cell-voltage spread
Internal-resistance trends
Charging curve
Battery and drivetrain warnings
Warranty history
Repair or module-replacement history
Climate and usage history
Whether the car can precondition and cool the battery correctly

A high-mileage vehicle with good thermal management and moderate battery stress can have a healthier pack than a lower-mileage vehicle stored at high SOC in a hot climate.

Questions Worth Asking

Has the battery received a documented health test?
Is the test based on measured energy or only a BMS estimate?
Does the vehicle still reach its expected charging power when properly prepared?
Are there unusual cell-voltage differences?
Has the pack or any module been repaired?
Does the warranty transfer to the next owner?
What capacity threshold does the warranty use?
Is the quoted capacity gross, usable, or current measured capacity?

A transparent battery-health certificate is far more useful than a dashboard photograph showing estimated range.

What Drivers Can Do

Battery degradation cannot be stopped, but unnecessary stress can be reduced.

Practical habits include:

Follow the charging guidance for the specific vehicle
Avoid leaving the battery near full for long periods when the range is not needed
Avoid storing the vehicle nearly empty
Precondition before high-power charging in cold weather
Use DC fast charging when it provides practical value
Prefer AC or lower-power charging when the car will already be parked for hours
Avoid unnecessary prolonged heat exposure when practical
Leave the battery at a moderate SOC during long-term storage
Keep thermal-management and charging software updated
Do not obsess over occasional full charges or normal road-trip fast charging

Owners should use the battery to make the vehicle useful. Avoiding every high SOC, deep discharge, or fast charge can create more inconvenience than meaningful battery-life benefit.

For practical ownership advice, see How to Extend EV Battery Life.

What Matters Most

EV battery degradation is driven by the interaction between time, temperature, State of Charge, energy throughput, and battery design.

The most important distinctions are:

Temporary range loss is not necessarily degradation
Capacity fade and power fade are different
Calendar and cycle ageing overlap
High temperature is a major long-term stressor
Cold becomes particularly relevant during charging
Frequent high-power DC charging is associated with faster degradation, but remains a normal EV use
Time spent at extreme SOC matters more than occasionally passing through it
Battery ageing is rarely perfectly linear
Degradation is not the same as component failure
Dashboard range is not a battery-health test

Modern EV batteries are generally proving durable, but there is no single percentage that describes every vehicle. Chemistry, cooling, cell design, BMS calibration, climate, and use pattern determine how each pack ages.

For warranty thresholds and the options available after vehicle service, see EV Battery Warranty and EV Battery Lifecycle, Repair, Second Life and Recycling.