EV battery degradation: capacity loss, power fade and ageing

Battery degradation permanently reduces the energy a battery can store or the power it can deliver. Understanding the difference helps explain why an ageing EV may lose range, charge more slowly or reach its power limits sooner.

What degradation changes

Capacity fade reduces the charge or energy available between defined limits. Charge capacity is measured in ampere-hours (Ah); for driving range, the relevant quantity is usually usable energy in kilowatt-hours (kWh). The distinction between charge, energy and power is covered in Battery basics.

Suppose a battery delivered 80 kWh when new and now delivers 72 kWh over the same operating window, at the same temperature and under the same test conditions:

Energy retention = 72 kWh ÷ 80 kWh × 100 = 90%

That is a 10% loss of usable energy. At an unchanged consumption of 20 kWh/100 km, the corresponding calculated range falls from 400 km to 360 km. These are illustrative figures, not measurements of a particular EV.

Keep the reference window consistent. Comparing a new battery's gross capacity with an older battery's measured usable energy mixes two different quantities. Software changes to the accessible window can also change the energy a driver can use without reversing cell ageing. Battery buffers explains those boundaries.

Power fade concerns how quickly the battery can supply or accept energy. As internal resistance rises, the voltage falls further under discharge current and rises further during charging. This can bring a cell group to its voltage limit sooner. In a simplified resistive model:

Resistive heating power = current² × resistance

At the same current, twice the resistance produces twice the resistive heating power. The vehicle may need to reduce current to stay within voltage and temperature limits. A battery can therefore retain much of its measured energy while losing useful power capability. Capacity and resistance are separate measurements in battery assessment. Procedure for Assessing the Suitability of Battery Second Life Applications after EV First Life

A temporary reduction in performance has other possible causes. A cold pack, a high starting state of charge (SOC) or a charging-station limit can reduce charging power; weather and cabin heating can reduce driving range. Compare like conditions before attributing a change to permanent ageing. Battery charging

How batteries age

Calendar ageing develops with time, including while the car is parked. Cycle ageing accompanies charge moving through the cells. They interact: cycling can expose fresh surfaces to chemical reactions. Lithium ion battery degradation: what you need to know Smith and colleagues' experiments with 32 graphite/NMC cells examined how temperature, charge level, depth of discharge and charging rate shaped this ageing. Lithium-ion battery life model with electrode cracking and early-life break-in processes

Lithium becomes unavailable

A graphite anode develops a protective solid-electrolyte interphase, usually shortened to SEI. This surface layer limits further reactions between the electrode and electrolyte. Continued growth consumes lithium and electrolyte, leaving less lithium available to move between the electrodes. This is one form of loss of lithium inventory. Lithium ion battery degradation: what you need to know

Lithium plating is another route. During charging, lithium can deposit as metal on the anode surface instead of entering the electrode as intended. Some can become unavailable for subsequent cycling. Low temperature and excessive charging current for the cell's condition increase this risk. Lithium ion battery degradation: what you need to know

Electrode material loses function

Electrode particles can crack, lose electrical contact or undergo structural changes that reduce their ability to store lithium. This is loss of active material. Cracking can also expose fresh surfaces to the electrolyte, encouraging further side reactions. Lithium ion battery degradation: what you need to know

These processes overlap with rising resistance. Surface films, changes in the electrodes and restricted transport through the cell can affect both usable capacity and power. The proportions differ between cell designs and operating histories; a single capacity percentage cannot identify the underlying mechanism. Lithium ion battery degradation: what you need to know

The electrode materials themselves are explained in Cell chemistry and components. Preger and colleagues compared commercial LFP, NCA and NMC cylindrical cells under varied cycling conditions and reported the longest cycle life for the tested LFP cells; some lifetimes to 80% capacity were extrapolated because the cells had not yet reached that point. The advantage narrowed when measured by cumulative discharged energy, reflecting the LFP cells' lower capacity and voltage. These results compare specific cell designs: the authors also found that lifetime predictions could not be transferred reliably to other cells sharing the same chemistry, format and capacity. Degradation of Commercial Lithium-Ion Cells as a Function of Chemistry and Cycling Conditions

Temperature, charge level and cycling

Temperature and time at high charge

High temperature accelerates many of the side reactions involved in ageing. High state of charge can also promote side reactions through its effect on electrode potentials. Together, time, temperature and charge level influence storage ageing. Lithium ion battery degradation: what you need to know

The relationship varies with cell design and temperature. In Smith and colleagues' graphite/NMC tests, calendar fade did not rise steadily with state of charge. That finding supports using cell-specific evidence rather than assuming a universal percentage-to-wear curve. Lithium-ion battery life model with electrode cracking and early-life break-in processes

Temperature also changes what happens during charging. Warming a cold cell can improve lithium transport and allow a higher charging current before plating occurs. Rodrigues and colleagues demonstrated that charging limits depend on temperature, cell voltage, electrode design and current duration. Fast charging of Li-ion cells: temperature effects and lithium plating That explains why a car can deliberately warm its battery before a fast charge even though prolonged hot storage accelerates ageing. The vehicle's heating and cooling strategies are covered in Battery thermal management.

Depth of discharge and equivalent cycles

Depth of discharge describes the size of a charge swing. A change from 80% to 30% is a 50-percentage-point swing; 70% to 50% is a 20-point swing. The average charge level matters too: repeated 50–100% and 20–70% operation have the same swing but use different parts of the cell's operating range.

An equivalent full cycle (EFC) expresses accumulated throughput relative to a chosen full-capacity reference. For example, using discharged charge and a fixed 100 Ah reference:

Equivalent full cycles = cumulative discharged charge (Ah) ÷ reference capacity (Ah)

Two 50 Ah discharges, with recharging between them, add up to one EFC on this basis. Counting both the discharge and recharge in that formula would double-count the cycle. A two-way convention instead divides combined charged and discharged Ah by twice the reference capacity. Other reports use energy throughput, so check the definition before comparing cycle totals.

Equal throughput can cause different wear. As Smith and colleagues' tests demonstrate, cycle-life comparisons need the charge window, temperature and current, alongside the capacity-retention endpoint. Lithium-ion battery life model with electrode cracking and early-life break-in processes

Current and fast charging

Charging power in kW does not describe the load on each cell by itself. C-rate relates current to charge capacity: 100 A through a 100 Ah cell is 1C. Dividing battery-side kW by nominal kWh gives an approximation, affected by the relationship between actual voltage and the voltage used for the energy rating. Battery charging

Higher current increases resistive heating and can make lithium transport the limiting factor during charging. The permitted rate therefore changes as the pack warms up and fills. Cell design, cooling and the charge-control strategy determine how much current can be accepted; a charger's advertised power is not a battery-wear rating. The DOE fast-charging assessment describes these linked electrochemical and thermal constraints. DOE fast-charging technology assessment

What real-world data shows

Geotab's 2025 analysis covered over 22,700 EVs and 21 models, averaging 2.3% annual degradation. Its report is dated 28 April 2026:

These observational associations do not establish causation or individual forecasts. Geotab EV battery-health analysis

Among low-DC-charging vehicles only, annual degradation averaged 2.0% for those spending over 80% of their time below 20% or above 80% SOC, versus 1.4–1.5% for lower exposure. Both extremes were combined across driving, charging and rest; this does not establish equal effects or a universal safe time threshold. Geotab EV battery-health analysis

Geotab's hot-climate group averaged 0.4 percentage points higher annual degradation than its mild-climate group. The groups experienced more than versus less than 35% of days above 25 °C; there were insufficient cold-only vehicles to isolate their long-term climate effect. Geotab EV battery-health analysis

Geotab also reports that many EVs lose capacity faster initially, before the rate slows. Geotab EV battery-health analysis Later in life, interacting degradation mechanisms can produce an accelerating decline, often called a knee. A straight-line extrapolation from a short interval misses these changes. Lithium ion battery degradation: what you need to know

Interpreting battery health

State of health (SOH) compares a battery's present condition with a reference, usually its new condition. Check what the reported percentage describes: charge capacity, usable energy or a power-related measure. The battery management system estimates internal state from measurements and models; it does not directly measure one universal health percentage. Sensor accuracy, the available operating data and the estimation method affect the result. Analog Devices: state-of-charge and state-of-health estimation The measurements and calculations are explained in Battery management system.

A useful capacity comparison records:

• The quantity measured and its new-battery reference: Ah, gross energy or usable (net) energy.
• The starting and ending limits: the full net window or the displayed 100–0% window, including how energy available below displayed 0% is treated.
• Battery temperature, charge or discharge rate, and the test method.
• Whether the result is a BMS estimate or a controlled measurement.

Energy billed by a charger includes losses and auxiliary consumption downstream of its meter, so it cannot be substituted directly for energy subsequently delivered by the battery. Charging loss Likewise, a dashboard range estimate combines an energy estimate with consumption assumptions. A change in either can move the displayed range without an equivalent change in cell capacity. For a practical estimation method and its limits, see Checking battery health.

Tesla's US Model Y manual illustrates the distinction between an estimate and a dedicated test. It describes a BMS-based health evaluation and, on equipped vehicles, a separate test taking up to 24 hours on an AC connection. The test reports energy retention relative to new and may recalibrate the displayed range estimate. Tesla recommends using the test only if there is a concern about energy retention. Tesla Model Y: high-voltage battery health

Charge imbalance can also restrict accessible pack energy when one series group reaches a limit before the others. Cell balancing explains how correcting charge differences can recover that access, and why balancing cannot restore capacity lost through ageing.

Degradation, faults and continued use

Gradual capacity loss and a pack fault require different assessments. A battery with substantial retained energy can still have a defect that prevents operation. Conversely, a battery with reduced capacity can remain useful if it provides the range and power its owner needs. Battery assessment therefore considers operating condition and faults alongside capacity and resistance. Procedure for Assessing the Suitability of Battery Second Life Applications after EV First Life

A warranty capacity threshold is a contractual test criterion, not a physical point at which every battery stops working. See Battery warranty for the applicable terms and measurement requirements, and Battery lifecycle for continued use, repair, second-life applications and recycling.

For day-to-day use, follow the vehicle's own charging and storage guidance, including any chemistry-specific instructions. The practical reason to avoid unnecessary time hot and fully charged is the extra exposure to ageing conditions. Finishing a required full charge near departure reduces that exposure while preserving the range needed for the journey. Protecting your battery