Battery Buffer and Usable Capacity

An EV battery rarely makes its entire declared energy content available for traction. The Battery Management System (BMS) defines an operating window, while displayed 100% and 0% are driver-facing limits rather than the cells’ absolute electrochemical endpoints.

The capacity terms EV buyers see

Battery-capacity figures are easy to compare only when they describe the same quantity. Manufacturers, regulators, test organisations, and vehicle databases do not always use gross, nominal, net, usable, and available in exactly the same way.

The most useful distinctions are:

Gross capacity: The manufacturer-declared total energy content of a new battery before the normal driver-accessible operating window is applied.
Net or usable capacity: Terms manufacturers commonly use for the energy the vehicle permits the driver to use for traction under defined conditions.
Displayed 100–0% capacity: The energy available from displayed 100% until the dashboard reaches 0%.
Accessible below-zero reserve: Energy that remains available for driving after displayed 0% but before the vehicle stops providing traction power.
Inaccessible protective margin: Energy outside the driver-accessible window, reserved by the battery-control strategy.
Currently available energy: What the battery can deliver before shutdown under its present temperature, current demand, state of health, and cell-balance conditions.

UN Global Technical Regulation No. 22 defines usable battery energy as the energy supplied from the start of its certification test until the applicable break-off criterion is reached. This is an important reminder that usable energy requires defined start and stop conditions; it is not simply a label attached to the pack. (UNECE)

EVKX uses net capacity for all driver-accessible energy from displayed 100% until propulsion stops. If a vehicle can drive below displayed 0%, that below-zero energy is included in EVKX net capacity. EVKX records it separately as a usable buffer when the amount is known.

The resulting relationship is:

Displayed 100–0% capacity =
Net capacity − accessible below-zero reserve

The declared difference between gross and net capacity can be calculated as:

Declared gross-to-net difference =
Gross capacity − net capacity

That difference is useful, but it does not reveal how the reserved energy is divided above and below the accessible window.

Examples of gross and net figures

Some manufacturers publish both figures clearly:

The Audi Q6 Sportback e-tron performance is specified with 100 kWh gross and 94.9 kWh net. The declared difference is 5.1 kWh, or 5.1% of gross capacity. (Audi)
The Audi Q8 55 e-tron and SQ8 e-tron use a 114 kWh gross battery with 106 kWh net. The declared difference is 8 kWh, or about 7.0%. (Audi)
The Porsche Taycan with Performance Battery Plus is specified with 105 kWh gross and 97 kWh net. The declared difference is 8 kWh, or about 7.6%. (Porsche)

These figures apply to the cited vehicles, model configurations, markets, and technical-data versions. They should not be transferred to a related model or a different model year.

They also do not tell us:

How much of the inaccessible margin is above the usable window
How much remains below the traction shutdown point
Whether any energy is available after displayed 0%
The cell voltages corresponding to displayed 100% and 0%
How much energy an aged battery can store now

Gross capacity is itself a rated value, not a measurement of every joule between destructive electrochemical limits. Temperature, discharge rate, cell variation, battery age, and the manufacturer’s test method all affect a measured result.

How the operating window works

The buffer is not a separate compartment, module, or group of cells. The BMS creates the operating window by controlling how far the same cells are normally charged and discharged.

A simplified battery window can contain:

An inaccessible upper protective margin
Displayed 100%
The normal displayed operating window
Displayed 0%
A possible accessible below-zero driving reserve
The traction shutdown point
An inaccessible lower protective margin

The illustration is conceptual. Cell voltage, stored charge, and stored energy do not have a perfectly linear relationship, and the boundaries depend on chemistry, temperature, ageing, current, and BMS strategy.

Upper protective margin

The upper margin prevents normal charging from entering a cell-voltage region the manufacturer has excluded from routine operation. It can provide tolerance for cell imbalance, measurement error, temperature variation, and charging control.

Restricting the upper end can also reduce time spent at particularly high cell SOC and voltage. Experimental work on commercial lithium-ion cells shows that ageing depends on the SOC window, chemistry, current, and temperature; it does not support a universal rule that one buffer percentage suits every battery. (Journal of Power Sources Advances)

Lower protective margin

The inaccessible lower margin keeps the cells away from damaging undervoltage and gives the BMS room to disconnect the traction system before the weakest monitored cell group exceeds its permitted limit.

The lowest group matters more than the pack average. A battery may still contain energy in its stronger groups when one group reaches its lower voltage limit. The relationship between group imbalance and usable pack energy is explained in Cell Balancing.

An inaccessible lower margin is not the same as a below-zero driving reserve. The first remains unavailable after traction shutdown; the second can still propel the vehicle.

Why EV batteries reserve energy

The BMS must keep every monitored cell group inside approved voltage, current, and temperature limits. A restricted operating window helps it:

Prevent overcharge and excessive discharge
Allow for differences between cell groups
Maintain measurement and control margins
Reduce power safely as the battery approaches a limit
Account for temperature-dependent voltage behaviour
Manage SOC-estimation uncertainty
Balance range, performance, durability, and warranty targets

The operating window is only one part of battery protection. Cell chemistry, electrode design, cooling, charging strategy, power limits, pack construction, software, and driver use can matter as much or more.

A larger gross-to-net difference therefore does not prove that a battery is safer or will last longer. It may reflect conservative limits, a particular cell chemistry, a measurement convention, an allowance for variation, or a design decision made for that vehicle.

The BMS functions that enforce these limits are covered in Battery Management System.

What displayed 100% and 0% mean

Displayed 100%

Displayed 100% normally means that the battery has reached the upper end of the driver-accessible window. It does not usually mean that every cell has reached its absolute electrochemical maximum.

Three limits should not be confused:

The manufacturer’s inaccessible upper margin
The vehicle’s displayed 100% point
A lower charging limit selected by the driver, such as 80%

Selecting an 80% daily charge limit does not create or replace the manufacturer’s protective margin. It stops charging earlier within the normal usable window.

The presence of an upper margin also does not guarantee full regenerative-braking power at displayed 100%. Regeneration is constrained by the permitted cell voltage, charging current, battery temperature, cell balance, and the power the pack can accept at that moment. The BMS may reduce regeneration even though the gross specification suggests that some inaccessible energy exists above the usable window.

Charging power, taper, preconditioning, and charge limits are explained separately in Battery Charging and Charging Performance.

Displayed 0%

Displayed 0% is a manufacturer-defined point near the bottom of the driver-facing scale. Depending on the vehicle, traction may stop at that point or some accessible driving energy may remain.

As the battery approaches its lower limit, the vehicle can progress through several stages:

Low-energy and charging warnings
Reduced climate or accessory power
Reduced acceleration and maximum power
A very low or disappearing range estimate
Displayed 0%
A possible below-zero driving reserve
Traction shutdown

The exact order and thresholds are model-specific. High power demand can make a weak or cold cell group reach its voltage limit sooner, so the energy available after displayed 0% is not necessarily repeatable.

Below-zero reserve is still usable capacity

If an EV continues driving after displayed 0%, the energy it uses remains driver-accessible energy. It is not part of the inaccessible lower protective margin.

This distinction matters when comparing specifications. A vehicle with 80 kWh available from displayed 100% to shutdown, including 3 kWh below displayed 0%, has:

80 kWh net capacity by the EVKX definition
3 kWh accessible below-zero reserve
77 kWh between displayed 100% and 0%

The amount left after traction shutdown cannot be measured by simply driving the car until it stops. That test measures the accessible window only.

Never treat the reserve as guaranteed range

In a 2025 test of six EVs, ADAC found that none stopped immediately at displayed 0% and all continued for about 15–20 km under favourable test conditions. ADAC also warned that the reserve could be much smaller in cold weather, with an aged battery, or in less favourable conditions. (ADAC)

That result describes six individual test vehicles, not an industry guarantee. Tesla’s Model 3 manual gives the appropriate driver rule: do not assume any range remains when the display reaches 0%. (Tesla)

Why measured and displayed energy can change

SOC is an estimate, not a direct measurement

State of Charge cannot be measured directly like liquid in a tank. The BMS estimates it from current flow, voltage, temperature, battery models, and reference conditions. Coulomb counting is useful over short periods, but small errors accumulate; voltage-based corrections also depend on chemistry, temperature, recent current, and rest time. Reviews of SOC-estimation methods describe why production systems combine measurements and models rather than relying on one signal. (Electronics)

The dashboard adds another layer. It can map the BMS estimate onto a driver-facing scale, filter rapid changes, and incorporate a manufacturer-defined reserve.

One displayed percentage point should therefore not automatically be treated as exactly one hundredth of the battery’s kilowatt-hours. The relationship can differ because:

Battery voltage changes through the discharge
Displayed SOC may be filtered or mapped
The BMS can revise its capacity estimate
Heating, cooling, and other battery-side loads consume energy
Temperature and current affect the point at which a cell reaches its voltage limit
Cell imbalance can make one group end the usable window early

A sudden SOC correction does not prove that the physical buffer changed. It may be an estimation correction.

Available energy depends on conditions

The gross and net specifications describe a new battery under defined conditions. The energy the car can deliver during one journey can be lower.

Common causes include:

Low battery temperature
High power demand
Increased internal resistance
A weak or imbalanced cell group
Battery degradation
Battery heating or cooling loads
A recent software or SOC-estimation correction

Some energy that is unavailable under high load can appear to return after the vehicle rests. With less current, cell voltage recovers from the resistance-related drop that occurred during driving. Warmer cells may also deliver more energy before reaching their lower-voltage limit.

This does not mean that the BMS has permanently released its protective margin. It means the battery can meet its operating limits under the new conditions.

Energy drawn from a charging station is also not a direct capacity measurement. Charging losses occur in the cable, onboard charger, power electronics, battery, and thermal systems. A charger can therefore deliver more kilowatt-hours than the increase stored in the battery.

How usable capacity can change

Software can change the operating window

The BMS controls the operating window, so a manufacturer can revise it if the cell and pack limits allow. A software change can:

Make more or less of the existing battery accessible
Change the displayed SOC mapping
Revise power limits near full or empty
Alter the point at which charging or traction stops
Improve the BMS capacity estimate without changing the physical window

The original Audi e-tron 55 provides a documented example. Audi offered an update for eligible 2019 and 2020 vehicles that increased the net capacity of the existing 95 kWh battery to about 86 kWh, alongside other efficiency changes. The physical pack did not become larger; Audi revised how it was operated. (Audi)

That example does not mean that every EV contains capacity that can safely be unlocked. An operating window is validated for a particular cell, cooling system, voltage range, power demand, ageing target, and fault margin. Changing one limit can affect several others.

Buffer and battery degradation

Battery degradation reduces the cells’ physical ability to store energy and deliver power. It is different from an operating-window change.

Three effects can produce a lower measured capacity or changed dashboard behaviour:

Physical degradation: The cells store less energy or have higher resistance than when new.
Estimation change: The BMS revises its estimate of SOC or available capacity.
Operating-window change: Software changes the permitted upper or lower boundary.

These effects can occur together and are difficult to separate from one uncontrolled road or charging test.

A manufacturer could, in principle, make part of a reserved margin accessible and temporarily reduce the decline in net usable energy. Doing so would not reverse physical degradation. It would only expose more of the remaining cell capacity.

EVKX does not assume that a manufacturer releases reserve capacity to conceal degradation without model-specific evidence. Battery health should be assessed against a defined test procedure, temperature, software version, and the vehicle’s capacity when new. UN GTR No. 22 follows this principle by comparing measured usable battery energy with certified usable battery energy under a harmonised procedure. (UNECE)

Capacity loss, resistance growth, testing, and warranty thresholds are covered in Battery Degradation.

How to compare battery-capacity figures

Before comparing two EVs, check:

The exact model, battery option, model year, and market
Whether each figure is gross, net, or measured usable energy
Whether net capacity includes energy below displayed 0%
Whether the number applies to a new battery or a specific used vehicle
Whether both results use the same start and stop conditions
Whether the value came from the battery or from energy supplied at the charger
Whether the source is the manufacturer, a certification test, or an independent estimate

Do not infer net capacity from gross capacity using a typical percentage. Buffer strategies vary too widely. Do not infer gross capacity by adding an assumed reserve to a published net figure.

Likewise, do not estimate battery health from one short trip by dividing the trip computer’s energy use by the percentage lost. Dashboard rounding, auxiliary loads, temperature, voltage recovery, and SOC corrections can dominate a small interval.

The most defensible comparison uses manufacturer or certification figures with matching definitions. A capacity test on an individual vehicle should use a wide SOC window, controlled conditions, a known software version, and battery-side energy measurement where possible.

Running out of usable energy

The BMS stops traction before the cells reach an unsafe lower limit, but running an EV to shutdown can still make recovery difficult. The high-voltage battery may stop supporting the low-voltage system, the vehicle may be unable to begin charging without external low-voltage support, and transport may be required.

The accessible reserve below displayed 0% is therefore a warning margin, not part of a journey plan. Its size can change with temperature, battery condition, power demand, cell balance, and software. A driver should follow the warnings and charging guidance for the exact vehicle rather than relying on a zero-mile test from another car.

Protective margins are normal battery engineering. Clear specifications should separate gross capacity, net capacity, displayed 100–0% energy, and any accessible below-zero reserve so buyers know which quantity they are comparing.