Battery buffer and usable capacity

An EV’s gross capacity, net (usable) capacity and displayed 100–0% energy describe different quantities. Protective margins limit the operating window, while some vehicles also provide usable energy below displayed 0%.

Gross, net and displayed-window energy

Manufacturers and test organisations use terms such as gross, nominal, net and usable with different measurement conventions. In this chapter, capacity figures in kWh describe energy; their start and stop conditions determine what can be compared.

Gross capacity is the manufacturer’s declared total energy content before the driver-accessible operating window is applied. It is a rated figure under specified conditions, rather than all the energy that could be extracted by taking cells beyond their permitted limits.

EVKX defines net, or usable, capacity as all driver-accessible energy from displayed 100% until propulsion stops. This includes any accessible reserve below displayed 0%. Manufacturers may use a different boundary for a figure labelled net or usable, so check the source definition. The Gross and Net Battery Capacity glossary entry provides a short reference for these terms.

Displayed-window energy covers only the interval from displayed 100% to 0%. For matching conditions:

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

For a hypothetical battery with 80 kWh net capacity, including 3 kWh available below displayed 0%:

Displayed 100–0% energy = 80 − 3 = 77 kWh

The 3 kWh reserve belongs inside net capacity. The inaccessible lower protective margin remains outside it, even after propulsion stops.

Published gross and net figures

The following manufacturer figures illustrate the declared gap between gross and net capacity. The difference is calculated from each source’s own pair of values.

Those differences are 5.1%, about 7.0% and about 7.6% of the respective gross figures. They do not establish how the margin is divided above and below the accessible window, or how much energy remains after displayed zero. Those boundaries require separate evidence for the battery version.

How protective margins work

The battery management system (BMS) controls how far the same cells can be charged and discharged. It monitors cell-group voltages and other measurements to keep operation within permitted limits. Analog Devices: SOC and SOH estimation The protective margin is the part excluded from the driver-accessible window.

The diagram is conceptual: its voltage labels are illustrative, not universal EV limits. Its percentage scale should not be read as the dashboard SOC scale. “Brick Buffer” denotes the inaccessible lower protective margin; the diagram does not separately show an accessible below-zero reserve. Where that reserve exists, it lies between displayed zero and traction shutdown, inside net capacity.

Upper and lower limits

The upper margin leaves part of the rated range outside normal charging. Keeping cells away from the upper end can reduce their exposure to high charge levels, but the ageing effect depends on the cell and its operating conditions. Smith and colleagues’ experiments with graphite/NMC cells found that temperature, charge level, depth of discharge and charging rate shaped capacity loss; calendar fade did not increase uniformly with charge level. Smith et al.: lithium-ion battery life model

At the lower end, control limits restrict discharge before cell groups enter damaging undervoltage conditions. A group reaching its limit can end discharge for the whole pack while other groups retain energy. Cell balancing explains how charge differences affect access to pack energy.

A larger declared gross-to-net gap therefore provides too little information to rank battery longevity. The operating window needs to be considered alongside the cell design and its temperature and charging history. Battery management system covers the measurements and control decisions that enforce the limits.

What displayed 100% and 0% mean

Full charge and a driver-selected limit

Displayed 100% marks the top of the vehicle’s normal driver-facing charge scale. A driver-selected limit, such as 80%, stops charging earlier within that scale; it does not redefine the manufacturer’s protective margin.

Regenerative braking also charges the battery and must respect its permitted charging power. An inaccessible upper margin is not a promise of extra room for regeneration at displayed full charge. Tesla’s US manual for the 2024-and-later Model 3, for example, says regeneration can be limited when the battery is cold or fully charged. Tesla Model 3 (2024+) US manual: braking and stopping Charging taper and the battery and station limits on charging power are covered in Battery charging.

Displayed zero and the remaining reserve

Displayed 0% is a point on the dashboard scale. Propulsion may stop there or continue into an accessible reserve. As energy runs low, the vehicle can issue warnings and reduce power; the thresholds and sequence depend on the model.

In September 2025, ADAC reported a test of six EVs: the VW ID.3, Tesla Model Y, Kia EV6, Volvo EX40, BYD Seal and Nio EL6. None stopped immediately at displayed zero, and ADAC described roughly 15–20 km of further travel with reduced power. The tests used young vehicles on a flat route in good weather. ADAC warned that in winter or with aged batteries the reserve could be much smaller or almost gone. ADAC: six EVs driven beyond displayed zero

That measured driving reserve gives no value for the inaccessible energy left after traction shutdown. Driving until the car stops measures the accessible window under the test conditions.

Follow the vehicle’s warnings rather than planning to use a reserve. Tesla’s US manual for the 2024-and-later Model 3 says not to assume any range remains at displayed zero. It also explains that running out of range can stop support for the low-voltage battery, requiring an external low-voltage supply before high-voltage charging can begin. Recovery instructions are vehicle-specific. Tesla Model 3 (2024+) US manual: running out of range

Why available energy can change

Conditions and estimates

The energy available on one journey can differ from the rated net figure. Under load, a cell’s terminal voltage falls; higher resistance or greater current can bring the limiting group to its minimum voltage sooner. A lower current can reduce that voltage drop without changing the programmed operating window. Temperature also affects the voltage and resistance used to determine permitted power. MathWorks: battery power estimator

Displayed SOC is an estimate of charge, mapped onto the vehicle’s driver-facing scale. Energy also depends on voltage, so a percentage-point change need not represent a fixed share of kWh. An estimation correction can change the display without adding energy or changing the physical limits. Analog Devices: SOC and SOH estimation The estimation methods are explained in Battery management system.

Software changes to the window

A manufacturer can revise the permitted window for a particular battery. Audi Norway’s October 2021 announcement provides a documented example: eligible e-tron 55 quattro vehicles built from mid-September 2018 to late November 2019, covering MY2019 and MY2020, could receive an update making 86 kWh of their existing 95 kWh battery usable. Audi also changed motor control and thermal management, so the range benefit was not solely a capacity increase. Audi Norway: software update for early e-tron 55 quattro vehicles

That was an operating change to an existing pack. It does not establish that another battery has the same headroom for an increase. A software update may instead alter the displayed SOC mapping or capacity estimate, so a changed dashboard reading alone cannot identify a revised upper or lower limit.

Separating buffer changes from ageing

Three different changes can affect the result a driver sees:

• Cell ageing reduces storage capacity or power capability.
• A revised estimate changes the reported SOC or available capacity.
• A revised operating window changes how much of the remaining cell capacity is accessible.

Making more of the remaining capacity accessible would not restore capacity lost through ageing. Stable reported capacity alone does not establish that a manufacturer has released reserved capacity to offset ageing. That requires model-specific evidence. Battery degradation explains capacity loss and resistance growth.

Comparing capacity figures and measurements

For a useful comparison, establish:

• The exact battery version and software, and whether the figure describes a new battery or a particular used vehicle.
• The measured quantity and boundaries: gross, net, or displayed 100–0% energy, including treatment of any below-zero reserve.
• The test conditions and measurement point, including temperature, load and whether energy was measured at the battery or charger.

Gross minus net is a calculation from two specified values. It cannot supply a missing net figure through an assumed standard buffer percentage.

Charger-billed energy includes losses and auxiliary consumption downstream of its meter, so it cannot substitute directly for energy delivered by the battery. Charging loss explains those measurement boundaries.

A drive-based estimate can help track changes when the recorded energy and SOC cover the same period and the comparison uses the same energy window. Dashboard rounding, omitted consumption and SOC corrections limit its precision. How to check EV battery health with a capacity estimate gives the procedure and explains how to choose a matching new-battery reference.

For a warranty claim, confirm which capacity definition, test procedure and threshold the manufacturer accepts before paying for a test. Battery warranty covers capacity guarantees and defect coverage.