Buffer

A battery buffer is the part of the battery's total energy capacity that is reserved by the battery management system (BMS) and not made available for normal driving. This is why an EV often has a difference between its gross battery capacity and its net usable capacity.

• Gross capacity is the total energy the battery pack can store.
• Net capacity is the part of that energy the vehicle allows the driver to use.
• Buffer is the difference between the two.

The BMS controls this buffer to protect the battery and to keep it operating within a safe and efficient state-of-charge window.

Why EVs use a battery buffer

The main purpose of a battery buffer is to protect the battery pack and support long-term performance.

A battery should not be charged or discharged all the way to its true electrochemical limits during normal operation. Doing so can increase degradation, reduce performance, and in extreme cases damage the cells.

By reserving part of the battery capacity at the top and bottom of the SOC window, the manufacturer can:

• Reduce stress on the cells
• Lower the risk of overcharging and over-discharging
• Improve durability
• Maintain more stable power delivery
• Help the battery operate within its preferred voltage range

This reserved energy is not necessarily wasted. It is a design choice that trades some usable capacity for better protection, performance, and long-term battery health.

Top and bottom buffer

A battery buffer usually consists of two parts:

• Top buffer: energy reserved above the displayed 100% SOC
• Bottom buffer: energy reserved below the displayed 0% SOC

The bottom buffer is sometimes informally called a brick buffer, because it helps prevent the battery from reaching a state where the cells become deeply discharged and potentially unusable.

The diagram below shows how a top and bottom buffer prevent the battery from operating at the true extremes of the cell's charge window.

The size of the buffer depends on the battery design. It can vary with cell chemistry, voltage limits, cooling capability, degradation targets, and how conservative the manufacturer wants to be.

The BMS continuously regulates charging and discharging to keep the battery within the approved operating range.

Can the buffer change over time?

Sometimes manufacturers change the usable battery window through software updates. This can happen when they learn more about how a battery behaves in real-world use or when they want to improve durability, charging behavior, or reliability.

Changing the usable window is possible because the BMS controls the pack's operating limits. However, it is not something manufacturers can do freely without trade-offs. Expanding usable capacity by reducing the buffer can increase stress on the cells, while increasing the buffer can reduce the energy available to the driver.

Are buffers used to hide degradation?

A common claim is that manufacturers use battery buffers to hide battery degradation by gradually unlocking previously reserved capacity.

EVKX has not seen clear evidence that this is a normal strategy among mainstream manufacturers. In practice, changing the usable battery window to mask degradation would also change cell voltage limits and operating conditions, which can affect ageing, performance, calibration, and charging behavior.

If a manufacturer were to change the usable SOC window significantly, it could often be detected through changes in the relationship between displayed SOC and measured cell voltage.

Hidden usable reserve and displayed SOC

Protective battery buffers are one thing. Displayed SOC behavior is another.

In some EVs, the SOC shown to the driver is not a perfectly linear representation of the remaining usable energy. This means that one displayed percentage point may not always represent the same amount of available energy across the full battery range.

In particular, some EVs appear to keep an additional hidden reserve close to 0%, where the energy available between 0% and shutdown is larger than the energy represented by most other 1% steps on the display. This is sometimes referred to as a zero buffer.

The diagram below shows how the lowest displayed percentages can represent a larger share of energy than the rest of the SOC scale.

Manufacturers may do this to reduce the risk of drivers running out of energy unexpectedly. From a vehicle usability perspective, that can be a reasonable safety margin.

However, it also has a drawback: if the final displayed percentages contain more usable energy than expected, drivers may charge earlier than necessary because the displayed SOC appears lower than the practical remaining range would suggest.

This can make the vehicle feel like it has less usable range near empty than it actually does.

EVKX view on hidden reserve

EVKX considers a protective bottom reserve normal and reasonable. What matters more is how transparently the remaining usable energy is communicated to the driver.

A large hidden reserve near 0% can make the displayed SOC less intuitive. That may reduce trust in the range display, especially for drivers who plan charging stops based on percentage rather than estimated remaining distance.

Examples of hidden reserve near 0%

Bjørn Nyland has tested several EVs to estimate how much additional energy is available below the displayed low-SOC range before the vehicle stops.

Some example results:

See Bjørn's test results for more data or all his Zero Miles Tests.

Summary

Battery buffers are a normal and important part of EV battery design. They help protect the cells, improve durability, and keep the battery operating within a safer and more efficient range.

A separate issue is how the usable energy is mapped to the driver's displayed SOC. When the lowest percentages contain a larger hidden reserve, the car may be easier to live with near empty, but the displayed battery percentage can become less intuitive.