Battery buffers

The 0 to 100 % shown on your dashboard is usually not the full electrochemical range of the cells. Manufacturers typically keep buffers at the top and bottom, which affect how your charging habits translate into cell stress.

Gross versus usable capacity

Many EV specifications distinguish between gross (total) and net (usable) battery capacity. The difference is held back as buffers: usually a reserve at the bottom and often some margin at the top. When your car shows 100 %, the cells are generally not at their absolute maximum voltage.

Buffer sizes are often undocumented and can differ between models, model years and even software versions. Because buffers are set in software, they can change with updates, which may also shift the displayed range or SOH.

Why the top buffer matters for ageing

Calendar ageing tends to rise in steps as the graphite anode moves through its potential stages at higher SOC, and studies that limited the maximum SOC indicate that avoiding the very top extends lifetime. A top buffer therefore shields the cells from part of the most stressful range. As a result, a displayed 100 % in a car with a generous top buffer may be less stressful than 100 % on a bare lab cell.

This also means that lab results expressed in cell SOC cannot be mapped directly onto dashboard percentages. A displayed 80 % may correspond to a somewhat different cell SOC, depending on how the buffers are arranged.

The bottom buffer

The bottom buffer protects against deep discharge, which can harm some cell chemistries, and keeps a reserve for the car's own systems. It also means that a displayed 0 % does not mean the cells are empty, although it is wise not to rely on driving into that reserve.

How Voltgevity handles buffers

Where buffer information is available, Voltgevity converts displayed SOC into an estimated cell SOC before applying its ageing models. Where it is not, Voltgevity uses typical assumptions and widens the uncertainty range. This is one reason predictions for two cars with the same nominal capacity can differ.

Key takeaways

A displayed 100 % is usually not the cells' absolute maximum.
Buffer sizes are often undocumented and may change with software updates.
A top buffer reduces high-SOC stress, so dashboard SOC and lab cell SOC are not directly comparable.
Voltgevity maps displayed SOC to an estimated cell SOC and widens uncertainty where buffers are unknown.