When we talk about the lithium-ion battery in an electric car, we generally refer to two values linked to the amount of energy it can store: gross capacity and net capacity.

But what are they? More importantly, why do we make this distinction? When we talk about petrol tanks, we are essentially talking about the total capacity and, at most, the part devoted to what is known as the 'reserve'.

In the case of electric cars, the question is a little more complex. As we know, when lithium-ion batteries are fully charged or fully discharged, the cells tend to degrade more quickly. The difference between gross and net capacity is in fact a kind of safety buffer that manufacturers set aside to keep the battery in good health for longer.

A safety buffer

To all intents and purposes, even when the charge is zero, the battery is not completely discharged. It simply doesn't have enough energy left to power the car's motors and keep on running. Or at least, to continue driving normally. In fact, in many cases, when the zero charge is reached (or close to that value), the car continues to drive, putting in place a kind of protection that minimises energy consumption to enable the driver to reach a charging point or, at least, to get out of a dangerous area.

Volvo e StoreDot, batteria con anodo in silicio

The StoreDot battery with silicon anode tested by Volvo: it has a higher energy density.

The same applies when the display shows 100%. In this case, the battery is not fully charged. It has simply stored the maximum amount of energy it can use to move the car.

The few kWh between gross and net capacity are also used during charging. They are used to smooth the charging curve and optimise the process by which the battery receives electricity.

2020 Lexus UX 300e ricarica

A variable percentage

Once you've understood the function of this buffer, you'll realise that the difference between net capacity and gross capacity varies from one model to another, and that it represents a higher percentage in smaller batteries.

Let's take an example. The Mini Cooper SE has a battery with a gross capacity of 32.6 kWh and a net capacity of 28.9 kWh. These 3.7 kWh less represent 11.4% of the total. Still at BMW, the i7's battery has a gross capacity of 105.7 kWh and a net capacity of 101.7 kWh. The buffer in this case is 4 kWh, or 3.8%.

BMW i7 2022

The BMW i7's enormous battery 

  Gross capacity Net capacity Buffer
Mini Cooper SE 32.6 kWh 28.9 kWh 3.7 kWh /11%)
BMW i7 105.7 kWh 101.7 kWh 4 kWh (3.8%)

Some manufacturers, to avoid any confusion, prefer not to declare the gross capacity, but simply give the value of the kWh that can actually be used. This is what happened in the past with Porsche, for the Taycan, and what is still happening with Lucid Motors, for the Air.

This is of little importance. It's a well-known fact that, regardless of gross and net capacity, it's always a good idea to maintain a state of charge that doesn't fall below certain percentages and, if possible, doesn't frequently reach 100%. By doing so, you can be sure that the battery will last a long time and that its performance will remain virtually unchanged over time.

Gallery: Renault Megane E-Tech Electric