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What's the Difference Between Capacity and Energy?

What's the Difference Between Capacity and Energy?

2022-05-31

Although we use the word capacity in everyday conversation, it has a particular meaning to battery scientists, which can affect how we understand battery performance. In the simplest terms, a battery’s capacity describes how many electrons it can store for later use.

A battery’s capacity does not tell you the amount of energy it stores or the driving range it can deliver. Even with good capacity, it’s not possible to know how much energy the battery stores without knowing the voltage. This is because a higher voltage will deliver more energy for a given capacity. The math is simple:

Energy (Watt-hours) = Capacity (amp-hours) x Voltage (volts)

Let’s look at an example using the equation above — if a battery has a capacity of 3 amp-hours and an average voltage of 3.7 volts, the total energy stored in that battery is 11.1 watt-hours — 3 amp-hours (capacity) x 3.7 volts (voltage) = 11.1 watt-hours (energy). This is roughly enough to power a smartphone for a day[1]; thousands of such batteries make up an EV battery pack.


This diagram provides an analogy that illustrates the difference between capacity and energy. The capacity is represented by the amount of water at the top of the hill and the voltage by its elevation. Energy is extracted by the mill at the bottom of the hill. To know how much energy the mill will be able to use, you need to know both the altitude and the amount of water. In batteries, the principle is the same; voltage has as much influence on the energy of a battery as its capacity.

Cycle life: Energy or capacity?

EVs may see their driving range fade after many charging cycles (this effect is measured as cycle life). This fade can be caused by factors such as repeated fast-charging or extreme temperatures. This is why QuantumScape subjects its batteries to tests over many hundreds of cycles, with fast charging speeds and typical operating conditions.

Predicting a battery’s useful lifetime, then, requires understanding how the battery loses the ability to store energy over time (or many cycles). This is why at Green Tech, we always report the discharge energy retention in our cycle life tests. This is a measure of how much energy the battery can deliver each cycle, and in our battery, this fades very slowly: after 10000 cycles, our battery can still deliver well more than 80% of the original amount of energy.

When cycle life tests only include capacity retention, not energy retention, this only tells half the story. Without also knowing how the voltage changes over time, it’s impossible to tell the difference between a battery that can last hundreds of cycles and a battery that will be almost useless after just a few dozen.

This matters because accelerating the electric vehicle revolution requires new batteries that can deliver improved performance for drivers while also withstanding many years of charging cycles. A battery that can pack more energy into an EV isn’t useful if that energy fades quickly in a few months, and a battery that can charge quickly isn’t useful if fast charging compromises the lifetime and value of the vehicle. So, the next time you see battery reporting on cycle-life performance, make sure to check whether it is actual energy or just capacity. If only capacity is reported, you don’t have everything you need to understand how that battery would fade over time in a real-world EV.

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