AC, kW, SOH? Welcome to the dictionary of electric mobility

With electric mobility, several acronyms and units of measurement have appeared in front of our eyes much more frequently, which we rarely give much thought to. We only have to look at a charging station or a test in a specialised publication to ask ourselves: ‘But what does that mean?’. Come aboard on this journey into the dictionary of electric mobility.

So, let's start with AC and DC, which correspond to two types of electric current: Alternating Current (AC) and Direct Current (DC). If you're familiar with the names of Thomas Edison (1847-1931) and Nikola Tesla (1856-1943), two of the most prominent inventors humanity has ever known, you should know that they were responsible for leading the famous ‘War of the Currents’. Edison advocated direct current, which flows in a single direction, like a calm river following its course; Tesla, on the other hand, favoured alternating current, which changes direction many times a second, like a tide of energy coming and going.

AC ended up being the winner for most of the applications we know, largely because of its easier generation and, above all, its greater efficiency in transmission over longer distances, without energy losses along the way. On the other hand, DC is commonly used in applications where a constant voltage or current is required, such as battery-operated devices (including EVs), solar energy systems that generate DC power and some industrial processes.

But how does electric car charging work? Returning to the same dichotomy, the charger can be of two types: AC or DC.

The first one is more common at home or in public places, where the time factor is not so critical. When you charge a battery with AC, that energy must be converted to DC via a built-in device - a converter. It's like a chef who needs to turn the basic ingredients into a dish ready to serve. That's why AC charging tends to be slower.

On the other hand, DC chargers, are like fast-food restaurants, but with electricity. They directly supply direct current to the car battery, eliminating the need for conversion. These are the fast and ultra-fast chargers you see on motorways or at petrol stations, perfect for those in a hurry and need to get going quickly.

Let's move on to the magic acronyms that define power and consumption: kW and kWh. At first glance, they seem similar, but in fact they are very different.

The kW (kilowatt) is a unit of power for the energy generated. In the charging process, the more power, the faster the energy is delivered to the battery. This means, for example, that a 7 kW charger will charge a battery more slowly than a 50 kW charger.

This unit also tells us the power of an electric motor, which is decisive for the level of performance the car in which it is installed can achieve. As a unit of power, the kW is comparable to the horsepower (hp), the unit of measurement commonly used for combustion engines, and which has served as a reference since the first car was produced. One kW corresponds to 1.36 hp, which means that 100 kW in an electric motor is equivalent to 136 hp in an internal combustion engine.

And if KW expresses power, the kWh (kilowatt-hour) is the unit that defines quantity.

But quantifying a flow of energy is not as straightforward as in the case of fuel, where you can directly measure the litres that disappear from a tank. For this reason, to measure the electricity consumed (whether in cars or our homes), it has become customary to measure the power output over an hour.

To put it simply: kWh is a unit of energy measurement that indicates the amount of electricity used to operate a 1,000-watt appliance for one hour.

In other words, when a car indicates a consumption of ‘15 kWh’ at any given time, this means that, if the pattern of use (speed, acceleration, etc.) is maintained for an hour, the electric motor is delivering an average power of 15 kW during that period.

The kWh thus serves as the main indicator of the car's consumption, measured in kWh/100 km.

From here, it's easier to understand the logic behind the EV's battery capacity. In a vehicle with a 60 kWh battery, we can store more energy than in one with a 40 kWh battery, which means it can travel more kilometres on a single charge. For example: an average consumption of 15 kWh is equivalent to 400 kilometres in the case of a car with a 60 kWh battery, and 266 in another with a 40 kWh battery.

Another relevant unit is the Volt (V). If you're considering buying an electric model, you've certainly come across mentions of 400V or 800V batteries, values that refer to the electrical voltage of the system. By allowing for a more efficient battery architecture, wiring and motorisation, and being lighter, the 800V system allows for higher performance, in terms of charging speed and the car's performance and efficiency, especially over long distances. The first systems, which are the standard, are more affordable, less complex to produce and equip most electric cars. The 800V system, on the other hand, is found in higher-end models.

But there are still other relevant acronyms, such as SOC, which stands for ‘State of Charge’, the equivalent of the fuel gauge, showing how much of the battery's capacity is still available; or SOH (‘State of Health’), referring to the ‘state of health’ of the battery. The SOH test is an essential tool for determining the battery's condition, allowing you to measure its remaining capacity in comparison to the original one that left the factory.

From a user perspective, understanding these acronyms - AC, DC, kW, kWh, V, SOC and SOH - begins to unravel the world of electric mobility, which may seem complex, but is surprisingly intuitive once you immerse yourself in it.

The next time you hear about ‘150 kW fast chargers’, you'll already know that it's about efficiency and the speed with which that electricity makes its way into vehicles that is shaping the future.