Everything you always wanted to know about fast charging




What is fast charging? How does it work? How fast can it really get? Our charging expert Luc Bronk answers these questions below.

The basics

What is fast charging? And how is it different from ‘regular’ charging? All batteries — including those in electric vehicles — use Direct Current (DC) for charging and discharging. But the electric grid delivers Alternating Current (AC). Therefore AC from the grid needs to be converted to DC, so it can be used to charge the battery. This is done by an AC/DC converter.

This AC/DC converter is part of what we call a charger. Chargers can either be integrated into the vehicle as an onboard charger, or chargers can be external to the vehicle (for example, the fast chargers you find at Fastned stations). Today, virtually all electric vehicles have a small onboard charger. You can use a cable to connect the onboard charger to a regular AC socket in your garage or plug it into a charge point. The charge point delivers the AC required for the onboard charger to charge your battery. 

AC vs DC

The main difference between Direct Current and Alternating Current is that DC delivers a constant voltage. Think of a stable, straight line, where the electrons flow in one direction. As the name implies,  Alternating Current fluctuates. The voltage periodically changes from positive to negative and back again, creating a wavy line.

If you want to charge faster, the AC/DC converter and hence the charger need to be bigger. But a bigger charger is heavier, takes up more space in the car and adds complexity and cost to the vehicle. On top of that, every component in a vehicle needs to be automotive grade to ensure its reliable operation for the lifetime of the vehicle. So vehicle manufacturers usually choose a relatively small — and therefore slow — onboard charger to optimise between these factors.

Fast charging is different

An external charger that does the AC/DC conversion can be a lot bigger, heavier, more complex and more expensive than an onboard charger. But it is also a lot faster. That is why they are usually referred to as ‘DC fast chargers’ or just ‘fast chargers’. A typical fast charger delivers 300 kW which charges a vehicle about 25 to 80 times faster than an onboard charger. The next generation of fast chargers were introduced in early 2023 and deliver 400 kW. More on the impact of this later.

How fast charging works

A vehicle battery consists of many ‘cells’. A single cell is quite similar to a rechargeable battery you use at home, only bigger. A Lucid Air with an 112 kWh battery pack contains 6,600 individual cells. A BMW i3 with a 21.6 kWh battery has just 96 cells, but its cells are larger than the cells used by Lucid. Together with all wiring and packaging the cells form the battery pack as depicted below.

"Today’s battery packs are designed with fast charging capability"

BMW i3 battery pack

Today’s battery packs are designed with fast charging capability. For example, the powertrain of the BMW i3 is rated at 125 kW peak power and 75 kW continuous power while charging at 50 kW.

Battery life

Here’s something you might not know: car battery packs are never 100% utilised during daily use. And for good reason. Both the automotive and fast charging industries are working to make the most efficient use of car batteries. The latest generation of EV batteries is set up to maintain a charge level of between 10% and 90%. By keeping battery levels between these parameters, we can extend battery life and maximise (fast) charging capacity.

For example, the usable capacity of the 78,1 kWh Tesla Model Y Long Range battery pack is only around 75 kWh or 90 to 95% of the total capacity. The difference of 3,1 kWh is used as a reserve to ‘cushion’ the impact of charging and discharging. The battery pack automatically cycles between 5% and 95% of the battery pack. All of this is handled by the Battery Management System (BMS) and completely hidden from the driver.

There are many factors influencing battery life including: temperature, battery age, battery size, chemistry, duration of keeping a battery fully charged and the number of charge — discharge cycles. Research shows that frequent use of fast chargers hardly affects battery life when tested with the Tesla Model Y. The same degradation happens when charging with slow chargers. As a general rule, a battery will last longer when its size increases because fewer charge — discharge cycles are needed for the same mileage.

"Frequent use of fast chargers hardly affects battery life"

Why Amps and Volts matter

Power (expressed in Watts) is the product of voltage (Volts) and current (Amps). You need both for fast charging. Think of voltage (V) as the water pressure and current (A) as the size of the tap. If you want more water faster, you need to increase water pressure and/or increase the size of the tap. Our fast chargers do both but then for electricity.

Usually people call it fast charging when the speed is 50 kW or more. In terms of voltage and current that means 400V and 125A (400 * 125 = 50.000 W = 50 kW). 

Why 400V? That’s because of car batteries. Most car battery packs today operate at around 400V when fully charged, so that’s the sweet spot we’re aiming for.

Why 125A? That’s because of the chargers. Most 50 kW fast chargers can provide a maximum current of 125A.

 Luckily, fast charger development is in full swing, and our latest 400 kW CCS chargers can provide up to 500A and soon up to 600A. Talk about a bigger tap!

Good to know

When a battery pack is not fully charged, the voltage will be lower - e.g. 325V. Voltage will gradually increase while charging, so this has a positive effect on the effective charge speed (see the blue line in the graph below showing a fast charge session of a 30 kWh Nissan Leaf). The current can be increased or decreased by the fast charger based on the instructions received from the BMS (see yellow line in the graph).

Your car mostly determines the charge speed

There is a common misunderstanding that car batteries charge constantly at peak performance. This is not the case. In almost all situations, it’s the vehicle that determines the charge speed.

Here’s how that works: during fast charging there is continuous communication between the BMS and the fast charger. The BMS instructs the fast charger to set the charge speed. This speed is usually expressed in kilowatt (kW). Charging a car for 1 hour at 50 kW puts 50 kWh of energy into the battery pack. On average, an electric vehicle uses 1 kWh to drive 5 km. Some vehicles, like Tesla, also express the charge speed in kilometres of range gained per hour charging. So 50 kW equals about 250 km/hour (‘250 km of range charged in 1 hour’).

What influences charge speed?

Battery pack capacity

In general, a larger battery pack can be charged faster. So a Tesla Model S with a large 100 kWh battery can be charged at higher power than a BMW i3 with a 21 kWh battery. This is also the main reason why the current crop of plug in hybrid electric vehicles (PHEVs) cannot fast charge: their battery packs are simply too small. Most PHEV manufacturers do not include the additional hardware (e.g. extra inlet and wiring) in the car.

State of Charge (SoC)

When the battery is almost fully charged the charge speed drops to prevent the battery cells from overheating. Typically at 80–90% SoC the speed drops and charging will slow down further closer to 100% SoC. That is the reason why fast charging is most effective between 0% and 80–90% SoC. Also, as mentioned before, the latest batteries are even set up to maintain a charge level of between 10-90%.

Battery temperature

Battery cells operate most effectively between 20–25 degrees Celcius (68–77 degrees Fahrenheit). When battery temperature is too low or too high, the BMS reduces the requested current to protect the health of the battery cells. If the battery pack is equipped with a heating or cooling system (e.g. the BMW i3) the BMS will activate this system in order to control the cell temperature. Note that battery temperature is not only influenced by the outside temperature, but also by (highway) driving and (fast) charging as this will generally increase battery temperature.

Battery chemistry

When designing a battery, manufacturers have to make choices on size, weight, costs, life and performance of a battery. Depending on the target audience of the vehicle they could compromise battery performance over costs and weight for instance. Alternatively, a higher end vehicle will have better performance and may include battery temperature regulation, but it will also come with a higher price tag.

The evolution of the charge curve

This graph shows the charge curves of 2 generations of BMW i3 batteries (22 kWh and 33 kWh). The charge curve is different for each vehicle make and model. Find the charge curves for the most popular vehicles here.

"At Fastned all chargers can typically work at full power regardless of the number of vehicles charging at the same location"

Impact of fast chargers on charge speed

Fast chargers themselves could limit the maximum speed at which a vehicle can be charged. If a fast charger is rated at 50 kW it will never provide more power even though the vehicle might be able to charge faster. Also there might be limitations of the grid connection or multiple chargers on a single location could share power, which may result in a situation where a fast charger cannot provide full power.

High power charging

Currently there are just two open fast charging standards: “CCS” and “CHAdeMO”.

Combined Charging System (CCS) was developed by seven car makers and was originally designed for charging up to ~80 kW (at 400V). The standard is promoted by CharIN and is backed by a large number of car makers and charger manufacturers across the world.

CHAdeMO was already developed in 2010 by the CHAdeMO Association and is an initiative of Japanese car makers. The initial design allowed for charging at up to ~50 kW (at 400V).

Fastned is a member of both organisations and we offer these standards at all of our stations. At a Fastned location, you are unlikely to find a charger that cannot reach the peak power of your vehicle. As of 2023, Fastned has started installing 400 kW chargers. There are currently no passenger vehicles in the market that can charge at this speed.

Increasing range added per 10 minutes of charging

The race for faster charging is on

More and more vehicles charge with a higher peak. Currently, we see more vehicles that come with an 800V infrastructure in their battery. When the voltage is higher, it’s likely that the car can charge faster. In the summer of 2023, the Lotus Eletre debuted in the European market with a peak charging power of 350 kW. This is the fastest charging passenger vehicle to date (October 2023).

Meanwhile we also see the traditional car manufacturers entering the market with vehicles that can charge faster. Porsche is working on a new version of their Taycan that can charge with 320+ kW, and Audi will present their new Q6 e-tron with a peak charging capacity of 270 kW on a 100 kWh battery. Where the previous Audi models 2023 Q8 e-tron (170 kW) & 2021 Audi Q4 e-tron (175 kW) were not able to charge at these high speeds.

These announcements show that the race for faster charging is on. It’s also clear by now that a network of ultra fast charging stations is required to support the introduction of these next generation electric vehicles since most other forms of charging simply won’t be able to support the latest battery advancements.

Deploying next generation charging infrastructure

Many Fastned stations are equipped with 300 kW chargers. Fastned has grid connections that support charging 4-8 cars simultaneously at up to 300 kW. Other Fastned stations can easily be upgraded with a larger grid connection as well. More capacity can be added in the future by introducing on site battery buffering and/or by further increasing the capacity of the grid connections. The layout of our stations is already designed for maximum throughput of cars. We installed our first 400 kW charger in December 2022 and will continue the deployment of 400 kW chargers in the years to come. 

You can follow @fastned for the latest news and me @lucbronk (Commercial Activation Manager) or @rolandvanderput (Energy Sourcing Manager) for more insights into the fast charging business.