What is fast charging? How fast can it get? And why is a network of fast charging stations so important for the breakthrough of electric cars? Fastned Head of Network Technology, Roland van der Put, explains. For the full article, click here.
1. 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 a fast charger).
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. So a charge point is not actually a charger but an intelligent socket to plug in your charge cable.
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 any 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 very common fast charger delivers 50 kW which charges a vehicle about 5 to 15 times faster than an onboard charger. The next generation of fast chargers was introduced in early 2018 and delivers 175 kW and can even be upgraded later to 350 kW. More on the impact of this later.
2. 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 Tesla Model S with a 85-kWh battery pack contains 7,104 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 Tesla.
Together, with all wiring and packaging, the cells form the 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 fast charging is done at 50 kW.
The battery pack of a car is never used 100%. The usable capacity of the 21.6-kWh i3 battery pack is around 19 kWh or about 90% of the total capacity. The difference of 2.6 kWh is used as a reserve to ‘cushion’ the impact of charging and discharging. The battery pack automatically cycles between around 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 number of charge — discharge cycles.
Research shows that exclusive use of fast chargers hardly affects battery life when tested with the Nissan Leaf MY2012, which was one of the first full electric vehicles. And other research indicates that fast charging might actually be better for battery life. Battery technology will continue to involve which means that characteristics such as charge speed and battery life will keep improving as well.
As a general rule, a battery will last longer when its size increases because fewer charge — discharge cycles are needed for the same mileage.
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 kilometers of range gained per hour charging. So 50 kW equals about 250 km/hour (‘250 km of range charged in 1 hour’).
Why Amps and Volts matter
Power (expressed in Watts) is the product of voltage (Volts) and current (Amps). When charging at 50 kW this is typically done at 400V and 125A (400 * 125 = 50.000 W = 50 kW). Note that this means that the charge speed is influenced by both the voltage and the current.
You can compare charging electricity with running water from a tap. Think of voltage (V) as the water pressure and current (A) as the size of the tap. If you increase the pressure more water will flow, and the same is true when increasing the size of the tap.
The voltage is a characteristic of a battery. Most car battery packs today operate at around 400V when fully charged. But 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 below). Most 50 kW fast chargers can provide a maximum current of 125A, but our 175 kW CCS chargers can provide up to 375A.
What influences charge speed? Find out by reading the full article from Fastned here.