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What you should know about electric cars charging speeds

When trying to understand charging speed, most chargers just mention one thing: the power they can deliver to an electric vehicle. That says very little about how far it can drive with that power or how long it takes to charge it.

After all, the electric vehicle could be a large electric truck that has a large battery, consumes a lot of energy, and that takes more time to charge. Or it could be a Lightyear One, an efficient vehicle that has a smaller battery and takes less time to charge.

So how should we think about charging speed?

Before We Start: Some Definitions

So let’s first talk about charging speeds and some of the most important elements when talking about them:

  • A kW, which is short for kilowatt, is a measure of power. It is the amount of electricity flowing through a cable at any given time. Think of it as water flowing through a hose to fill a pool: the more water flowing through the hose, the faster the pool fills. Chargers usually provide 7kW, 11kW, 22kW of power;

  • A kWh, which is short for kilowatt-hour, is an amount of energy. If the battery were a pool, the kWh would indicate how much water it can contain in total. The more kWh a battery has, the more energy it can store. You will likely meet this metric when you read the description of the size of the battery;

  • The kWh/km metric calculates how much energy you need to drive one kilometre. It measures how efficient your car is: the lower it is, the more efficient the car;

  • The km/kW is a measure of how many kilometres of range your charger can add for every kW. If your car charges 1km/kW, and you’re charging at a 7kW charger for an hour, then you will have charged 7km of range.

Explaining terms like kW, kWh, kWh/km, km/kW

Large Battery Pack Example

Let’s take the two examples from before, but add some numbers this time: a truck with a 500kWh battery pack, and Lightyear One, with a 60kWh pack.

A truck with a battery around 8 times as large takes around 8 times as long to charge. If it is connected to a charger that delivers 11kW (which is on the slow-charging side if you’re driving a truck), it would take about 45 hours to charge the truck.

Illustrated charging speed differences

This arguably ‘fast’ charger may as well take literally forever to charge your vehicle.

What this means is that larger batteries take longer to charge, no matter what the power of the charging station is.

Low-Efficiency Vehicle Example

Now let's look at another example, this time a car driving on the highway.

There is a hitch though: It's so warm outside that the driver is planning on driving around with the windows open on the highway. Driving with the windows open takes up a large amount of energy due to increased wind resistance, so your cars’ efficiency will be lower compared to what it usually is.

This means that it uses up the battery quicker than if it had closed windows.

If the car uses twice as much energy than usual, a fully charged battery (which on paper should be able to get 600km of range) only reaches 300km.

Lightyear One charging frequency

If the car has to drive 600km, it only has to charge once if it has the windows closed, or twice if they're open. It has to charge twice as often to drive the same distance.

With an inefficient car, it is like constantly driving with the windows jammed open. This means the car has to charge more and more often for the same range.

How We Should Think About Charging Speed

If you talk about charging at an 11kW or 72kW charger, it doesn’t tell anything about how long it takes you to charge nor how much range you can get out of it. And the truth is that it depends on the vehicle driven and how it's driven: how big the battery pack is and how much energy it uses on the road.

Lightyear One efficiency

Instead, we should think of charging speed in terms of km/kW, a measure that ignores how big your battery is or how fast the charger is and only depends on the efficiency of your car. It tells you exactly how long you have to charge to drive a certain distance.

The lower that energy consumption is, the longer the distance the car can drive on the same amount of energy. If a car consumes 100Wh/km, it will charge the same amount of range as a car driving 200Wh/km in half the time.

If we flip those numbers around, we can see how much battery power it takes to drive a set amount of kilometres. If car A consumes 100Wh/km, it can drive 10 kilometres per kWh of battery (10km/kWh). If car B consumes 200Wh/km, it can drive 5 kilometres per kWh of battery (5km/kWh).

Now let's plug these cars into a charger. If you’re in the car that can drive 10km/kWh and you are at an 11kw charger for an hour, then you will charge 110km per hour at that charger. If the car can drive 5km/kW and you are at an 11kw charger for an hour, then you will charge 55km per hour at that charger.

Lightyear One technical sheet

It's as simple as that: multiply the energy efficiency of your car by the power of the charger you’re at to get the range you’re charging.

To charge quickly, there’s only one solution: an efficient EV, with a smaller battery. Now you know what to look for, when researching electric vehicles. So hit the open road, hopefully from behind the wheel of your own personal Lightyear One.

Read more about how to build an EV that has the longest range

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This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 848620