Power rating and charging time: keep calm and connect

A few basic notions

What’s the difference between kW and kWh?

People often get these two confused, but we can soon straighten that out. By way of illustration, let’s imagine a bath tub:

• The flow of water from the tap corresponds to the instantaneous power rating of the charging station in kW.
  –> Example: A 3-phase 11 kW AC charging station.
  
  
• The bath tub corresponds to the battery capacity measured in kWh. The quantity of water transferred to the bath tub corresponds to the quantity of energy delivered to the electric vehicle in kWh.
  –> Example: In theory, if I plug my car into an 11 kW AC charging station for 1 hour, I’ll gain 11 kWh.
  

Keep in mind that kW measures the instantaneous power at the charging station, whereas kWh measures the energy consumed or delivered to the vehicle by the charging station. 

What’s the difference between AC and DC charging?

Charging with alternating current (AC)

Alternating current is the current you find in the mains power supply. EV batteries can only store electricity in the form of direct current (DC).  

An AC/DC converter under the bonnet of your car converts alternating current into direct current before it’s stored in the battery. This conversion requires additional charging time. AC charging stations usually offer normal to fast charging (up to 22 kW) but can sometimes go as high as 43 kW.

Charging with direct current (DC)

Charging stations delivering direct current offer rapid to ultra-rapid charging (up to 350 kW). Here, the charging station itself is equipped with a powerful AC/DC converter which delivers direct current and instantaneously tops up the vehicle’s battery.

Are you familiar with the technical characteristics of your electric car?

A whole new world opens up to new EV drivers: charging. You really must be familiar with your vehicle’s technical characteristics if you want to have the upper hand during the charging process. Guidance on the intricacies of charging is rarely given when you buy from a dealer. This often leads to incomprehension and frustration for new EV enthusiasts. 

There are 4 things you need to know inside out to get to grips with charging your electric vehicle: 

1. Battery capacity

This is the amount of energy, expressed in kWh, that the battery can store during the charging process and deliver to the motor. It varies between 15 and 200 kWh. What you need to remember is that the greater the battery capacity, the more time you need to charge up at a charging station. 

2. The type of on-board charger 

You need to know the characteristics of the vehicle’s on-board charger (single-phase or three-phase) since this is the factor that determines the power rating tolerated by your car in AC or DC.

With alternating current, the power tolerated usually varies between 3 and 22 kW. With direct current, it can go as high as 270 kW (for now). 

In other words, at a charging station delivering more kW than the vehicle can cope with, the charging speed is limited since it adapts to the vehicle. 

Let’s take a couple of theoretical cases:

• Sarah drives a Tesla Model 3 and plugs into a three-phase 22 kW AC charging station. After an hour, she’s expecting to have topped up 22 kWh, but discovers she’s only gained 11 kWh.
  
  Sarah’s first reflex is to blame the charging station which she thinks is boasting a power rating higher than what it can actually deliver. However, the problem doesn’t lie with the infrastructure, but with her car. Sarah doesn’t know that the TM3 can only take 11 kW maximum in alternating current (AC). 
  
  

• Paul drives a Renault Zoé R110 (latest version). He plugs his EV into the same charging station as Sarah and tops up 22 kWh in one hour. 
  
  In this case, Paul’s Zoé can accept the charging station’s maximum power rating and charges faster since the on-board charger accepts 22 kW AC.
  
  

You can easily find out the charging time required by your vehicle in relation to the charging station thanks to a charging time simulator available for free on Automobile Propre (French web page translated into English with Google Translate).

3. Charging cables that match your car 

If you don’t use the right sort of charging cable (on charging stations over 22 kW, a cable is necessarily attached to the charging station), the power rating can also be limited. 

You must therefore be equipped with the charging cables adapted to your EV and the mode of charging you prefer.

4. Battery level and temperature 

Additional factors (last ones – promise 😅) influence the amount of energy delivered by a charging station at a given instant T. A Battery Management System (or BMS) communicates with the charging station and adapts the power rating depending on the charging status and the battery temperature. 

If the optimum conditions are not combined, the maximum power delivered by the charging station and tolerated by the car will not be attained. 

Keep in mind: 

• If the battery level before charging is less than 10%, the charging rate will be low. 
• From 80% on average, the time required to attain 100% will be at least as long as it took to get from 20 to 80%. Generally speaking, it is not recommended to top up your vehicle 100%, unless you’re going on a long journey.
• The ideal battery temperature lies between 20 and 25°C. This is why charging will take longer during heatwaves and cold spells. 
  
  
  

To sum it all up

Before driving off in your new electric vehicle, you must know at least:

• the power rating tolerated by your EV when charging with AC and DC, expressed in kW,
• the battery capacity, expressed in kWh,
• additional factors which may influence charging, such as the internal and external temperature of your vehicle, the type of cable used and the battery level and status.

Once you are familiar with your EV’s characteristics and the ABC of charging, you will be able to select the charging stations that correspond best to your vehicle. This means that you can optimise the charging time and avoid a few unpleasant surprises when the bill arrives.

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