Why does an electric car take so long to charge?

Ginny Buckley

15 Jun 2023

Electric car drivers are understandably fascinated by charging times. A car with a short range and slow charging could be a source of constant frustration, while a model which can charge quickly can shave hours off a long journey. 

You might reasonably wonder why there are so many variations. The biggest factor in understanding recharging times times is whether the electricity used is AC alternating current (as used domestically), or DC direct current (usually reserved for things like rapid chargers). With AC charging, the box on the wall is just providing power to the car’s on board charger. With DC the charger is ‘off board,’ sitting by the side of the road, so it doesn’t use a vehicle’s on board charger. 

The built in charger limits the speed at which AC inputs can be converted to the DC needed to charge the battery

AC is what comes out of the grid, but it needs to be converted to DC before it can be fed into your battery. This process needs some hardware, which is like a big version of the charger you get with a laptop.

Most cars these days will have on-board converters which will accept 7.2kW from a wallbox, but some models will take up to 22kW from a suitable AC charger as they have more effective on-board hardware.

When you use a rapid charger, the power is converted to DC inside the charger (the big unit you plug into) rather than the car. As the charger’s converter is bigger and more efficient, it bypasses the bottle neck of the car’s on-board charger and means the energy can get into the battery faster.

Rapid chargers bypass the bottleneck of the car's on-board AC-DC converter and provide a DC feed straight into the battery

Public DC chargers can deliver vastly more power to the battery than a domestic connection. For example, while your house can deliver 7kW, some DC chargers can deliver up to 350kW - 50 times more!

But here’s where it gets a bit complicated (again). You might reasonably think that plugging in to a 350kW charger will allow you to charge your car 50 times faster than if you plugged into your home wallbox. Unfortunately, it’s not quite as simple as that. All electric cars limit the amount of power that can be fed into the batteries. Recharging creates heat in the cells, and too much heat can potentially damage the battery. To maintain battery health, electric cars carefully manage the flow of power from the charger to the battery pack. 

Let’s look at an example. The Citroen eC4 or eC4X, comes with a DC rapid charging maximum of 100kW. If you plug in to a 50kW DC rapid charger, then the level of charge is limited by the charger and you’ll only get 50kW maximum. Plug in to a 125kW charger or faster and the car will limit the incoming power to 100kW. 

Also bear in mind that the power delivery from a rapid charger varies during the charging session. This is to protect the battery from getting too hot. When the battery has reached 80% capacity, the car draws far less power as it prepares to ‘top up’ the last remaining cells. It’s a bit like pouring a cup of tea - you ease off the flow as you reach the top of the cup. 

Another factor that can affect your charging times is air temperature. Electric car batteries work at all temperatures, but like us, they perform at their best when they are warm. If a battery pack is too cold or too hot, the chemical reactions within the cells don’t happen as efficiently. This means that that a cold pack or a hot pack will take longer to charge. 

Fortunately, modern battery management systems and heat pumps such as the ones Citroen fits to its eC4 and eC4X models, means that the car can keep the battery pack in the ‘goldilocks’ zone of temperature. 

Rapid chargers come with a variety of speeds but your car will limit the level of power it can charge at

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