Independent, Expert EV Reviews & Advice Since 2006
Fastned Oxford Charging Hub

Buying an EV? Why you need to check its Charge Curve first

If you’re buying an electric car, being aware of its driving range is likely to be one of your top priorities, but there are two other essential things that EV buyers need to know about. One is the EV’s maximum charge rate, in other words how fast it can charge; some EVs can charge at up to 300 kW, yet others only have a maximum charge rate of less than 50 kW, which is a big difference. The other important, but generally overlooked issue, is how the EV’s charge rate slows down as the battery charge increases.

The good news is that Green Car Guide’s electric vehicle reviews include an EV’s maximum charge rate, and they also include the charge curve for an EV, courtesy of Fastned.

Most people will charge an EV at home overnight if they have off-street parking, but if you need to use the public rapid charging network, then you’ll need to know how quickly your car can charge.

If you visit a 100 kW rapid charger and your car can charge at 100 kW, then many EV drivers expect their car to charge at 100 kW for the duration of the charging session. However nothing could be further from the truth.

An EV could start charging at 100 kW when using a 100 kW rapid charger, although typically it will start charging at a slower rate, but the speed at which an EV charges will reduce as the car’s battery charge increases. This is true for every EV, but the ‘drop-off’ in an EV’s rapid charging speed varies for different EVs. So how do you know what the drop-off in the charging speed is for your EV?

This is where a brilliant solution from charging company Fastned comes in. Fastned, based in the Netherlands, and currently aiming to build 1,000 rapid charging stations around Europe, publishes charge curves for all EVs that use its network.

As an example, the charge curve for the Kia EV6 is shown below. The Kia EV6 is one of our favourite EVs, for all sorts of reasons that you can read about in our Kia EV6 review, and one of its many brilliant points is that the EV6, thanks to its 800V electrical architecture, has a maximum ultra-rapid charging rate of 233 kW.

KIA-EV6 charge curve

Kia EV6 charge curve

Fastned’s charge curve shows that the EV6 can indeed charge at between 200 kW to 250 kW, however this is until the battery charge reaches just over 50%. At that point the charge rate drops from around the maximum of 233 kW to around 175 kW, then the charge rate continues to drop in ‘steps’ until the battery nears an 80% state of charge, when the charge speed then drops off a cliff.

This charge curve demonstrates how the maximum charge rate for an EV is not sustained throughout an entire charging session. However the Kia EV6 is one of the best EVs in terms of charging speeds – many EVs have an even more pronounced drop-off.

So why does Fastned produce its charging curves? There have been instances when people have bought a new EV, used a rapid charger, and they’ve been disappointed by how slow the car charges compared to its advertised maximum charge rate. So Fastned – along with Green Car Guide – believes it’s important to educate EV buyers about how an EV’s charge rate slows down during a charging session, and so it took the decision to produce and publish charge curves.

If you’re a car manufacturer, and you see a Fastned charge curve for your EV that shows a significant drop-off in charging speed, are you going to be upset? Apparently not, says Fastned – the EV manufacturers also see the charge curves as being really useful to educate drivers about how batteries behave when being charged.

What’s the secret to being able to charge as quickly as possible? Apart from starting to charge with a low state of battery charge rather than a high state of charge, battery pre-heating is key. Fastned has another chart for the Kia EV6 – without battery pre-heating activated – showing charge curves when charged at different temperatures (see below) – the charge rate is fastest when the temperature is 25 degrees C, and it slows down significantly as the temperature gets colder.

Kia EV6 charge curve at different temperatures

The solution is battery pre-heating systems, which increasing numbers of EVs now feature. Conversely, if a battery gets too hot, then it won’t be happy if asked to do repeated rapid charges; the latest liquid-cooled batteries, rather than the older air-cooled batteries, help to address this problem.

Using the public rapid charging network in the UK used to be a challenge, as EV drivers needed multiple cards and/or apps for different charging networks. Most rapid chargers in the UK should now take contactless payment, which, at last, makes life much easier for drivers. However the current situation is that if you use rapid chargers at motorway service stations, typically there are only around two or four chargers, and there are more EVs queuing than there are chargers. Access is also almost impossible in many cases for any vehicle that’s larger than a car.

Fastned also has a solution to this problem, as the company is building ‘drive-through hubs’ that can charge up to 18 vehicles. It has 217 charging locations open in Europe at the time of writing and is opening more than one new site each week – including at locations in the UK. Fastned’s site at Oxford, which opened earlier this year, is part of Energy Superhub Oxford – Europe’s most powerful EV charging hub – initially having ten charging bays with 300 kW chargers, but with plenty of room to grow.

Most of Fastned’s sites have a roof to keep EV drivers out of the rain, and if the sun is shining, the roof features solar panels which provide one-fifth of the electricity for the chargers in summer, or one-tenth of the electricity in winter. If, like many people, you often find charge points hard to find at service areas due to lack of signage, Fastned’s chargers address this problem by having a canopy that lights up at night.

Fastned is even building facilities to address the next big issue – the lack of ultra-rapid charging for electric trucks.

The next significant development in charging will be the ability to drive up to a chargepoint and plug in your car with the charge starting automatically without any need to use an app, a card, or contactless payment – something that Fastned has enabled its customers to do for the last five years. In 2017, Fastned developed and open-sourced its game-changing ‘Autocharge’ technology.

So thanks to companies such as Fastned there’s hope that the public rapid charging infrastructure can keep pace with the ever larger numbers of exciting new EVs coming to market.

Paul Clarke Green Car Guide

Paul Clarke