More than a century of fossil-fuel motoring has resulted in significant technology and infrastructure developments. As a result, few motorists plan their journeys around refuelling stops. Today’s internal combustion engine (ICE) cars comfortably cover several hundred kilometres on a single tank of fuel, refueling takes minutes and forecourts are plentiful.
Electric vehicles (EVs) are different. Batteries typically have capacities in the tens of kilowatt-hours – rising into three figures at the higher end of the market. To fully charge them in a way that compares to refuelling an ICE car requires two things: Technology capable of delivering this amount of power in a matter of minutes, and batteries able to accept that rate of charging. The reality is, it makes most sense to charge today’s EVs when they’re not in use for extended periods, rather than stopping mid-journey when the tank is empty, the way we do with ICE cars. This means we need charging points in the places where we park our vehicles most often: our homes, public car parks and office car parks.
To charge an EV, regardless of model, alternating current (AC) from the grid needs to be converted to direct current (DC). The most basic approach is an AC/DC converter in the vehicle, enabling even a single-phase household charge point to charge the car at up to 7.4 kW. Even so, a full charge of a 52 kWh EV battery, such as the one in the new Renault Zoe, can take eight-and-a-half hours1.
Higher up the scale, there are AC charge points that can deliver 22 kW, resulting in significantly faster charging. In the aforementioned Zoe, Renault claims a 22 kW charge point can deliver around 275km of range in two hours2. Faster still, 43 kW AC chargers are available too – though you’ll need an EV model that supports this charge speed.
With many vehicles sitting in car parks while their owners work or shop, the refuelling rates that can be achieved with 22 and 43 kW AC chargers will be acceptable for most drivers.
Journey length and fast DC chargers
A recent European study looked at driving patterns in France, Germany, Italy, Poland, Spain and the UK. It found the average distance motorists in each country drive varied from around 40km per day in the UK to 80km per day in Poland, typically split between two or three journeys3. This kind of driving in an EV such as the current Nissan Leaf, which can achieve up to 270km4 on a single charge, would mean many people can go several days between plugging in. The challenge arises on longer journeys, such as family holidays. A couple of hours charging every 200km significantly extends the journey time.
Fast DC chargers are the solution. Current specifications define a maximum power envelope of 350 kW at up to 920 V. Depending on the EV, such a charger could add several hundred kilometres of range in under 10 minutes – which comes close to ICE refuelling times. However, this type of charger requires 400 V AC, three-phase grid supplies, meaning significant infrastructure work is required to install them. It also requires an EV capable of accepting these kinds of charge speeds.
How DC charging works
An AC/DC stage, typically using the highly efficient Vienna topology, converts the incoming AC to a fixed or variable DC voltage. This is passed to a DC/DC stage that supplies the EV battery. Such enormous power delivery is typically implemented using a modular design. Several converters, each capable of between 15 and 50 kW, operate together to deliver the total power required. IGBTs, SiC diodes and SiC MOSFETs are the power devices of choice here, because you need to achieve total system efficiencies of at least 95%5. A loss of just 1% in a 150 kW charger equates to 1500 W that must be dissipated as heat.
Losses in the charging cable and connector alone can run into hundreds of watts. To dissipate this energy, you can pump liquid coolants based on a water-glycol mix through the cable and connector, as part of the charger’s overall heat management system6.
Will EVs be superseded?
The improvements we’re seeing in EV battery capacity, coupled with the fact that relatively few people regularly make journeys that exceed the maximum range of today’s entry-level EVs, mean that the oft-cited issue of range-anxiety is increasingly a non-issue, especially as more charge points are rolled out.
The speed of charging remains the challenge, though as we’ve highlighted, there are practical solutions available, and we’re seeing increasing numbers of high-speed chargers appearing across our road networks. The question remains though: could technologies such as hydrogen displace electric, before electric displaces petrol and diesel?
The next blog in the series will look at: Hydrogen Fuel Cells
- To read the previous blogs in this series follow the links below.