What is DC Charging? What’s The Difference Between AC and DC EV Charging?

DC Charging FAQ

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What Is DC Charging?


It is true, in that, the majority of electric cars are charged at home, and usually overnight, via a dedicated AC (alternating current) EV charger. In general, we do not use DC (direct current) EV chargers for home charging. However, for public charging we use both AC and DC EV charging, but with a greater emphasis on DC electric car chargers.

One of the key reasons for the preference of DC EV chargers, for public EV charging, is that, DC EV chargers offer faster charging speeds. Put another way, an onboard EV battery can be charged faster using a DC electric car charger compared to an AC EV charger. There are a number of reasons for the higher charging speeds for DC EV chargers. But one of the key factors is the location of the ‘converter’ that converts AC power, into DC, to be stored in the onboard EV battery. The power from the grid is always AC power.

When using a residential EV charger like myenergi zappi, the power is being converted from AC to DC, to be stored in the EV battery. Electric cars incorporate a ‘converter’ usually referred to as an ‘onboard charger’, to convert AC into DC when the electric car is charging. AC EV chargers do not incorporate a converter. The converter is always located within the electric car. Depending on the type of electric vehicle (EV), the onboard AC charger can vary between single-phase and three-phase.

Though most of the recent battery-electric vehicles (BEVs), incorporate a single-phase onboard AC charger (7.4 kW AC), many also offer a three-phase onboard AC charger (up to 22 kW AC). In contrast, plug-in hybrid electric vehicles (PHEVs) usually incorporate between a 3.6 kW AC and 7.4 kW AC onboard AC charger. This is not surprising, given the smaller on board EV battery in a PHEV.

Most homes in the UK are power by a single-phase power supply. Therefore, even if the electric car has a 3-phase (22 kW) onboard charger, it will be limited to single-phase EV charging. Conversely, if the building is supplied by 3-phase power supply, and the onboard AC charger is limited to single-phase, the EV will only charge at single-phase. Most commercial buildings in the UK are powered by three-phase power supply.

For those new to electric driving, an onboard EV battery, is in some respects similar to a fuel tank in an internal combustion engine (ICE), petrol or diesel vehicle. The larger the fuel tank capacity (litres), the more the fuel required for filling. Similarly, the larger the capacity (KWh) of the onboard EV battery, the more the energy required to recharge!

In AC EV charging, the impediment to achieving faster charging speeds is limited by the capability of the onboard charger (converter). In DC charging, though, there is a need to convert grid power (AC) into DC, it is done within the DC charger and not in the electric vehicle. In DC charging, power is sent directly to the EV battery, bypassing the onboard EV converter. Hence the higher performance of DC EV chargers. In general, AC charging session take between 4 – 12 hours, while DC charging sessions between 15 minutes and 60 minutes. It is also worth noting, that public DC charging costs significantly more than residential AC electric car charging.

The majority of AC EV chargers are between 7 kW AC – 22 kW AC, but can be up to 43 kW. In the case of DC chargers, the majority are between 50 kW DC -250 kW DC. There are four primary charging speed categories for electric cars: slow, fast, rapid and ultra-rapid. Of course, rapid and ultra-rapid chargers offer the fastest charging speeds, and these are often located at motorway services. Like for AC charging, DC charging speeds are limited to the onboard DC charging capability of the electric car. If an EV has a 50 kW DC charging capability, the maximum charging speed will be limited to 50 kW DC, even if the DC charging station offers a higher DC charging capability.


Charging Times (Overview)
Slow charging AC (3 kW – 3.6 kW):6 – 12 hours (dependent on size of EV battery & SOC)
Fast charging AC (7 kW – 22 kW):3 – 8 hours (dependent on size of EV battery & SoC)
Rapid charging AC (43 kW):10%-80%: 20 mins to 60 mins (dependent on size of EV battery & SoC)
Rapid charging DC (50 kW+):10%-80%: 20 mins to 60 mins (dependent on size of EV battery & SoC)
Ultra-rapid charging DC (150 kW+):10%-80% : 20 mins to 40 mins (dependent on size of EV battery & SoC)
Tesla Supercharger (120 kW – 250 kW):10%-80%: up to 25 mins (dependent on size of EV battery & SoC)

Most DC charging sessions are up to 80% state-of-charge (SoC). This is to protect the onboard EV battery. Interestingly, the last 20% can take as long as it takes to charge up to 80%. Therefore best to charge up to 80% and charge up to 100% in the convenience of your home, overnight. Not all electric cars offer DC charging capability. In fact, most PHEVs do not offer DC charging, while most BEVs offer DC charging. Again, not surprising, given the smaller size of the onboard EV battery in a PHEV.

At an average, pure electric cars incorporate a DC charging capability between 50 kW DC to 100 kW DC. Of course, some pure electric cars offer far faster charging capability. As an example, the all-electric Tesla Model Y offers DC charging up to 250 kW DC. Put another way, the EV can add 150 miles in 15 minutes.

DC EV chargers vary in performance between 50 kW DC an up to 250 kW DC. The Tesla Supercharger network is a case in point, with most Tesla DC EV chargers offering up to 250 kW DC. Tesla is now in the process of introducing V4 of its Supercharger, capable of DC charging up to 350 kW DC.

Tesla is not the only public DC charging network in the UK. Others include: InstaVolt, Osprey, GRIDSERVE, bp pluse and more. The majority of these operators offer rapid and ultra-rapid DC charging. According to zapmap, as of December 2022, the UK has 6,902 rapid and ultra-rapid charging devices across 3,996 locations.


What Is The Difference Between AC and DC EV Charging?


AC EV Charging DC EV Charging
AC/DC converter in the EVAC/DC converter in the DC EV charger
Charging speeds limited by the onboard EV converterCharging speeds not as limited by the converter incorporated in the DC charger
Longer charging timesShorter charging times
Cheaper charging tariffs Higher charging tariffs
More widely available Less widely available
Can charge at home Cannot charge at home

EV Glossary
Alternating Current (AC):What is alternating current? Though we may not be familiar with this term, we use alternating current everyday in our homes to power our appliances! Alternating current is a type of electric current, in which the direction of the flow of ‘electrons’ switches back and forth at regular intervals or cycles. When an electric car is charged at home, the type of electric current used, is alternating current.
Direct Current (DC):What is direct current? Direct current (DC) is a type of electric current that flows in only one direction i.e. uni-directional. DC enables the constant flow of electrons from an area of high electron density to an area of low electron density. DC is quite common in our day-to-day lives. Many of the appliances we use on a regular basis that are operated by batteries, use DC. A mobile phone, a laptop, a torch light etc. In electric cars, the onboard EV battery also uses direct current to store energy.

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Author

Ashvin Suri

Ashvin has been involved with the renewables, energy efficiency and infrastructure sectors since 2006. He is passionate about the transition to a low-carbon economy and electric transportation. Ashvin commenced his career in 1994, working with US investment banks in New York. Post his MBA from the London Business School (1996-1998), he continued to work in investment banking at Flemings (London) and JPMorgan (London). His roles included corporate finance advisory, M&A and capital raising. He has been involved across diverse industry sectors, to include engineering, aerospace, oil & gas, airports and automotive across Asia and Europe. In 2010, he co-founded a solar development platform, for large scale ground and roof solar projects to include the UK, Italy, Germany and France. He has also advised on various renewable energy (wind and solar) utility scale projects working with global institutional investors and independent power producers (IPP’s) in the renewable energy sector. He has also advised in key international markets like India, to include advising the TVS Group, a multi-billion dollar industrial and automotive group in India. Ashvin has also advised Indian Energy, an IPP backed by Guggenheim (a US$ 165 billion fund). He has also advised AMIH, a US$ 2 billion, Singapore based group. Ashvin has also worked in the real estate and infrastructure sector, to including working with the Matrix Group (a US$ 4 billion property group in the UK) to launch one of the first few institutional real estate funds for the Indian real estate market. The fund was successfully launched with significant institutional support from the UK/ European markets. He has also advised on water infrastructure, to include advising a Swedish clean technology company in the water sector. He is also a member of the Forbury Investment Network advisory committee. He has also been involved with a number of early stage ventures.

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