While refuelling an internal combustion car comes down to a filler nozzle that is the same for everyone, with an electric car the matter is rather more involved. The traction battery stores and releases energy exclusively as direct current (DC), whereas the electricity grid that reaches homes and service areas distributes energy as alternating current (AC). Between the socket and the battery there must therefore be a converter, which turns alternating current into direct current and brings it to the voltage required by the battery pack. The fundamental difference between the charging modes lies precisely in a detail that looks trivial: where this converter is physically located. From that stem two broad families:
- Alternating current (AC) charging
- Direct current (DC) charging
Alternating current (AC) charging
In this case the charging point, or the domestic wallbox, simply supplies energy as it arrives from the grid, taking care of the safety of the installation, of the dialogue with the vehicle and of metering. Conversion takes place on board the car, by means of the on-board charger. This is a component that has to be carried around for the entire life of the vehicle and which, for reasons of weight, bulk and cost, is therefore sized with care: it is the on-board charger, and not the charging point, that sets the maximum power that can be drawn on alternating current. Hence a frequent misunderstanding, because connecting a car with a modest on-board charger to a far more powerful charging point brings no benefit whatsoever.
With the appropriate variations from model to model, the power levels involved fall into these orders of magnitude: a few kW with the cable fitted with a household plug, an emergency solution given that domestic sockets are not designed to deliver power continuously for many hours; figures in the region of 7 kW with a wallbox on a single-phase supply, the most common configuration in Italian homes; figures of 11 or 22 kW on three-phase installations, typically in company or apartment-block settings. These are power levels that translate into times measured in hours, but that is precisely where they belong: alternating current charging takes place while the car is parked anyway, and it is also the gentlest option as far as the battery is concerned.
Direct current (DC) charging
Here the logic is reversed. The converter is no longer on board the car but built into the charging point, which is consequently a far larger and heavier piece of equipment, often with a dedicated cooling system. The charging point supplies direct current straight away, and this reaches the battery pack without passing through the on-board charger, which is simply bypassed. With the bottleneck of the on-board electronics out of the way, power levels can rise considerably: from around 50 kW at the older installations up to the 150, 350 kW and beyond of the more recent stations.
One point that is often misunderstood deserves clarifying, however: the power quoted for a charging point represents an upper limit, not a constant value. The power actually drawn depends on the charging curve of the car, that is on the relationship between the power accepted and the state of charge of the battery: the management unit progressively reduces the flow as the pack fills up, in order to keep temperatures in check and preserve the cells. This is why manufacturers habitually quote times referring to the 10 to 80% range rather than to a full charge. The temperature of the pack also plays a part, which is why many cars start an automatic thermal preconditioning routine when the station is set as a stop in the navigation system.
The connectors used in Europe
Unlike in the early days of electric mobility, the European landscape today is essentially standardised. The connectors that may be encountered are basically these:
- Type 2 (Mennekes)
- CCS Combo 2
- CHAdeMO
- NACS
Type 2 (Mennekes): this is the reference connector for alternating current charging in Europe, adopted as the EU standard and commonly known as Mennekes after the German company that helped develop it. It takes the form of a circular socket with a flattened top and includes, besides the power contacts, those dedicated to the dialogue between vehicle and infrastructure, through which the charging point tells the car the maximum current available. It is found on domestic wallboxes and on virtually all public AC charging points.
CCS Combo 2: the acronym stands for Combined Charging System, and the name describes the concept precisely. It is not an alternative connector to Type 2, but an extension of it: two large-section contacts dedicated solely to direct current are added below the Type 2 socket. The design advantage is obvious, since the car carries a single inlet for both charging modes. It is the reference standard for rapid charging in Europe.
CHAdeMO: a direct current charging standard of Japanese origin, historically among the first to spread on a large scale and adopted in Europe above all by the previous generation of Japanese cars. It requires a separate inlet, in addition to the one for alternating current. With the rise of CCS Combo 2, Japanese manufacturers have aligned their models for the European market as well: CHAdeMO is now in clear retreat and survives on some of the existing charging points, often alongside CCS, for the benefit of the cars still on the road that are equipped with it.
NACS: short for North American Charging Standard, this is the connector developed by Tesla and subsequently opened up to other manufacturers, who have adopted it in growing numbers. It is compact and handles both alternating and direct current through the same contacts. It deserves a mention for the sake of completeness, but it concerns the North American market: in Europe Tesla itself has used Type 2 and CCS Combo 2 for years, in compliance with EU standards.
The charging networks
Alongside the technical side there is the infrastructure side. The Tesla Supercharger network is the historically best-known example of a proprietary high-power network, conceived to make long motorway journeys practical. In Europe the stations use the CCS Combo 2 connector and a growing number of them have also been opened up to cars of other makes.
On the Italian front Free To X deserves a mention, a company born within the Autostrade per l’Italia group, which has built more than a hundred high-power stations along the motorway network it holds under concession, spaced on average around fifty kilometres apart and capable, in the most recent configurations, of delivering power in the order of 300 kW. The approach is in line with what European legislation requires, since it calls for high-power stations at regular intervals along the main road corridors.
In conclusion, the distinction between alternating and direct current is not a subtlety for insiders, but the criterion that shapes the entire experience of using an electric car. Alternating current is the charging of the long stop, inexpensive and gentle on the battery, and it covers the majority of the kilometres driven; direct current is the tool for the journey, to be used for a short, targeted stop along the way.