Electric Car: Always Charged to the Max?

Driving requires energy – and that applies to electric cars just as much as it does to combustion engines. However, different rules apply at the electric charging station than at the diesel or gasoline pump. What does “filling up” actually mean with regards to an electric car? Does it use alternating current or direct current? And when is the best time to pull the plug? We asked DEKRA experts.

People who rely on public infrastructure for charging should look for a higher AC charging capacity of 11 or 22 kW when purchasing an e-car. Photo: DEKRA Automobil

In today’s circumstances surrounding electromobility, the use of e-cars depends heavily on the refueling infrastructure. If you’ve got a small drive battery, you might think twice about going out for a long drive. In this case, you would have to allow two hours or more just to refuel at the charging station – unlike a comparable combustion engine, which can be refueled quickly. But how do you calculate the charging time? By simply dividing the station’s charging power by the traction battery’s capacity? That would be too simplified a calculation. When refueling, supply and demand have to match. If the charging station offers a respectable 150 kW of charging power, but the car can only handle 50 kW, the extra power is of little use. Incidentally, according to the German Automobile Association (ADAC), an electric car counts as fit for long-distance driving if it has a rechargeable range of 200 kilometers in 30 minutes.

What charging options are available for electric cars?

“Connecting to the household socket with 230 volts (mode 2) is an emergency solution in which a battery is charged with the same charging power as for example a kettle,” explains Michael Ringleb, product manager for electrical engineering (ELT) at DEKRA. “In this charging mode, the vehicle is connected via a charging cable provided by the manufacturer or accessory retailer.” Most electric cars today are charged with alternating current at private wallboxes and charging stations in public areas. This charging mode 3 usually involves a so-called type 2 plug. Charging mode 4 stands for electric charging with direct current (DC). Here, the Combined Charging System (CCS) has become the European standard. The combo plug combines a type 2 plug with two additional power contacts for DC charging in one housing – the e-car can tap direct current at the fast charger and alternating current at the AC charging station.

Good to know: Expansion of charging infrastructure. As of May 2021, the German Federal Network Agency lists 36,894 standard AC charging points with customary charging capacities of 22 kW. There are also 6,099 charging points for DC fast charging. However, less than two percent of all charging points currently have a charging capacity of at least 100 kW, which is considered a prerequisite for the use of e-cars on long distances. With the “Fast Charging Act for nationwide fast charging infrastructure for pure battery-electric vehicles” that has just been presented, the German government now wants to move into the fast lane – by 2023, 1,000 fast charging hubs that enable charging with over 150 kilowatts shall be available in this country. The expansion of the private charging infrastructure is progressing well. According to the Federal Ministry of Transport, around 385,000 funding applications for 470,000 charging points have been submitted since the start of the funding program for private wallboxes.

For whom is an AC charging wallbox suitable?

Having your own charging station in the garage, carport, or private parking space can be the deciding factor when purchasing an e-car. Politicians have also recognized this – the federal government subsidizes the installation of a wallbox with a maximum of 900 euros per charging point. The prerequisites are a charging capacity of 11 kW (ex works or throttled), an intelligent control system, and the station’s exclusive use of renewable energies. In fact, three-phase models with 11 kW charging power are in high demand in Germany. In most cases, the 22 kW versions would be overkill. However, the large wallbox would be a perfect fit for the Renault Zoe Z.E.50 – the small French car has a power intake of 22 kW and could thus deliver enormous charging speeds.

What’s the best way to use public AC charging stations?

People parking their cars on the street have to rely on public charging infrastructure to recharge their batteries. That doesn’t necessarily constitute a disadvantage, as municipalities, retailers, supermarkets, and service providers increasingly offer additional charging options. “The charging capacities of AC charging stations are generally 22 kW. However, the same restrictions apply here as with charging at a private wallbox – charging power at the AC pole can only be as high as the onboard charger in the vehicle allows,” reports Andreas Richter, engineer at DEKRA’s Electromobility Competence Center. So the lower the onboard charger’s charging capacity, the longer the car has to be attached to the charging cable to draw a corresponding amount of energy. Conversely, this means that a short stop at the public charging station during a shopping trip doesn’t significantly increase the amount of power in the battery. People who rely on public infrastructure for charging should therefore look for a higher AC charging capacity of 11 or 22 kW when purchasing an e-car. Otherwise, a visit to the charging park is a good way to get a refill in the form of direct current – provided the e-car is equipped for this.

What about DC charging at the charging park?

DC charging stations supply the battery directly with direct current using the integrated rectifier. Their power spectrum ranges from 50 kW for simpler systems to 475 kW for high-performance charging stations. Unlike with AC charging, the onboard charger is left out of the equation. Instead, it’s the battery management system that decides at what power the direct current flows into the battery. The charging station usually provides full power up to a battery capacity of 80 percent. At this point, the system basically slams on the brakes. As a result, the remaining 20 percent only flow slowly into the battery until it’s fully charged. In practice, it’s advisable to use fast charging at the DC charging station only up to a charge level of 80 percent. Otherwise the time to wait until the battery is full increases enormously. For how long charging with high currents is possible also depends on the manufacturer’s charging strategy. Measurements by the ADAC have shown that the Audi e-tron charges at the highest power of just under 150 kW over almost the entire range up to 80 percent battery capacity. In contrast, the Mercedes EQC continuously reduces its charging power of 110 kW at just under 40 percent battery level.

What are charge losses and when do they occur?

Charge loss means that less energy arrives in the battery than is tapped at the charging station. In a recent series of measurements, the ADAC identified charge losses of ten to 24 percent, depending on the vehicle. This phenomenon becomes visible the moment the on-board computer indicates a lower value for the amount of electricity in the battery than displayed by the charging station. The reasons for this gap are physical. “When electricity flows through a line, there are always electrical resistances at work that cause heat loss. The losses occur in the charging station itself, for example, but also in the charging cable, the on-board charger, and the traction battery,” explains Andreas Richter. The bottom line is that this loss of charge can significantly extend charging time at the charging station. Charge losses can also play a role in calculating refueling costs. If you rely only on the on-board computer’s data, you end up with a lower sum than you actually have to pay at the cash register. Charge losses are therefore always borne by the user.

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Technology explained: What role do onboard charger and charging capacity play?

Wallboxes and public AC charging stations use three phases to supply the vehicle with power from the grid. A three-phase wallbox with 11 kW charging capacity provides 3.7 kW of power via each phase. How much of this the vehicle can use depends on the built-in onboard charger, which converts the alternating current to direct current on its way to the battery. Two regulating variables are important: the number of phases and the system’s charging capacity. A three-phase charger with 11 kW could thus call upon the wallbox’s full power. A single-phase system, on the other hand, could only receive a maximum of 3.7 kW. The same applies if the charging capacity is 7.4 kW, but only one phase is available. These days, many electric cars drive around with chargers that only charge with one phase. However, newer models increasingly have two- or three-phase units on board. The VW ID.4 and Opel Corsa-e, for example, both come with a two-phase charger and a charging capacity of 7.4 kW – meaning they could draw their full power from a wallbox with 11 kW charging capacity.

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