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Regulars' table knowledge: technology facts about electric cars

When charging is not good for the battery, do you know the memory effect, can it be compared 1:1 with a petrol tank? Here we answer good questions about e-mobility so that you can shine with expert knowledge at the regulars' table.

You still knew your way around with combustion engines, for example you knew why shifting up early saves fuel and why displacement cannot be replaced by anything but ... yes, exactly! Many new questions arise when it comes to electric cars. We answer the most important of them. You can see the e-car innovations of 2022 in our picture gallery.

Why is the power storage unit also called a battery - shouldn't it be called a rechargeable battery?

In principle, an understandable idea, after all, the term "battery" is used in German for cells that are only used once and then, in the best case, are recycled. The word "battery" in this country, on the other hand, stands for rechargeable cells. However, this difference does not exist in English, which is why the term "BEV" (Battery Electric Vehicle) has become established internationally for purely electric vehicles. So both designations are correct.

Why is the battery of an electric car more than just its tank?

Basically, the comparison with a tank is not entirely wrong: the larger the battery, i.e. the more energy it stores, the further an electric car can go. However, a petrol engine with 100 hp doesn't care whether it gets fuel from a 30 or 80 liter tank - it will always produce its 100 hp. It's different with an e-car. Here, the system performance results from the interaction of the electric motor, power electronics and battery. The latter in particular is often the bottleneck that limits the acceleration of an electric car. The following applies here: the larger the battery, the higher its current delivery capacity. ,

A battery that is twice as large also delivers twice as much power. This becomes clear, for example, with e-cars, which are specified with different driving performance depending on the battery capacity, although identical motors are installed. Also interesting: A combustion engine is an energy converter that first converts the chemical energy contained in petrol and diesel into thermal energy through combustion. The heat generated leads to an increase in volume and pressure in the cylinder, causing the piston and crankshaft to move. An electric motor, on the other hand, gets its energy from the battery in bite-sized form as electrical energy. The fact that no additional energy conversion takes place is one of the reasons why e-cars are much more efficient than combustion engines.

Gross, net and in general: How much power does my battery hold?

, The amount of energy a battery stores is usually measured in kilowatt hours (kWh). However, the information sometimes varies for one and the same battery: Mercedes specifies 80 kWh for the EQC, although the memory actually takes up around 85 kWh.However, the last 5 kWh cannot be run empty because this would put too much strain on the battery, which would affect its durability. The range of an electric car results from the net capacity of the battery, while the gross figure is more of technical importance. But how can it be that 90 kWh of electricity flows into a completely empty 80 kWh battery when charging? Isn't the real capacity perhaps larger? No, this phenomenon is due to load losses. Normally, when current flows through a conductor, a resistance arises, in which electrical energy is converted into heat and thus dissipates uselessly. How high the charging losses are depends on many factors. Typically it is around 10 to 15 percent. ,

What can I do to ensure that my battery charges as quickly as possible at fast charging stations in winter?

, With up to 350 kW, DC fast chargers offer enough power for short charging processes. However, most e-cars can only cope with 50 to 200 kW, and even these often do not make it. How much is actually put through to the battery is determined by the charging electronics in the car. And an important factor is the temperature of the battery. He feels most comfortable - a curious parallel to humans - between 20 and 35 degrees. At low temperatures, the electrochemical processes in the battery cells are delayed. The ions can no longer be stored in the battery so quickly, so the electronics reduce the charging power so as not to damage the cells. If you drive your e-car parked overnight in sub-zero temperatures to the nearest quick charging station in winter, you will be surprised how slow the charging process starts. It would be better to drive a few kilometers first so that the battery warms up. Some e-cars can also precondition their batteries. Its active heat management is able to heat the energy storage on the way to the charging park. If you use it, you won't have to wait as long.

Why does the time of charging also play a role in battery aging?

, The wear and tear of a battery depends on two factors: its age and the number of times it has been charged and discharged. Even if an e-car just stands around, its battery ages, so that the maximum capacity and thus the range decreases. Charge states (SOC=State of Charge) close to zero and close to 100 percent are particularly stressful for the materials inside. If there are no longer journeys, the battery feels most comfortable with an SOC between 30 and 70 percent. An e-car should therefore not be slavishly returned to the wall box after each trip. , If you are going on a longer route, it is best to fill up to 100 percent SOC just before the start of the journey. The departure time can often be set on the on-board computer in order to control the charge accordingly. Aging processes are also temperature-dependent, heat promotes wear enormously.The worst thing you can do to an electric car battery is charge it to the brim and then park the vehicle in direct sunlight.

Why is there no memory effect in electric car batteries?

, The memory effect is a major problem with battery types such as nickel-cadmium (NiCd) or nickel-metal hydride (NiMH) batteries: If you only use part of their energy and recharge it early, the battery will eventually "forget" the unused ones areas and reduces its capacity. The effect is caused by the formation of crystals on the cadmium cathode. However, NiCD and NiMH cells are hardly (still) found in high-quality products. The lithium-ion batteries in today's e-cars have no memory effect. Whether it occurs depends on the cell chemistry used.

Why do e-cars with 800-volt technology sometimes hardly charge faster than some 400-volt models?

Double the voltage, half the charging time? In theory, that sounds plausible: a typical high-power charger on the highway can charge up to 500 amps. If you multiply current (A) and voltage (V), you get the charging power. A maximum of 400 V x 500 A=200 kilowatts can therefore flow on a 400-volt system. With 800-volt technology, the charging capacity is theoretically doubled to 400 kW. However, most batteries cannot cope with such high charging capacities. Even the 800-volt pioneer Porsche Taycan is limited to 270 kW, and these 270 kW only flow at the beginning. In order not to put too much strain on the battery, the charging power is successively reduced depending on the charge level. More important than the theoretical maximum value is therefore what power can be absorbed over a longer period of time, for example through effective cell cooling. The voltage level of a single lithium-ion cell is always 3.7 volts anyway – regardless of whether it was integrated into a 400 or 800 volt system. The main advantage of the 800-volt technology is the lower ohmic losses in all cables, which increases efficiency and allows thinner cable cross-sections, among other things.

Why do some e-cars charge much slower than promised, especially on wall boxes?

, Strange: Many of the AC on-board chargers designed for 7.4 kilowatts in electric cars only use 3.7 kW on standard 11 kW wall boxes. Is it a defect? Or do you have to change something on the car or wallbox? Neither - it's because the car isn't designed for multi-phase charging. Wall boxes usually pass on all three phases (= power lines) of the house connection to the car. The 11 kW therefore flow over three phases of 3.7 kW each. However, if the charger in the car only supports single-phase charging, only 3.7 kW arrive. If you want to achieve the full 7.4 kW, you need a 22 kW wall box that outputs 7.4 kW via each of the three live lines.In Germany, however, the Unbalanced Load Ordinance prohibits loading a single phase so heavily. More than 4.6 kW are not possible if the other phases remain unused.

Why is the cd value even more important for electric cars than for combustion engines?

Since electric cars can recuperate, the vehicle weight plays a smaller role in consumption than with combustion engines. A higher mass balances more kinetic energy back into the battery when braking. However, irretrievable losses of driving resistance are all the more noticeable. From approx. 80 km/h the displacement of the air costs the most energy, the air resistance increases with the square of the speed. , A electric car with very good aerodynamics therefore gets further on the Autobahn. Electric vehicles also have to be more efficient with the energy they carry. A 100 kWh battery, as installed in some luxury electric vehicles, has an energy content of just around ten liters of diesel fuel. Due to the higher efficiency (about three times as good) of electric motors and recuperation, electric cars still have more than decent ranges - as long as the aerodynamics are right! ,

You can find a good overview of electromobility on our Electric car topic page .


Many new questions arise when it comes to electric cars. "Why is that different from the petrol engine?", "What about the memory effect?" and many more. We answered the most important of them.


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