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Technology explains: what is important when comparing e-cars

Battery capacity, charging power, engine
Subscriptions & booklets
  1. Battery capacity and range
  2. Battery capacity
  3. temperature dependence
  4. charging power and Loading time
  5. CO2 balance
  6. Electric motor types
  7. Recuperation

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Energy management using the example of a Tesla Model S P85

Problem: With the consumption, which electric cars indicate very precisely on the on-board computer, the range cannot be extrapolated so easily. Because many manufacturers only give thatGross energy content of your batteries. The actual battery size that can be used in practice is therefore an estimate. Audi, for example, specifies an energy content of 95 kWh for the E-Tron. That is the gross value. Less the 13 percent mentioned above, 82-83 kWh are more likely to be used.

Another difficulty: Some manufacturers give the net value. Mercedes, for example, calls the EQC 80 kWh - net. This allows the real range to be extrapolated with consumption values ​​from practice, but you have to be careful before making a comparison with the E-Tron - and first determine the net range of the Audi battery.


The gross and net energy content of car batteries differ by a buffer amount that must not be used to protect the battery. The net range is best suited for comparing batteries of different models. It can also be used to calculate the range in practice with real consumption values.

The actual performance of an electric Cars and their charging speed strongly depend on the current capabilities and the size of the battery.

How much power or energy a battery can deliver depends largely on its current capabilities. These values ​​follow very simple formulas as the example of a lithium polymer battery for a model car shows:

Energy content=(nominal) voltage (V) x capacity (Ah)

Example : 7.4V x 5 Ah=37 Wh

Power=voltage (V) x current(A)

Example: 7.4V x 10 A=74 watts

Maximum power=voltage x maximum current

Example: 7.4V x 300 A=2220 watts

(simplified formulas for sometimes very complex relationships)

A=ampere, unit for current intensity

V=volt, unit for electrical voltage

Ah=amp-hour, unit for capacity

W=watt, unit for power

Wh=watt-hour, unit for energy

To assess how much a quick charge stresses a battery, the capacity of a battery can be set in relation to the charging current.

The example battery has a capacity of 5 Ah. If this is now charged with a constant charging current of 5 A, the cell is charged after one hour. The ratio of capacity and charging current is 1 (the electrical engineer says “1 C”). If the cell were charged with twice the current of 10 A, the ratio would be 2 (2 C). In many cases, a current that is twice the capacity is the limit for fast charging. How much electricity a battery can handle with a quick charge without suffering major damage depends not only on its size, but also on its cell chemistry.

In the automotive sector, the capacity of a battery is usually not used in Ampere-hours (Ah), but its energy content is given in watt-hours (Wh). Since Ah and Wh are proportional with an assumed constant voltage, this calculation can also be carried out with watts and watt hours.

That means: If a Tesla battery with 85 kWh energy content with 135 kW power is connected to a direct current Column is charged, this corresponds to 1.6 C - an even more moderate value. If an only 50 kWh battery were charged with this power, this would correspond to 2.7 C. That would destroy the battery in the long run. Porsche and Tesla are currently planning significantly higher charging capacities of 200-250 kW. If the cell chemistry is designed for it, manufacturers can allow it. Nevertheless, a battery is more stressed by high currents than by low currents and therefore ages faster.

Roughly, it can be said that a battery can deliver and tolerate more maximum current with increasing energy content. The installation of a more powerful electric motor therefore only produces more power if the battery also grows. A higher charging speed is only possible if the battery also grows.


The actual performance of an electric car and its charging speed depend heavily on the Current capabilities and the size of the battery. Anyone who only charges their car at the fast charging station must expect the battery to age more quickly.

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Temperature ranges of a lithium-ion battery

At low temperatures the usable capacity decreases, as the internal resistance increases (especially drastically at minus temperatures) and thus less current flows. This applies to both delivered and consumed electricity, so that a battery is damaged if it is loaded with high charging currents at sub-zero temperatures. Then metallic lithium is deposited on the cathode (plating) because it can no longer absorb the lithium ions quickly enough. Which, by the way, also happens at higher temperatures when too much power is placed on the battery. Above 40 °, the battery ages particularly quickly and loses energy particularly quickly when it is stored - the battery only likes it cool for storage.


In the case of an electric car, consumption and charging time depend heavily on the outside temperature. Batteries not only deliver less electricity when it is cold, they can also be irreversibly damaged when charged with high currents. An active thermal management of the battery is therefore important for a long service life of the battery and good current capabilities.

auto-motor-und-sport .de Important: Performance is not the same as charging time!

In addition, the maximum is especially for direct current fast charging Charging power often not available in the full range up to 80 percent state of charge. The maximum charging power fluctuates depending on temperature and state of charge factors. Even more interesting than the pure charging power or the time for an 80 percent or full charge is the question: How many kilometers can I charge in half an hour, for example? This figure is in turn dependent on consumption: An Audi E-Tron consumes 26 kWh /100 km and can theoretically recharge a range of 290 kilometers in half an hour with 150 kW charging power. A Kia e-Niro with 15 kWh /100 km consumption shovels 330 km range into the battery in half an hour with just 100 kW charging power.

With AC-based charging, the speed is not quite as important as it in most cases takes place at home and overnight. However, a modern electric car from 50 kWh battery should have at least 11 kW three-phase charging power.


The theoretical maximum charging power is only a rough guide something about how much time has to be spent at the charging station. More important is: What is the true loading speed? Which charging options are offered? Consumption also plays a role.

The carbon footprint of electric cars with small batteries compared with combustion and hybrid vehicles.

Electric cars and vehicles with combustion engines can only be fairly compared on the basis of a well-to-wheel comparison, i.e. from the energy source to the wheel. The data for the carbon dioxide load on one kWh according to the German electricity mix are always well-to-wheel - i.e. with all emitters. According to the Federal Environment Agency, the generation of one kWh of electricity currently produces an average of 489 g of carbon dioxide.

For combustion engines, the usual consumption figure is tank-to-wheel. So that just takes into account how muchCO2 is produced when the fuel is burned while driving. Therefore, a fair comparison of emissions also includes the energy required to produce the fuel. There is also an EU study from 2007: Each liter of gasoline produces approx. 19 percent more CO2 during production. With diesel it is about 20 percent.

According to the IFEU Institute in Heidelberg, 125 kilograms of CO2 are produced during the production of one kWh battery. There are also values ​​in the range of 200 kg, but these are extremely controversial. Depending on the size of the car, an electric car generates a few tons less CO2 for the production of the rest of the car compared to a combustion engine.

Companies such as Mercedes or VW offer exact environmental certificates that show the emissions during production.

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The carbon footprint of electric cars with large batteries compared to combustion vehicles.


How environmentally friendly the car really is has a lot to do with the CO2 produced during production - and with how the electricity is generated. A comparison of different concepts only works well-to-wheel. In principle, the following applies: the smaller the battery in an electric car, the more environmentally friendly it is in terms of production and thus also in terms of its overall balance. Comparisons of the CO2 balance show that electric cars with smaller batteries already emit less CO2 over their life cycle with the current electricity mix than cars with combustion engines. In addition to the decreasing CO2 load on power generation, there is further potential for savings in the low-CO2 production of batteries.

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Advantages and disadvantages of the individual electric motor types.

The PSM has the worst environmental balance because it relies on a rare earth (neodymium) for its magnet. In China, neodymium is extracted with the release of high greenhouse gases.


The type of electric motor has an impact on the performance and efficiency of electric cars , on the recuperation concepts and on the environmental balance.

Example of recuperation for the smart electric drive


The recuperation strength and strategy has a major influence on the efficiency of the electric car under certain driving conditions. The potential energy gain through recuperation is greatest in the city and in the mountains, technically it also depends on the motor configuration.


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