Pauses on the fast charger can vary in length, even if the same car refills the same amount of electricity. Here we explain how that happens.
High charging capacities are the new trump cards of car manufacturers. Tesla boasts 250 kW in the Model S Plaid, and Porsche even boasts up to 270 kW in the Taycan. The top values only represent a short-term peak value. In practice, the only question that counts is: How long does my electric car need on the fast charger to have enough electricity for the next stage? The explanation is not easy!
Not all charging stations are the same
How quickly an electric car battery can be charged at a DC station – i.e. a fast charger – depends on many factors. First of all, from the charging station itself. How much power does it offer and under what conditions? What happens if another car is connected to the same charging station? For example, while the Ionity stations on the motorways are designed to keep their charging power constant even when frequented, other charging stations sometimes reduce their power. Especially when another car is attached to the same column. Many charging stations are kind enough to share this on the display. The topic of possible charging power is therefore much more complex than the striking kilowatt inscription on it.
Because in addition to the promising kW specification, there is another indication on the charging stations: "The charging power is controlled by the car!" More specifically, the car and the pillar talk about what is or is not possible with electricity. At least the basis has to be right for this, so that the column offers at least as much power as the car demands at most.
The car decides how much electricity flows
Once all that is in place, the electric car can finally decide how many electrons will get going. We talk about performance all the time. Actually, it's all about the amperage, or simply "the current." And from this, after multiplying by the voltage, you get the electrical power. However, the voltage is absolutely determined by the battery structure. In order for current to flow and a battery to be charged, the charging voltage must be higher than the battery voltage. The analogy of voltage as the (sea) height of a water reservoir helps to understand this. Current flows like water from higher to lower point. The current strength (amount of water), on the other hand, can be freely regulated via the tap. When it comes to the battery, four key factors decide how much current is allowed to flow.
The size of the battery determines the "C rate"
The larger a battery is, the more current it can handle when charging. If you connect two battery cells in parallel, the total capacity doubles (the voltage remains the same).But not only that: Because both are parallel, they share the current flow like brothers and the load on the individual cell does not increase, although twice the current and thus twice the power is applied. A BMW iX whose 111.5 kWh battery (gross) is almost 33 percent larger than that of an i4 with 83.9 kWh will, roughly speaking, also tolerate a third more charging current. Because this ratio is so important, the relation between the charging current and the capacity of the battery is given by the so-called C-factor. It allows the charging capabilities of a battery to be assessed regardless of its size. Strictly speaking, the C-factor is defined for charging current in amperes to capacity in ampere-hours. In the electric car sector, the ratio of charging power in watts to energy content in watt hours is taken.
Voltage in the battery: 400 or 800 volts
There are two main voltages in electric cars: 400 and 800 V. The majority of electric vehicles use a 400 volt battery. A high voltage and a low current are advantageous for low-loss current transport. The reason why our domestic electricity is chased across the country by high-voltage lines with several hundred thousand volts - the losses increase with the square of the electricity, higher voltage means less electricity. In the same cable, when the current doubles, the losses are four times higher due to the resistance. If, on the other hand, the voltage is doubled and the current remains the same, the losses also remain the same – although twice the power is transported in both cases. This also applies to the supply lines in the car.
Can this also be transferred directly to the battery? No. A battery in electric cars is made up of up to several thousand individual cells and each of these individual lithium-ion cells has a nominal voltage that cannot be changed by external wiring. It is in the range of 3.7 V for normal cells (depending on the cell chemistry) and varies with their charge. Therefore, increasing the voltage of the battery at cell level is of no use at first. But now the charging station comes into play again. The super fast chargers according to the CCS standard can charge with 400 and 800 volts. However, the maximum current is technically limited to 500 amperes. That means a maximum of 200 kW (400V x 500A) for 400 volts and a maximum of 400 kW (800 V x 500 A) for 800 volts. In practice, 400-volt electric cars can easily exceed this value. A BMW i4 achieves a peak output of 206 kW. The 800-volt electric vehicles are currently limited to a maximum of 350 kW as the power limit. So far, however, no electric car has exhausted this. Speaking of which: we are always talking primarily about Li-NMC batteries here, not about lithium iron phosphate.
Battery temperature is essential
Only a warm battery can be charged quickly. He behaves like a summer vacationer: from 20 degrees he starts to feel good, between 30 and 40 degrees it really goes off.From 50 degrees it has an overheating problem and urgently needs to be cooled. In practice, the possible charging capacity drops noticeably below 20 degrees. Loading a battery that is cold at minus degrees with high charging currents means its death relatively quickly. That's why there are active battery heaters today - the experts call them preconditioning - which first heat up the battery before it is charged. When driving, the battery also reaches higher temperatures by itself due to the heat loss in the cells. For really high currents, however, this is not enough for Taycan, Tesla and Co. The e-car has to be actively informed that a quick charge is due in half an hour and that it should now be heated. It also charges without preconditioning, but not as fast. If, as in the case of Hyundai and Kia, there was no pre-heating in the past, this can lead to very low charging capacities in winter. However, the new models retain this important function.
Battery charge level (SOC) crucial
This is given as a percentage of the maximum usable energy content. Experts call it state of charge, SOC for short. Batteries usually achieve the highest charging performance when charging begins below 20 percent SOC - this varies depending on the vehicle model. The more a battery fills up, the more difficult it becomes to quickly store ions inside. Similar to how it takes a car more time to find a space in a crowded parking garage than in an empty parking garage. There is even sheet metal damage in the battery, since metallic lithium is increasingly being deposited on the poles (so-called plating).
The same effect also happens with cold battery temperatures. Above 80 percent filling level, it is simply no longer recommended to load particularly quickly. This means two kinds of stress for the battery: On the one hand, the strong difference in potential between the poles causes an imbalance and thus stress - so a battery should ideally have around 50 percent charge and thus balanced potential for longer storage - and the almost full battery can do that can no longer process an enormous amount of electricity smoothly. If you drive off immediately after charging, you can also charge up to 100 percent. However, this takes forever, even with DC chargers. That's why a reasonable electric car driver only does this with slower AC charging and when an hour more or less doesn't matter.
Conclusion
The charging power promised by the manufacturer is initially just a number on paper. Because in practice, very different conditions determine the actual current that flows from the charging station. If you want to spend less time on the fast charger on long journeys, you should think about preconditioning if possible.
