How long does it take to charge an electric car?
Battery capacity and the charging current of an electric car
The most important terms associated with charging time are explained below, followed by an exemplary determination of the charging time with a charging power of 22 kW, which is also supported by the hesotec charging stations of our eSat series.
Just as a bucket can hold a certain amount of water, a battery can store a certain amount of electricity. The water flows into the bucket through an inlet pipe with a fixed cross-section. The rate at which the water can flow through the pipe is fixed to ensure that the water does not splash over the edge of the bucket or burst the pipe.
The battery’s supply line is not a pipe but a copper line with a suitable cross-section that ensures that the cable does not get too hot while the battery is charging. Control electronics prevent too much current from flowing at once. When the bucket or the battery is full at the end, the water or the power supply is switched off.
The battery capacity and the strength of the charging current are crucial factors in determining the charging time.
Difference between charging stations for electric cars
Standard charging stations/wallbox for electric cars
For example, a 22 kW charging station is operated with a three-phase alternating current also used with stove connections. It is also known as a three-phase connection. Since this type of connection is usually available in any building, there is generally nothing standing in the way of installing a 22 kW charging station, which ensures that the electric car can be recharged overnight.
Standard charging stations are also known as wallboxes.
Fast charging station
Ultra-fast charging stations
How to calculate the charging time of an electric car
The charging time can be determined as follows.
“Standard charging” is not dependent on direct current or alternating current charging but only on the charging power in relation to the electric car’s battery capacity. If the charging power is less than or half the battery capacity, it is considered slow charging. Example: Battery capacity 44 kWh and charging power up to a maximum of 22 kW (however, this is not quite the correct physical expression).
For slow charging, the time required for a full charge from 0 % to 100 % SOC can be approximated with the following formula:
Battery capacity in [kWh] / (charging power in [kW] * efficiency 0.9) = charging time in [h].
To simplify matters, the efficiency of the charger is assumed to be 90 % (= 0.9). Depending on the vehicle model and charging power, the efficiency can range from 80 % to 95 %.
When charging with a higher charging power in relation to the battery capacity, the charging time is influenced by further factors than previously described.
Today’s lithium-ion batteries cannot be charged at their maximum charging power throughout the entire charging process. The charging current is reduced as the charge level increases. This effect can be disregarded during slow charging because the charging current is generally low.
With fast charging, maximum power is only possible up to about 75 % SOC of the battery. From this point on, the time required to fully charge the battery increases over-proportionally (see Fig. 1). In practice, the use of battery capacities between approx. 10 % and 80 % SOC for long-distance journeys have proven successful in avoiding long charging times. In most cases, direct current fast charging stations switch off even before the battery is fully charged.
The following calculation formula is suitable for determining the approximate charging time required for a fast charge from 0 % to 80 % SOC:
Battery capacity in [kWh] / charging power in [kW] = charging time to 80 % in [h].
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