How long does it take to charge an electric car?

How long does it take to charge an electric car? Learn everything about the charging time and how to calculate it.
1. October 2020

Battery capacity and the charging current of an electric car

Those considering the purchase of an electric car are mainly concerned about range and charging time. The charging time of an electric car depends on several factors, such as battery capacity, maximum charging current, charging technology, surrounding temperature, state of charge and type of charging station and cannot be determined in general terms. However, if specific key data are available, it is possible to determine a rough reference value.

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

Standard charging stations are available with different charging powers between 2-22 kW. The most common are Type 2 alternating current standard charging stations with 11 kW or 22 kW, such as the hesotec electrify eSat series.

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

Fast charging stations are available with a charging power of around 50 kW or more, most of which are found in public places. The electric vehicle’s charging technology must support the higher charging power to enjoy a shorter charging time. The higher charging currents require a different power supply technology and are rather unsuitable for private use.

Ultra-fast charging stations

The development and distribution of mainly public ultra-fast charging stations with up to 350 kW (as of 2019) are steadily advancing in Europe. They are of little relevance for private use or in company parking lots. A pioneer in this field is Tesla with its Supercharger charging stations, where charging was available free of charge until 2018.

How to calculate the charging time of an electric car

As already mentioned, the charging time is mainly dependent on the battery capacity (SOC) of the electric car and the charging power of the charging station.

The charging time can be determined as follows.

Standard charging

“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 %.

Fast charging

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.

charge diagram

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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