As electric cars and rechargeable hybrids become more common, car-specific questions on the subject will also become more common.

Like, for example, how long does a particular electric car have to be kept in the charger in order for the car’s empty energy tank, or driving battery, to be full again?

In this respect, the indicative time is obtained when the car's traction battery capacity is divided by the available charging power.

For example, a 20 kWh traction battery and 7.4 kWh charging power give a charging time of 2.7 hours, with a calculation formula of 20 kWh / 7.4 kW = 2.7 h.

Before calculating, you must therefore know how much power the car and the charging station can charge the battery and what size driving battery the car is equipped with.

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So let's take a hypothetical example, a fully electric car with a battery capacity of 20 kWh. Its internal AC charger is capable of charging from two phases at 7.4 kW and from the quick charger it is capable of charging at a maximum power of 50 kW.

First of all, the calculation must take into account that the battery is not charged at full power, but the power decreases when the battery is full. This is why manufacturers often report charging times to 10%… 80% capacity, which in itself is a very time and cost effective way to continue the journey.

Less than half a day from the cleaning ladder or half an hour with a quick charger

Roughly the charging time of the example can be calculated with the basic formulas of electrical engineering as follows:

If the car is charged from the suko plug with the charging cable supplied with the car, the current of which is limited to 8 A:

P = U * I = 230 V * 8 A = 1840 W = 1.84 kW

E = P * t => t = E / P

T = 20 kWh / 1.84 kW = about 11 h

If the car is charged from a basic charging point (7.4 kW):

T = 20 kWh / 7.4 kW = about 3 h

If the car is charged from a fast charging point (50 kW):

T = 20 kWh / 50 kW = approx. 30 min

In addition to the energy going to the battery, part of the charging current also goes to the car's low voltage circuit. When charging, energy is also required for the controllers, the battery cooling, the holding current of the battery contactors and so on. This energy is removed from the charging current going to the battery, thereby slightly extending the charging time.

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Especially when charging at higher currents, the temperature of the battery also significantly affects the capacity to receive current. Thus, as the temperature decreases, the charging current decreases. In addition, in severe frosts (typically -15 degrees Celsius or less), part of the charging power is used to heat the battery to prevent damage. The exact limit depends on the car model, but in a frost of -30 degrees, for example, an 8 amp charging current is not enough to even start charging if the battery has had time to cool down after driving.

The respondents were Tuukka Heikkilä, M.Sc. other Finnish experts in the field of automotive technology.

You can find more of these set of electric car questions and their answers with the answers below.