Inner workings of a charging station

What does a charging station actually look like from the inside?

A charging process is started quickly from the outside. Plug in the cable, authorise if necessary, and off you go. But if you are a bit of an enthusiast, you have probably wondered what goes on underneath the casing. We have the answer and will show you which safety barriers the current has to pass through before it reaches the battery.



FI circuit breaker

First, the current passes through the residual current circuit breaker (FI). This registers fault currents, i.e. it monitors the circuit and trips if an AC fault current occurs. This ensures that there is no danger to life and limb from the station in the event of defects in the supply line or the station.

Installation contactor

After the FI, the current flows through the installation contactor, which is similar to a remote-controlled “on/off switch”. The installation contactor starts and stops the current flow when it is externally instructed to do so, e.g. by phase current detection.

Phase current detection

The electricity network in buildings consists of three “phases”, which must always be balanced. This is a requirement set by grid operators, as excessive load on a single phase would make the power grid unstable. To ensure that this stable state is maintained, the phase current detection measures whether the vehicle is drawing too much current from one or more phases without permission. If this causes the power grid to become unbalanced, the phase current detection sends a signal to the installation contactor, thus terminating the charging process.

DC fault current module

Batteries always draw direct current (DC). The AC current from the mains must therefore first be converted before it arrives in the battery. This usually does not happen in the charging station, but in the car. While the FI switch checks the mains connection on the AC side, the DC protection module protects the charging station against fault currents on the vehicle side. This prevents fault currents from the vehicle from reaching the house connection and damaging the electrics and electronics there.

In some chargers, instead of the DC protection module, an “FI switch type B” is installed, which identifies both AC and DC fault currents. However, if the FI switch trips, the charging station must be restarted manually. The advantage of the DC protection module is that it does not immediately flip the FI switch in the event of problems on the vehicle side, but checks the situation again after some time and, in most cases, resumes operation without human intervention.

Energy meter

If no fault currents have been registered and everything is in order, the electricity flows through its last station: the meter. It is only from here that kWh consumption is measured. Therefore, the energy consumption of the charging station itself is not included in the measurement. If there is an invoice with payment processing at the end of the charging process, drivers only have to pay for the electricity that has actually arrived in the battery.

Logging gateway

The energy values from the meter are signed and stored in the logging gateway. This is absolutely necessary for operation in compliance with calibration law . This ensures that the energy values are not manipulated from the meter to the billing service provider.

Emergency capacitor

In the event of a power failure, the operation of the charging station would be interrupted. Drivers would encounter the problem of the plug lock still being active and they would not be able to disconnect the charging cable. This is where the emergency capacitor comes into play, initiating the unlocking process, so that the plug can be removed.

EVCC (Electric Vehicle Charge Controller)

The EVCC is like a little brain inside the Wallbox. It handles communication with the vehicle, communicates with the DC residual current module and the phase current detection and enables the charging process via the installation contactor.


Most modern charging stations also have a second computer that reads the data from the EVCC and the energy meter and sends it to a cloud via a SIM card. This allows users’ activity to be managed within one software and for charging processes to be billed. In addition, the second computer makes it possible to control several charging stations and thus distribute the available electricity intelligently to the charging vehicles.


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