How bidirectional charging with e-cars works

Due to the switch from conventional energy sources to renewable energies such as solar, wind and hydropower, the possibility of electricity storage will become particularly important in the future. Corresponding storage systems can, for example, absorb solar power on sunny days and feed it back into the grid at night or on days when there is little sun to meet demand. However, this does not necessarily require only large central energy storage systems. Electric cars that support bidirectional charging can be used as temporary storage spaces.

Bidirectional charging allows energy exchange in two directions. The electricity can therefore not only flow into the electric car battery, but also back into the respective household or grid via the wallbox. By the Charging the e-car the alternating current from the power grid is converted into direct current, as electric cars only run on direct current. If the electricity is to be fed back into the grid, it must be converted into alternating current again. This is done by means of a changer either directly in the car or in the wallbox.

In bidirectional charging of an electric car, the alternating current (AC) from the mains is converted into direct current (DC) by the charger or by a special converter in the car. If the electricity stored in the electric car is to be reused in the house or recharged into the power grid, a bidirectional charger can convert it from direct current back to alternating current.

E-car drivers who also use their cars as battery storage systems are among the pioneers in this country. Their number is not recorded. However, in order to bring bidirectional charging into series production, regulatory requirements are needed in addition to further research.

For this reason, the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC) developed the ISO 15118 standard and are responsible for its further development ISO 15118.20. This interface regulates the communication between the vehicle and the charging station. Among other things, the charging process according to the "Plug & Charge" principle should be as simple and convenient as possible, without an app or RFID card for authorization. The vehicle automatically identifies and authorizes itself at the charging station, which then starts the charging process. Billing runs in the background. ISO 15118 uses encryption methods and security certificates to ensure that communication between the vehicle and the charging station and the stored contract data of the users are protected.

Since 2015, enercity has been conducting practical tests to verify the potential of e-cars as mobile electricity storage systems. With intelligent charging boxes, we are already helping our customers to draw their electricity as efficiently as possible – namely at times of energy peaks that are automatically detected as such.

Only so-called DC wallboxes, which have a DC connection, have so far been considered for bidirectional charging. The prerequisite for bidirectional charging to be possible on both the DC and AC sides in the future is the nationwide implementation of smart meter gateways, the intelligent metering systems, and the ISO 15118.20 communication standard.

As of December 2022, charging stations that support bidirectional charging are only available from a few manufacturers. Our selection is not an evaluation, but shows which providers are already intensively following the topic.

• Quasar from Wallbox
• Two-way-digital by Sun2wheel
• sospeso&charge from Evtec

In addition, VW also has a DC BiDi wallbox in the test phase, which will be used for communication and power transfer. According to the company, all ID models with a 77 kWh battery will soon be able to supply electricity again. Vehicles already delivered are to receive an over-the-air update.

There are different applications for bidirectional charging with electric cars:

• Vehicle to Grid (V2G) technology connects the electric vehicles with the Smart Grid, the intelligent power grid. If the car is not used for a longer period of time, the idle time can be used to release the decentrally stored electricity and thus stabilize the grid. In the future, e-car drivers could even be paid to make their vehicle available as an energy storage device. This would further reduce the ownership costs for e-cars.
• With the Vehicle to Home (V2H) principle, the electricity supplied is not used publicly, but privately in one's own home to cover one's own needs – even for several days if necessary. A calculation example: According to the Federal Ministry for Economic Affairs and Climate Action (BMWK), the average daily requirement of a family of four is 11 kilowatts. An electric car with a battery capacity of 50 kilowatt hours could cover the energy requirement for about four days.

To be able to apply this technology, a bidirectional wallbox and an electric car that supports charging in both directions are needed.

Bidirectional charging – these models can already do it:

• Polestar 3
• Nissan Leaf
• Nissan e NV-200
• VW ID.5
• VW ID. Buzz
• Honda e
• KIA EV6
• Hyundai Ioniq 5
• MG 5

Since 2022, several electric cars have been on the market that can charge bidirectionally with the standard CCS connector established in Germany and Europe - for example the Honda e, Hyundai Ioniq 5 and KAI EV6. Previously, bidirectional charging only worked via a CHAdeMO connector. Before the newer CCS solution, the Japanese fast-charging system was the only system that enabled data communication between the charging infrastructure and the electric car by sending commands when energy should be extracted from the car or "delivered".