Bidirectional charging 


In the ongoing quest for more dependable and sustainable means of producing energy, the problem of efficient energy storage solutions has long been something of a stumbling block. But now, a joint research project between Audi and the Hager Group is looking at ways to incorporate the electric car into the domestic grid by making it a part of the storage solution as well as a means of transport.

In places like Germany, renewable energy account for more and more of the electricity generation, with more than 50 percent of the German electricity mix in the first half of this year produced by renewable energy systems. One of the major problems with renewable energy though remains, that being the lack of constant supply because of weather factors – the sun doesn’t always shine and the wind doesn’t always blow. This is compounded by a lack of capacity to store the generated energy that the grid cannot use. 

But, as the number of registered electric cars increases, the number of mobile energy storage units also rises, offering great potential, provided that the storage capacity can be used intelligently. 

“Electric mobility is bringing the automotive industry and the energy sector closer together,” says Martin Dehm, technical project manager for bidirectional charging at Audi. 

“The battery of an Audi e-tron could supply a single-family home with energy for around one week independently. Looking ahead, we want to make this potential accessible and make the electric car part of the energy transition as an energy storage device on four wheels.” 

Essentially, the high-voltage battery of the electric car is not only charged via the wall box at home but can also supply energy back to the house as a decentralised storage medium. If the customer has a photovoltaic system, the electric car serves as a temporary storage medium for the domestically generated eco-electricity and when the sun isn’t shining, the vehicle can supply the stored electricity back to the house. Bidirectional charging at home or Vehicle to Home (V2H) – has great potential to reduce the home owner’s electricity costs and increase network stability.

“Using the battery of electric vehicles to contribute to climate protection while lowering electricity costs at the same time is a vision that we have found fascinating since the very beginning. And we have found an ideal partner in Audi,” says Ulrich Reiner, project manager at Hager Group. 

Of course this all sounds very simple, but to actually make it a reality requires significant technical ability and of course interaction between different technical components in terms of infrastructure and in the vehicle. 

An Audi e-tron with near-series charging technology was used in the research project, and was charged using a DC wall box with a charging capacity of up to 12kW and a flexibly extendable home storage unit with a capacity of 9 kWh. 

While it could provide additional flexibility in possible series production, it is not a necessary requirement for bidirectional charging. 

Thanks to the DC voltage level in the overall grid, the connection between the PV system and the vehicle does not require an inverter and is thus a particularly efficient solution. 

Bidirectional charging focuses mainly on use cases where home owners use their own photovoltaic system to benefit from cost-optimised charging with their domestically generated electricity. The electric car stores the excess electricity from the PV system that is not used. If the customer has variable rates, the electric car can supply the entire house in phases where electricity prices are high. At night or during non-productive times of the rate, the car then uses inexpensive electricity to charge up to the desired target SOC (state of charge). 

“Maintaining mobility is at the centre of our attention. Customers therefore don’t need to restrict themselves in order to make bidirectional charging suitable for everyday use,” Dehm says.

“The intelligent charging management manages the optimum use of the battery, thereby maximising the cost-effectiveness of the overall system. The system is very easy for customers to use – all they have to do is plug in the car, and the rest happens automatically.”