Relevance of this field of application

With a share of 20 % of greenhouse gas emissions in Germany, the transport sector is the third largest emitter after the energy sector and industry and a central field of action for climate policy. At 59 %, car traffic accounts for the largest share of transport-related greenhouse gas emissions.¹

The German government is focusing on electromobility for the energy transition in the passenger car sector. This includes battery electric vehicles (BEV) in particular, but also hydrogen-based drives with fuel cells (fuel cell electric vehicle, FCEV). With the current electricity mix, CO2 emissions can be reduced by 16% and 27% respectively with a BEV compared to a passenger car with an internal combustion engine. By increasing the share of renewable energies in the electricity mix and by making progress, especially in the service life and recycling of batteries, this value could be significantly improved.² Furthermore, electric vehicles can help to reduce other problems caused by road traffic, in particular road noise, the (local) emission of air pollutants such as particulate matter and nitrogen oxides, and dependence on mineral oil imports.

From a global perspective, the need for action increases: in 2019, there were almost 1.2 billion passenger cars worldwide,³ and in 2035 this figure is expected to reach two billion.⁴ Around 12% of global CO2 emissions are attributable to road traffic, 60% of which is accounted for by passenger transport.⁵

Use of battery storage

The electric traction battery, a high-voltage battery with a nominal voltage of 400-1,000 volts, is the central element of a BEV. Its characteristics determine range and charging times, which are key factors for practicality and comfort and thus for the market success of the vehicles. With a weight of several hundred kilograms and capacities of 20-100 kWh, it is also the most expensive component of electric cars.

In principle, the traction battery can be placed in different areas of the vehicle body. It is usually installed in the vehicle floor. It also serves as a storage location for the recuperation energy, which is recovered from the kinetic energy of the vehicle during braking. The on-board electronics are usually supplied from a separate (low-voltage) battery. In principle, alternating battery models are possible, but hardly pursued at present. Instead, the storage unit is usually permanently installed. It can be charged at the home charging station ("wall box") or at public charging stations.

The use of a traction battery is also common in electric vehicles with fuel cells: this can be charged both by the fuel cell and by recuperating braking energy. The battery is used in driving situations that require a lot of engine power. Depending on the context of use, BEVs or FCEVs may appear advantageous: For city cars, for example, the battery-electric drive is likely to be more attractive in terms of purchase costs, refuelling options and energy efficiency. For taxis, on the other hand, hydrogen may be the preferred option due to its greater range.⁶

Performance requirements

Central motives for the promotion of electric cars are climate and resource protection concerns. It therefore goes without saying that the battery storage systems used must meet high environmental standards: Their production should require as little energy and (critical) raw materials as possible; their performance parameters (specific energy, performance over time, deep discharge capability) should enable "a lot of use with little battery" in operation; and their cycle stability should allow many years of use.

Furthermore, safety aspects play an important role. In normal operation and especially in accident situations, battery storage should not be a fire or explosion hazard. The same applies with regard to parking and charging times. Finally, a third performance factor is the fast-charging capability: it determines the duration of the individual charging processes and thus decisively the comfort for the vehicle user. However, fast charging can be a significant driver of battery ageing.

Up to now, traction batteries have usually been designed so large that fast charging without ageing damage and a long range are possible at the same time. In the future, the efficiency potential of electric cars will only be realised if fast-charging capability, low battery ageing and a long range can be achieved with batteries that are as small as possible.