The 48VDC vehicle electrical system

Reading time: approx. 7 minutes – The 48V electrical system offers the possibility of reducing the CO2 emissions of vehicles with combustion engines and thus making an active contribution to environmental protection. In this article, we look at the components required in the 48V vehicle electrical system and the different vehicle electrical system topologies.

The 48V electrical system is regarded as a key technology on the road to e-mobility and is being used for the first time in a production vehicle with the Audi SQ7 TDI. However, the idea of a higher vehicle electrical system voltage is not new. As early as 1990, a 42V consortium was set up out of fear that the 12V on-board electrical system would no longer be able to reliably supply the increasing number of electronic consumers. However, with a few exceptions in Japan and the USA, this promising approach failed to gain acceptance due to a lack of standards. The following section will take a closer look at the reasons and challenges faced by car manufacturers when using a 48V electrical system.

Reasons for a 48V electrical system

While the 42V consortium in the 1990s endeavoured to supply the comfort and driver assistance systems desired by customers, the idea of a higher on-board electrical system voltage is different today. The CO2 limits demanded by the European Commission can no longer or hardly ever be achieved with conventional combustion engines. Alternative drive concepts such as purely electrically powered or hybrid cars are associated with major technical challenges due to voltages above 60V. An on-board power supply system with 48V offers the opportunity to bring simpler hybridisation to market maturity quickly and cost-effectively and to keep pollutant emissions below the set limits.

48V vehicle electrical system topology

The use of a 48V system does not replace the familiar 12V vehicle electrical system, but complements it. Many established components in the vehicle can only be operated with 12V, which would make a complete switch to higher voltages unnecessarily expensive. The integration of a 48V vehicle electrical system can be achieved using two different concepts:

– Isolated solution

If the use of roll stabilisation or electric turbochargers results in short-term high power requirements, these can no longer be covered by the 12V network. For this reason, 48V lithium-ion batteries or capacitors are supplied from the 12V mains by means of a unidirectional DC/DC converter. This solution represents the simplest implementation and has no impact on emissions and fuel consumption, as the energy is supplied conventionally via an alternator.

– Combined grid

In contrast to the isolated solution described above, the energy transfer is reversed here. A unidirectional or bidirectional DC/DC converter supplies the 12V side from the 48V grid. This is connected to the starter-generator, a regulated electrical machine, and starts the engine. When the combustion engine is running, the electric machine supplies the 48V on-board electrical system.

The advantages of a 48V electrical system

A fundamental advantage of a 48V on-board electrical system is the lower power loss during energy transmission due to lower currents. This also results in a corresponding savings potential when designing the electrical conductors. This also enables economically viable recuperation, energy recovery and storage during braking. The electric machine works as a generator. However, when high torque and power are required, this system can also support the combustion engine by operating the electric motor in the opposite direction as an electric motor supplied from the 48V storage unit. This process is also known as boosting. By completely decoupling the petrol engine from the vehicle electrical system, it can also be switched off completely while driving. In contrast to conventional idling, there are no losses in the combustion engine when coasting. All in all, this can result in significant fuel savings.

Disadvantages and challenges in a 48V vehicle electrical system

As the standard 12V vehicle electrical system is largely specified up to 60V, the so-called SELV voltage (Safety Extra Low Voltage), many cable and plug-in components can be used, taking into account the clearance and creepage distances. However, there is an increased risk of arcing at load-carrying disconnection points, which means that the contact must be prevented from opening under load. Arc detection may also be necessary. Electrolysis when electrolyte-containing liquid enters can also be problematic, which makes appropriate protection against moisture necessary. Furthermore, the necessary safety precautions can result in high expenditure and enormous costs.

Components required in the 48V vehicle electrical system

 Increasing the on-board electrical system to 48V requires a number of new electrical components (see also “Power electronics in electric cars”). The most important of these are

– The electric machine

In contrast to the 12V generator, which is usually designed as a claw-pole machine due to the system, the 48V machines must also function as an electric motor. The new requirements allow different designs of electrical machines. A distinction must be made here between various synchronous and asynchronous electrical machines. Torque, efficiency and power density vary depending on the input and output parameters, so that different concepts are likely to be used in practice.

– Inverter

The inverter, a bidirectional inverter, converts the AC voltage into DC voltage when the electrical machine is in generator mode and vice versa when it is used as an electric motor.

– DC/DC converter

The DC/DC converter forms the interface between the 12V and 48V vehicle electrical system. As a rule, the DC/DC converter works as a step-down converter and charges the 12V battery. Exceptions to this are use in vehicles with the above-mentioned island topology and starting the engine at extremely low temperatures. The lithium-ion batteries used for 48V storage systems sometimes no longer have the capacity to start the engine at temperatures below -20°C, so the energy must be taken from the more resistant 12V battery. In this case, the converter works as a boost converter.

– BMS

The 48V battery requires a battery management system for safe operation. This ensures that the individual cell voltages are equalised, monitors the temperature and state of charge and communicates with the control units.

Conclusion

Although 48V vehicle electrical system technology reduces fuel consumption and therefore CO2 emissions in vehicles with combustion engines, the current implementations by car manufacturers are only a transitional solution. Purely electric propulsion is still only possible with a hybrid or a purely electrically powered vehicle. However, the 48V innovations are paving the way for alternative drive concepts and will accelerate their entry into series production.