The shift to 48-V electrical systems in vehicles could mean big changes for electronic components.
Andreas Mangler, RUTRONIK Elektronische Bauelemente GmbH
Ever-stricter limits for CO2 emissions have made one thing clear: The current CO2 limits are practically unattainable with internal combustion engines. Many countries will further tighten their requirements in the coming years: Europe will reduce the current limit of 130 g/km to 95 g/km in 2020, while the U.S. is set to follow suit in 2025 with a limit of 102 g/km. As a step toward meeting these goals, mild hybrid variants incorporating starter/engine/generator functions represent an important step towards the total electrification of the powertrain.
One of the basic prerequisites for mild hybrids is the 48-V electrical system. It is preferable because a 12-V electrical system would require high currents to power the necessary electrical functions.
Many suppliers are retaining the 12-V electrical system for the time being. But according to the VDA (German Association of the Automotive Industry), around four million vehicles will be fitted with 48-V subsidiary electrical systems in 2020. By 2026, this figure will rise to some ten million vehicles, or roughly one in every ten vehicles worldwide.
The result will be a complete change in the architecture of the vehicle. Because the voltage is four times higher, the electrical currents involved can drop while keeping the power unchanged.
In planning such electrical systems, the primary requirement is to ensure the electric motor can easily switch to generator mode when necessary. This mode switch lets the vehicle recover braking energy to charge the battery. During this emission-free sailing/coasting, the vehicle runs using energy from the battery. Entering and exiting a parking space under pure electric power, as well as a CO2-neutral e-boost, are also possible. And the 48-V subsidiary electrical system creates a basis for further innovations without forcing suppliers to rely on larger engines.
The Mild Hybrid
In contrast to full-hybrid models that operate at voltages of up to 360 V, mild hybrids are configured without a charging function. Because both powertrains are not fully decoupled, neither permit pure electric driving. They do permit coasting, an automatic start-stop system, and smooth start up. However, coasting alone reduces CO2 emissions by up to 12% depending on how measurements are made and driving style.
To see some advantages of 48-V systems consider the regeneration process. During regeneration, between 3 kW and 12 kW of energy is recovered. On a 12-V system, 12 kW would entail handling no less than 1,000 amps! In a 48-V electrical system, the same energy levels would entail handling a maximum of 250 amps. However, the higher voltage requires an added focus on safety requirements. To comply with the low-voltage directive, designers must avoid voltage peaks in excess of 60 V. In all, protection against overvoltage and undervoltage in 48-V systems can be more involved than in ordinary 12-V schemes.
The amount of power regenerated depends on the size of the engine among other things: Drag losses rise and the regeneration power falls in line with the number of cylinders. For this reason, designers generally use a fixed coupling between the internal combustion motor and the starter generator for smaller engines. For larger engines, a separated, active decoupled connection makes more sense as it allows braking energy to feed back into the battery.
Thanks to the lower currents in the 48-V electrical system, cables with significantly lower cross-sections can be used. This practice can reduce the vehicle weight by up to 10 kg, which also helps lower CO2 emissions. What’s more, mechanical components such as the steering system can be replaced with – smaller and lighter – electric units.
Among the first items to be added to the 48-V subsidiary electrical system are those that consume a lot of power: candidates include positive temperature coefficient (PTC) auxiliary heaters, seat or front windshield heating systems, as well as the climate control system. Over the medium term, they will be joined by other components such as electric steering, chassis stabilization equipment, audio power amplifiers, and LED lighting, which can then operate at higher voltages. In the long term, even smaller power consumers will join them.
These changes also frequently result in enhanced driving comfort. For instance, if the front and rear windshield heating consumes 1.5 kW at 48 V, the fan can rotate at a lower speed. The PTC auxiliary heater in the 48-V system is also available with full power at idle speed immediately after the engine starts. This allows it to heat the interior compartment, engine, and transmission before the engine warms up. This fast heating is all the more important in hybrid vehicles as the electric motor generates virtually no waste heat.
In the context of the engine’s automatic start/stop system, the 48-V electrical system also allows swift, repeated smooth starting without the internal combustion engine, as is necessary in stop-and-go traffic. If the compressor for the climate control system is on the 48-V system, the PTC heater, ventilation and fans, as well as a contact heater, can continue to operate for up to about three minutes, even during an automatic engine stop.
The key components of the 48-V electrical system are the start/stop generator, the battery – currently based on lithium-ion technology – as well as the electric steering and heaters. The starter-generator and the power electronics can be implemented with a conventional claw-pole machine (so named because poles of the rotor look like fingers of two hands interlocked with each other. The coil mounts axially inside and field current is supplied by slip rings and carbon brushes.) or a synchronous machine (essentially the same as a synchronous motor where the magnetic field of the rotor is supplied by direct current or permanent magnets).
The claw-pole variant is more closely aligned with the 12-V standard, enables economical mass production, and is easier to integrate into new vehicles. However, the V-belt has a negative influence on the torque transmission, and the lifetime of the slip-ring system is limited. In the synchronous machine, the permanent magnet ensures maximum power density. Because it does not use a slip ring, it is practically maintenance-free. The disadvantages are the higher costs, more complex integration, and the machine’s use of rare earth elements.
The battery system includes the battery cells themselves, a cooling system, a specific housing and stack structure, cell balancing electronics, and a sophisticated battery management system. The latter uses various diagnostic functions such as temperature monitoring, overvoltage and undervoltage monitoring to keep battery cells within their safe working range. In this regard, a whole range of design goals must be taken into account, especially rapid charging at 12 kW and compliance with crash requirements.
Lithium-ion batteries meet most of these requirements. Cylindrical cells also offer the advantage of coming in economical standard sizes that can connect in parallel and in series as a stack. Determination of battery cell state-of-health (S-o-H) poses an additional challenge. Present-day battery management systems adhere to strict documenting procedures to record the charging and discharging operations. In the process, they accumulate vast quantities of data because of the complicated Coulomb counting procedures involved, to draw conclusions regarding the state of the battery.
SoH status necessitates the measurement of complex impedances under real-world conditions. Today the techniques for these measurements are neither economical nor amenable for mass production. It is precisely in this area of battery management systems that Rutronik, together with leading universities, is actively driving research and development.
A core component of the 48-V subsidiary electrical system is also the bidirectional DC/DC converter that connects both systems. This unit typically offers an output of 3 kW with peaks of up to 3.5 kW for two seconds and is passively cooled to hit efficiencies of at least 96%.
Various suppliers already offer equipment meeting the requirements of 48-V systems. For instance, tier-1 supplier Valeo has developed subsystems to support 48-V powertrains. Other suppliers such as Continental, Bosch, Preh or Delphi are preparing their own.
However, neither mild nor full-hybrid vehicles roam the roads in large numbers, so the segment is relatively uninteresting for the distribution sector – for the time being. For when it comes to meeting CO2 regulations, there is no alternative. And with help of tax incentives and the roll-out of charging points, electric vehicles are on the rise.
Component suppliers are meeting the current challenges across the wide range of automotive applications, especially in the area of e-mobility. Other areas of focus include car-to-car communication, infotainment, driver assistance systems and lighting, as well as the continuously rising safety requirements according to ISO 26262, which also covers on-board electrical applications.