Currently, 48V systems are mainly used in mild hybrid and low hybrid electric vehicles. Hitachi's newly developed lithium-ion battery pack further expands the application of 48V systems in the field of hybrid vehicles.
The concept of a 48V system
To more effectively conserve energy and reduce emissions, and improve fuel economy, a new system has been found between modifying the engine itself and replacing it with a completely new powertrain: the hybrid system. Currently, the most widely used system is the gasoline-electric hybrid system, which adds an electric motor to a gasoline/diesel engine vehicle, allowing both engines to work together. The earliest mild hybrid system added a belt-driven starter motor (BSG) to the starter motor (usually 12V) of the traditional internal combustion engine. This motor is a stop-start integrated motor used to control the engine's start and stop, thus eliminating engine idling, reducing fuel consumption and emissions—this is what is known as the 12V mild hybrid system, which also has an automatic start-stop function.
The 48V system is an improvement on the 12V mild hybrid automatic start-stop system. The system has a voltage of 48V and uses a power-type lithium-ion battery with less than one kilowatt-hour of energy to replace the traditional lead-acid battery. It also uses a BSG motor to replace the traditional starter motor and generator, which is equivalent to an enhanced automatic start-stop system. In addition to the automatic start-stop function, it can also provide auxiliary power when necessary.
Source of 48V system
The concept of the 48V system originated in 2011 when several German automakers, including Audi, BMW, Daimler, Porsche, and Volkswagen, jointly launched and formulated the LV148 standard. In June 2013, the European Union approved the 2020 CO2 emission standards, which require that the average CO2 emissions of new cars be controlled at 95 grams per kilometer by 2020; European automakers that fail to meet the CO2 emission limits in the future will face fines of up to €95 per gram per vehicle exceeding the limits. This mandatory standard has prompted automakers and parts suppliers to reduce vehicle fuel consumption through different technological approaches. The 48V system, with its significant energy savings and emission reductions at a limited cost, has become a focus of attention in the industry.
Advantages and key technologies of 48V systems
Infineon believes the development of 48V systems is based on the following points:
While 12V start-stop technology is low-cost, its CO2 emission reduction is limited.
High-voltage hybrid power systems reduce emissions significantly, but increase system costs considerably.
48V at a cost close to that of a 12V start-stop system;
It offers performance close to that of high-voltage hybrid technology;
CO2 emissions can be reduced by up to 15% or even more;
The system cost target is less than 1,000 euros;
It can provide loads other than the power system with a more advanced and energy-efficient design.
To illustrate the advantages of a 12V mild hybrid system, Hong Yinglin, Delphi Packard Asia Pacific New Energy Product Manager, gave us an example: "Take air conditioning, for instance. In traditional cars, when waiting at a red light, the engine is idling, and the belt-driven air conditioning compressor can continue to work, allowing the air conditioning to still provide cooling. However, for cars with start-stop functionality, the system needs to switch to electric air conditioning when waiting at red lights; otherwise, passengers would have to endure the lack of air conditioning while waiting at red lights or in traffic jams. An electric air conditioning compressor requires a minimum power of 2.5kW. If powered by 12V, the power grid would struggle to handle it, but 48V solves the problem. Furthermore, the 48V side can also power the active suspension, electric power steering, and electric heating systems."
Currently, the mainstream in the Japanese market is strong hybrid electric vehicles equipped with high-voltage batteries of 100V or higher, using lithium-ion rechargeable battery modules of 100V or higher. While this significantly improves fuel efficiency in hybrid electric vehicles (HEVs), the cost of specialized and high-voltage components is high. Using a 48V system also increases costs, but compared to the cost of pure electric vehicles or strong hybrid vehicles, a 48V system can save 40% to 60% in costs, and 48V lithium-ion rechargeable battery modules can also improve fuel efficiency by 10% to 15%.
One of the key aspects of the 48V system specification is lithium-ion battery technology, which offers a higher (3 times) energy density than lead-acid batteries while reducing size and weight. 48V lithium-ion batteries also offer better charging performance, allowing for more efficient storage of regenerative braking energy and providing more power for the increasing number of electronic loads in vehicles, such as heated front seats, heated windshields, and rear shock absorbers.
Current Status of 48V Lithium-ion Battery Systems
One of the key aspects of a 48V system is lithium-ion battery technology. Compared to lead-acid and nickel-metal hydride batteries, lithium-ion batteries have higher energy density and better charge/discharge performance. A single lithium-ion battery cell has a voltage as high as 3.7V. To meet the voltage requirements of electric vehicles, individual cells need to be connected in series and parallel to form a battery pack, satisfying the high energy and high power demands of electric vehicles. Generally, the automotive power system treats the battery pack as a single cell. During the start-stop process of driving, the entire battery pack undergoes frequent charging and discharging. Since the battery pack is composed of individual cells, the absolute consistency of the individual cells cannot be guaranteed before leaving the factory. If a cell in the battery pack, after multiple charge/discharge cycles, cannot periodically balance its state of charge with the other cells, it will eventually enter a deep discharge state, leading to damage and ultimately battery pack failure. Therefore, a Battery Management System (BMS) is needed to monitor and measure the overall condition of the battery pack and individual cells, including voltage, current, and state of charge, to replace individual cells in a timely manner, extending the battery pack's range and lifespan.
Hitachi's newly developed 48V lithium-ion battery pack integrates the BMS substrate, battery cells, relays, and fuse packs into a single enclosure, offering high assemblability and reliability. This battery pack can support the efficient operation of the engine and motor in hybrid vehicles under various conditions, including vehicle acceleration, deceleration, and energy regeneration during braking.
Currently, 48V lithium-ion battery packs on the market are generally composed of 12 to 13 individual cells and are mostly used in small pure electric vehicles such as electric bicycles, electric motorcycles, and electric tricycles, as well as power tools. Major manufacturers of automotive lithium-ion power batteries include CALB, CITIC Guoan Mengguli, and Wanxiang Group.
In the hybrid vehicle sector, 48V lithium-ion battery systems are primarily used in mild hybrid or semi-hybrid systems, and are currently in the research and development and limited production stages. Driven by the demand for more energy-efficient vehicles, major automakers are increasingly developing vehicles equipped with 48V automatic start-stop systems. A report by IHS, a leading global industry research firm, states that by 2025, 48V mild hybrid vehicles will account for more than half of all hybrid vehicles, with 95% of mild hybrid vehicles adopting 48V solutions.
The 48V lithium-ion battery pack developed by Hitachi can be used in mild hybrid electric vehicles. It has higher input and output power, and the battery cells have excellent low-temperature performance. The output density is 1.5 times that of previous Hitachi battery cells. It also has good fuel economy and high cost performance, which will promote the popularization of mild hybrid vehicles in China and Europe.
