Integration of 48V and high voltage large batteries with 12V small batteries

In the design vision of Model Y, one of the ideas is to cancel the 12V small battery, or integrate the 12V small battery into the whole vehicle. The driving force of this integration is mainly to maximize the use of the vehicle space as much as possible. Secondly, with the centralized load demand of L2 and above autonomous driving systems leading to redundant backup, it may be necessary to take both 48V and high-voltage batteries into consideration to meet the requirements. This article focuses on this demand. Note: Since the battery system of pure electric vehicles currently does not have enough space to hold more power, it is used to the fullest in the short term. When the volume energy density breaks through a certain bottleneck, it is only a matter of time to put a 12V battery in the large battery pack of BEV. The situation of PHEV is similar to that of BEV. The battery is so large that it occupies the storage space, so the demand is not obvious in the short term.

01 Integration in HEV

HEV models are something that various car companies are quietly working on, but they are not talking about it much at the moment. If the 48V system is also considered a low-cost HEV, the entire market is slowly upgrading. Since these cars do not require a small low-voltage battery to start the engine, the capacity is getting smaller and smaller. In a sense, they are mainly used as a backup power source for the entire system, and they all have a DC-DC to convert the energy of the 48V or high-voltage battery. Therefore, in this type of battery, there is the greatest hope and demand to integrate the two batteries. The biggest advantage of using lithium batteries here is their small size and light weight, and the biggest bottleneck is cost. In this regard, the biggest demand is to analyze the load requirements for the 12V system in future HEV vehicles, including rated current, required instantaneous power, and operating conditions, especially to determine the worst driving conditions.

The key loads in this situation are used to sort out the usage restrictions of the 12V battery and the 12V distribution system (which is often designed based on the electrical system model). Of course, the biggest increase is the power demand brought by the core controller of the entertainment system and the autonomous driving controller after the EE architecture is zoned. The autonomous driving controller must also consider the power requirements of the perception system and the power required by the execution system (EPS and brakes) respectively.

In fact, we have already seen practical examples of integrating two batteries together on the HEV platform of Hyundai Ioniq Hybrid and Kira Niro. The 12V small battery uses a lighter and smaller lithium battery to reduce fuel consumption. The shorter distance can also reduce the cable impedance of the DCDC and 12V battery system, so the entire layout of the low-voltage battery must be close to the HV battery (the 12V lithium-ion battery and the HV battery are integrated in a battery frame) 

Figure 1: Modern HEV lithium electrification transformation 

Note: In this design, Hyundai uses a reset button. When the 12V battery is completely discharged, the HV battery can charge the small battery without any problem. In fact, there are many strategies that can be used here. Timed detection may be more effective. Keeping the 12V small battery charged can prevent the car from being unable to start.

Figure 2: Modern battery block diagram 

As shown in the figure below, the main task is to analyze and decompose different instantaneous loads and continuous loads, and to select a more appropriate strategy for the operating time of this electrical system.

Figure 3 12V requirements for conventional vehicles 

02 12V load demand brought by autonomous driving

As shown in the figure below, the vehicles currently being developed for autonomous driving are generally at least HEV. In fact, the core reason is that the power of the 12V generator is limited to a certain extent. In order to ensure the steering and braking of the vehicle during operation (at least these must be prepared), including the overall perception and decision-making process, the 12V power required is generally high. Therefore, even in the HEV system, we often need to configure an independent battery and independent DCDC power supply for this autonomous driving and ADAS to ensure the overall operation. 

Figure 4 System modified from HEV 

If there is a greater demand in the future, can we merge these two 12V batteries into one, and then divide them into two independent DCDCs to cooperate with a 12V power supply, and then cooperate with a redundant power distribution architecture to realize the management of the entire system? 

Figure 5 Using a 12V battery and integrating the system 

Conclusion: Let me start with the question of the advantages of domain control and autonomous driving in BEV. In fact, with the transformation of the power system, the overall architecture can be replicated in 48V/HEV and PHEV. It is nothing more than using the large battery as a redundant energy unit, and making adjustments in HEV battery charging, engine power generation strategy, etc.