Solving BEV Weight Issues via a New Power Architecture

Author : Noa Margolin, R&D Engineer, Vicor Corporation

22 March 2024

Figure 1: Conversion to a 48V system will reduce vehicles’ total current draw from over 250A to under 75A without impacting on the constituent electronic/electrical content
Figure 1: Conversion to a 48V system will reduce vehicles’ total current draw from over 250A to under 75A without impacting on the constituent electronic/electrical content

With many battery-electric vehicles (BEVs) being as much as 33% heavier than their counterparts based on internal combustion engine (ICE) technology, it is clear that they have a serious weight problem. This was highlighted by the US National Transportation Safety Board, which noted that; “The Ford F-150 Lightning is between 907kg to 1360kg heavier than the non-electric version.”

In the following article looks at how the use of advanced high-density power modules can help alleviate the situation.

In a modern day context, vehicle weight is a significant design factor - affecting the ability of automotive OEMs to achieve the desired range for their vehicle models, as well as their options for adding electronic systems to enable greater functional capabilities. It should be noted that carrying extra weight has serious safety implications too. The US National Bureau of Economic Research found that adding a further 453kg to a vehicle increases accident fatality risk by about 47%. As such, weight reduction is one of the most formidable R&D challenges the industry has yet faced.

Figure 2: There are 2 distinct PDNs used in passenger cars today - the 12V centralised architecture and the fast-growing 48V zonal architecture
Figure 2: There are 2 distinct PDNs used in passenger cars today - the 12V centralised architecture and the fast-growing 48V zonal architecture

Part of the extra weight BEVs carry is due to the additional mass of heavy batteries and electric traction motors - although these are becoming more efficient and can thus deliver greater performance per unit of weight. One of the most promising areas for weight reduction lies in the power delivery network (PDN). Since the 1960s, vehicles have relied on a 12V system. Over the years, manufacturers have continued to add new electronics and features (such as heated seats, air conditioning, etc.) to provide extra comfort, safety, security and autonomy. This has led to an inevitable increase in current demand (with today’s vehicles drawing around 250A) and a consequent rise in the thickness of wires that carry this current. 

Today’s BEVs are each powered by a primary high-voltage battery, typically 400V or 800V, that needs to deliver power not only to the electric traction motor but also to a large number of low-voltage loads. To meet this challenge, OEMs are increasingly looking at replacing the legacy 12V system with a 48V system.

Figure 3: Power modules deliver a faster transient response than 12V lead acid batteries - creating a virtual battery that can replace the heavy, legacy12V battery
Figure 3: Power modules deliver a faster transient response than 12V lead acid batteries - creating a virtual battery that can replace the heavy, legacy12V battery

Moving to 48V is the obvious answer for BEVs
In this architecture, the PDN steps down the high voltage to the auxiliary batteries to supply the vehicle’s many subsystems. Moving to a 48V system allows a lower current - because Ohm’s Law states that Power = Current x Voltage. Thus, for the same power delivery, a 12V source will call for 4x as much current as a 48V source. This means that the 12V wire is also around 4x thicker than a 48V wire will be. Another aspect of this shift is moving to a zonal architecture, where functions within a vehicle are grouped by location into several zones. Each zone has its own zonal gateway controlling specific functions and subsystems within that zone, such as engine management, lighting, or safety sensors. The gateways are close to the devices they control, allowing connecting cables to be relatively short.

Since 1908, which is the year that the first mass production car was introduced (the celebrated Model T), current demand has grown dramatically. It has followed an exponential curve with the addition of more sophisticated vehicle electronics. In the 1960s, OEMs increased the voltage from 6V to 12V, causing currents to drop for the first time in 60 years. Today, most OEMs still use the 12V bus, despite the demand for more current. 2023 saw Tesla become the first OEM to announce a full transition to busing 48V throughout the vehicle, resulting in a dramatic drop in the associated current demands.

Figure 4: The centralised housing can be reduced when using power modules and a zonal architecture - because heat can be dissipated more efficiently at the endpoints where 48V is converted to 12V loads
Figure 4: The centralised housing can be reduced when using power modules and a zonal architecture - because heat can be dissipated more efficiently at the endpoints where 48V is converted to 12V loads

A zonal architecture supported by high-density power modules to step down the voltage will be able to help lower weight in key 3 ways. Perhaps the main way is that it allows transition from thick wire harnesses to much thinner ones, which can cut harness weight by up to 85%. In addition, there is no need for low-voltage auxiliary batteries - these can be removed and replaced with power modules. Lastly, using such power modules optimises the thermal management system of the vehicle, reducing system weight by as much as 33% in the process. Furthermore, a zonal architecture will improve power system efficiency too. 

The demise of the 12V centralised architecture
The 12V centralised system consists of one bulky ‘silver box’ housing a set of discrete components, including all the DC/DC converters to take the high-voltage supply to 48V and 12V. As well as needing thick, heavy wires to carry the 12V current, legacy DC/DC converters are also inefficient, producing significant amounts of heat that often require heavy liquid cooling systems to dissipate.

Table 1: A 48V zonal architecture used in conjunction with high-density power modules will save about 18kg of weight overhead in compact BEV SUV models
Table 1: A 48V zonal architecture used in conjunction with high-density power modules will save about 18kg of weight overhead in compact BEV SUV models

48V high-density power modules can be used at the endpoints to convert to 12V at the points-of-load (PoLs). Allowing the OEM to gradually transition the 12V load devices to 48V, this method can help achieve the benefits of using 48V with minimal disruption to the system architecture. Because wires can be thinner, lighter and significantly cheaper using a 48V system (refer to Figure 2) they are easier to route within the vehicle. Additionally, heat losses associated with the DC/DC converters are evenly distributed throughout the vehicle, allowing the use of chassis-mounted heat conduction and convective air cooling. 

In the traditional 12V centralised system, discrete components create a high ambient temperature inside their silver box housings. Using high-density power modules instead creates less heat, allowing the PoLs to be efficiently air-cooled on the chassis and enabling the liquid-cooling system to be slimmed down by up to 7%. 

Table 2: Enhanced range offers drivers more distance per charge - reducing the regularity of recharges needed per year
Table 2: Enhanced range offers drivers more distance per charge - reducing the regularity of recharges needed per year

Cutting weight presents new opportunities
As already outlined, the adopting of a lower weight, zonal architecture brings a number of valuable benefits. For example, a zonal architecture supported by high-density power modules can reduce vehicle weight up to 18kg (see Table 1). Automotive engineers can make use of this extra weight allowance to pack in more battery cells - with the driving range of an electric powertrain sports utility vehicle (SUV) thereby being increased by as much as 4,000 miles per year thanks to this additional charge storage capacity, with no net weight gain being witnessed. This is significant - because the average person in the UK drives around 7,400 miles per year. Therefore, using a 48V zonal architecture could result in the yearly recharging time for the average BEV being reduced by somewhere in the region of 30% (see Table 2 for more details), which will mean greater convenience for such vehicles’ drivers.

Innovate to eliminate
Clearly, BEVs are becoming increasingly overweight. This trend is not sustainable and will contribute to a suppression of the overall growth in the global market for these vehicles. Avoiding this trend will require the widespread adoption of 48V zonal architecture implementations using small, lightweight PoL converter units. The use of power-dense modules capable of attaining elevated levels of efficiency is the best choice for 48V-to-12V conversion.
 
Employing a zonal system in BEVs can increase the distance they can travel each year without calling for more recharging time. Alternatively, it can facilitate the inclusion of additional safety or comfort features. As more electronic/electrical loads are added to vehicles, automotive OEMs need to be creative to save weight while increasing performance. Vicor’s compact power modules, architectures and topologies offer these OEMs flexible, scalable BEV powertrain solutions for high-voltage power conversion throughout their vehicles. This means that automotive brands can gain the maximum benefits from embracing the 48V zonal architecture - the future power delivery network for the automobile industry.


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