Today’s vehicles are loaded with electronics, sensors, control units, safety features, infotainment and advanced driver-assistance systems (ADAS), as well as dozens of small and big traction motors. Providing, managing, and delivering the required electrical power is critical and challenging. One reason is that it’s also necessary to dissipate the resultant heat, an unavoidable consequence of so much energy.
The demand for more power reached a point where it could no longer rely on a 12-V lead-acid battery. That battery cannot provide the power, and even if it could, the heavy-gauge cables needed to keep losses to an acceptable level add to the cost, weight, volume, routing issues, and assembly constraints.
If you’ve seen the miles of cabling and their large connectors in a modern car, much of it 12-gauge thick and inflexible, you know there’s a problem.
The history of change, with more to come
Of course, the power needs of today’s cars are several orders of magnitude greater than those of 100-plus years ago. How much power does a vehicle need (traction motor aside)? Figure 1 shows the inexorable and dramatic rise of energy use, except for a slight dip in 1970 (possibly due to the Arab oil embargo and gas shortage?)
Figure 1. The acceleration in a vehicle’s electrical use over time has been astonishing. (Image: ResearchGate)
The first vehicles had no battery at all. The driver hand-cranked the engine to start the car to initiate rotary motion. The spark-plug power came from a magneto, a crude but effective generator attached to the engine. The cranked car created its own energy for the spark plug once it was cranked and turned over.
When the electric starter was introduced between the 1910s and ’20s, it used a 6-V battery. The key to using a modest 6-V battery for the starter and the radio was that both were not used simultaneously. The power cable ran from the battery to the starter, and the radio was short and direct, minimizing losses.
By the’50s, the load on a vehicle’s electrical system increased with more lighting, air conditioning, and power steering. Many of these options were powered mechanically by belts running off the car’s engine to minimize electrical demands, with electricity used for control but not raw power.
Eventually, the industry switched to 12-V batteries to provide more available energy, reduce losses, and allow the use of thinner cables carrying higher voltages but at lower amperage (Figure 2).
Figure 2. Battery size, technology, and energy density have increased and advanced along with vehicle size. (Image: Continental Battery)
The 21st century
Flash forward to the 21st century, and the 12 V runs out of steam. The IR drop — the voltage drop across a conductor due to its internal resistance (R) when current (I) flows through it — and voltage losses become problematic, as does the associated heat dissipation, which adds to the thermal load.
Due to the 12-V issues, many newer ICE vehicles and nearly all EV/HEVs have a 48-V battery. This battery serves loads more efficiently and is powered by a higher voltage while retaining the 12-V source for lower current loads.
The 48-V dc/dc converter is typically designed to allow electrical energy to flow bidirectionally from one battery to the other as needed, under the direction of a sophisticated battery management system (BMS) that optimally balances the energy sources versus load demand (Figure 3).
Figure 3. The dual 12-V/48-V system has a high-level battery-management diagram that belies its actual sophistication. (Image: MPDI)
However, it’s not just the basic battery voltages that have increased. System demands have pushed designers to redo the basic topology of how the battery voltage and power get distributed and converted down to the lower-voltage rails for the electronics — while offering higher voltages for higher-power loads, such as accessory motors, audio amplifiers in the tens and hundreds of watts, and more.
Designers used a distributed topology with a central node as cars became “electronics and computers” on wheels. With this architecture, they could add new functions by adding new electronic control units (ECUs). Over the years, these additions expanded to between 50 and 100 (or more) ECUs and as much as 4 km (2.5 miles) of wiring harness, and often more.
This approach is simple and direct. However, a distributed architecture is no longer suitable due to the sheer volume of the wiring, electrical inefficiency in power distribution, reliability considerations (including cybersecurity), and cost and safety considerations. Additionally, each function requires a network connection and dc power.
Going “zonal”
As distributed architectures have become overwhelmed, designers are shifting to “domain” architectures with a partly decentralized power distribution network. In this approach, functions — such as ADAS, infotainment, and telematics — are logically grouped, each with its own processor.
One disadvantage is that it can increase wiring and connections, lead to power losses, and add to a vehicle’s weight and costs. Somewhat counterintuitively, this architecture is ultimately the most efficient way to organize vehicle electrical and power distribution systems.
Looking ahead, there’s a shift to fully centralized “zonal” architectures. With this architecture, systems are logically and physically grouped into zones that can be efficiently organized. In each zone, a single, powerful processor manages all the functions, and the zone is powered by one power distribution unit instead of an array of highly localized units. There’s also one network connection for the zone (Ethernet and others).
Moving the electronic control unit closer to actuators and sensors means less wiring and fewer connections. Optimized device placement significantly shortens cable run lengths, and functional integration eliminates some cables. Thinner, flexible cabling for lower currents at higher voltages can replace heavier, round, stranded-wire cables. This reduces BOM cost, simplifies cable runs, and is more conducive to robotic handling, assembly, and installation in production.
Zonal implications include easier implementation of higher-voltage systems. While 12-V power systems have been the norm for decades, zonal architectures require 48 V to support higher power consumption and redundancy requirements (Figure 4).
Figure 4. Moving to 48 V enables lighter, lower-cost wiring harnesses with significantly reduced power losses due to resistive heating. (Image: Vicor Corp)
Under the zonal architecture, many legacy loads will still be supported by a 12-V battery or its functional equivalent derived from a 48-V battery. In the mild-hybrid system, a 48-V battery is used alongside the traditional 12-V battery; in a full-hybrid design, the 12-V battery is eliminated, and 12 V is derived from the 48-V battery.
However, it may be possible to eliminate the 12-V battery in EVs/HEVs and the 48-V battery. All the lower-voltage rails can be derived from the higher-voltage vehicle battery packs (400 or 800 V). Key to battery elimination is the availability of small, efficient dc/dc converters, which regulate the 48-V output down to 12 V. They can be located closer to the load rather than at the 48-V battery. This reduces cabling losses while supporting legacy 12-V loads.
So, why not simply eliminate the 48-V battery? That would save further space, reduce weight, and cut costs.
Long-established legacies are difficult to displace, especially when the supply chain, manufacturing, costing, and post-sales support considerations are well known. The technical capabilities of the available dc/dc converters and regulators must also be adequate.
Moreover, batteries respond reasonably well to load transients (primarily sudden surges), a natural and unavoidable aspect of automobile operation. Their transient response is limited by their internal resistance and the resistance and inductance of the cables. Until recently, the transient response of the dc/dc converter in dynamic load conditions was not fast enough.
Figure 5. It is possible to eliminate the 48-V battery in EVs/HEVs and derive the needed 48 V from the 400/800 V battery pack. (Image: Vicor Corp)
Now that a 48-V battery (Figure 5) has been introduced (courtesy of Tesla’s Cybertruck), time will tell how quickly more changes occur — just know that they will.
Click here for a more in-depth discussion on zonal vehicle architecture.
References
- Cadence Design Systems, “What Is Zonal Architecture? And Why Is it Upending the Automotive Supply Chain?”
- Vicor Corp., “48V systems: What you need to know as automakers say goodbye to 12V”
- Vicor Corp., “Electric Vehicles: 48V is the new 12V”
- Vicor Corp., “Tesla Cybertruck will eliminate 12V electrical components”
- TE Connectivity, “Connectivity in Next Generation Automotive E/E Architectures”
- Infineon, “Automotive power distribution system”
- Clore Automotive, “The Evolution of the Automotive Battery”
- Continental Battery Systems, “Car Battery Evolution – From Old-Tech to MIXTECH”
- MDPI, “Characteristics of Battery Management Systems of Electric Vehicles with Consideration of the Active and Passive Cell Balancing Process”
- ResearchGate, “A Systematic Approach to the Development of the Automotive Electrical Power System Architectures”
- Inside EVs, “Tesla Confirms The Switch To 48 Volt System”
- Texas Instruments, “Processing the Advantages of Zone Architecture in Automotive”
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Filed Under: Batteries, FAQs, Power Management