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Designing Dual 48-V/12-V Battery Automotive Systems

The future of 48V/12V battery systems in automobiles now lurks just around the corner. Most major automobile manufacturers across the globe have been working on proving out their systems for the past few years, and it’s evident that their implementation will be relatively near term. This is a necessary and crucial step in the long and arduous journey to the fully autonomous passenger vehicle, which doesn’t require a human at the controls and has true autonomous driving.

Nevertheless, this doesn’t mean the 12-V battery is going away—there are far too many legacy systems in the installed vehicle base for this to occur. What it does mean is that autonomous cars will have both a 12-V battery and a 48-V battery (Fig. 1).

1. Next-generation cars will be powered by a 12-V and a 48-V battery.1. Next-generation cars will be powered by a 12-V and a 48-V battery.

A vehicle’s internal systems will either run off the 48-V lithium-ion (Li-ion) battery or the 12-V sealed lead-acid (SLA) battery—but not both. In addition to having two separate charging circuits for these individual batteries due to their respective chemistries, there must also be a mechanism that enables charge to move between them without causing any damage to the batteries or any system within the vehicle. An added benefit is that having two batteries also allows for redundancy should one of them fail during operation.

While this certainly complicates the design of the various electrical subsystems within the vehicles, there are some advantages to be gained. According to some auto manufacturers, a 48-V-based electric system results in a 10% to 15% gain in fuel economy for internal combustion engine vehicles, thereby reducing CO2 emissions.

Moreover, future vehicles that use a dual 48V/12V system will enable engineers to integrate electrical booster technology that operates independently of the engine load, thereby improving acceleration performance. Such compressors are already in the advanced stages of development and will be placed between the induction system and the intercooler, using the 48V rail to spin-up the turbos.

Globally, fuel-economy regulations have been tightening, while autonomous-driving capability with connectivity continues to proliferate in new automobiles. Accordingly, the 12-V automobile electric system has reached its usable power limit. As if these changes aren’t already enough, there has been a significant increase in automotive electronic systems. These changes, coupled with related demands for power, have created a new spectrum of engineering opportunities. Clearly, the 12V lead-acid battery automotive system with its 3kW power limit must be supplemented.

Furthermore, new automobile standards impact how these systems need to work. A newly proposed automotive standard, known as LV 148, combines a secondary 48-V bus with the existing automotive 12-V system. The 48-V rail includes an integrated starter generator (ISG) or belt start generator, a 48-V Li-Ion battery, and a bidirectional dc-dc converter, which can deliver tens of kilowatts of available energy from the 48- and 12V batteries. This technology