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Goodbye 12 volts... hello 42 volts!

Radical changes to car electrical systems are just around the corner.

by Julian Edgar

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Water-cooled alternators are being fitted to some vehicles, combined starter/alternators are being developed, and a standardised 42-volt electrical system is about to be introduced.

New Electrical Loads

The electrical loads of modern cars have increased dramatically over the last 10 or 15 years. Luxury cars - especially - are huge consumers of electrical power. The current model BMW 750iL has a maximum electrical load of a staggering 428 amps (5.9kW)! In this car, over half of the maximum power load is from the short-term electrical heating of the catalytic converters, with the heating used so that they come up to operating temp quickly.

While cat converter heating is one of the most major of the new electrical loads, other automotive technologies being developed also involve substantially increased electrical demands. These technologies include:

  • electromagnetic solenoid operation of the valvetrain (strongly tipped for BMW engines in the next 12 months or so);
  • large vehicle electrically assisted power steering;
  • brake by wire;
  • ride control systems.

In fact, US automotive supplier Delphi predicts that within 20 years, the electrical power consumption of a typical car will have reached 10kW without any form of electrical propulsion being employed, and more than double this if the electrical power is used to aid the engine. Ten kilowatts represents a current flow of 725 amps at 13.8 volts, a current flow that's very hard to handle. Doubling the voltage will halve the current flow for the same power, so it's not surprising that moves are afoot to raise the standard voltage of car electrical systems.

42 Volt Systems

About 40 years ago automotive electrical systems moved from a 6-volt standard to 12/14 volts. Now the change to 42-volt systems - which use a 38 volt battery and 42 volt alternator output - is being proposed. Two multi-company committees are working on the new standard. At the Massachusetts Institute of Technology, the Consortium on Advanced Automotive Electrical/Electronic Components and Systems includes General Motors, Ford, DaimlerChrysler, BMW, PSA-Peugeot/Citroen, Renault, Volvo and automotive electronics suppliers Delphi, Bosch and Siemens. In Europe, Sican - an organisation in Hanover, Germany - is working with major German carmakers and component suppliers to formulate the new 42 volt standard. The commitment to the new standard is high; for example, the French automotive component company Valeo has eight of its nine component divisions working on products using 42 volt technology.

The commercial risks to a car manufacturer of swapping to 42 volt technology and at the same time undertaking a major re-design of all the electronics in the car means that initially, dual 12/42 volt systems are likely to be introduced first. As Delphi state, "The increase in voltage means rethinking and possibly redesigning everything from light bulbs to major components". DaimlerChrysler: "We have decided to additionally retain a 12 volt supply so that components in standard use today can remain operable."

As indicated, the major benefit of the higher voltage is in the reduced current flows that are then possible for the same power consumption. Wiring bundles could be as much as 20 per cent smaller, in turn reducing cable mass and so benefiting fuel consumption and emissions. Says DaimlerChrysler, "We see the development of a 42-volt net not only as a technological necessity, but as a contribution to lessening the environmental burden."

A number of approaches to the introduction of 12/42 volt car systems has been proposed:

Single voltage generation and single voltage energy storage - a 42 volt alternator charges a 36 volt battery which services 36 volt loads, with a DC/DC converter used to charge a 12 volt battery that services 12 volt loads;

Single voltage generation and dual voltage energy storage - a 42 volt alternator charges the 36 volt side of a dual 12/36 volt battery, with a DC/DC converter used to charge the 12 volt portion of the battery;

Dual voltage generation and single voltage energy storage - where a dual 14/42 volt alternator charges two separate systems, one 12 volts and the other 36 volts;

Dual voltage generation and dual voltage energy storage - where a dual 14/42 volt alternator charges a dual 12/36 volt battery.

Benefits to current technology from using 42 volt systems are:

Current Technology

Benefits of 42 volt Architecture

Electric power steering

More power, improved fuel economy

Electric brakes

Redundant power supplies

Power windows, power seats, power hatchback lifts

Reduced size and mass of motors; more efficient operation

Heated catalytic converter

Lower emissions; quicker light-off time

Heating, ventilation, airconditioning blower motors and cooling fans

Greater efficiency; smaller/lighter units; flexible packaging

Mobile multimedia

More power available for video, mobile phones, navigation systems, audio amplifiers, fax machines

Electric water pumps

Improved efficiency; longer service life

Selected engine management system components (eg exhaust gas recirculation valves, ignition systems, control actuators)

Reduced size and mass; increased performance

Fuel pumps

Reduced size and mass

Heated seats

Faster heating, more efficient operation; increased power

Car technologies that are yet to be introduced but which would also substantially benefit from higher car voltages are:

Future Technology


Electric supercharging

Higher engine efficiency

Ride control systems

Improve ride, handling and vehicle stability

Brake by wire

Improved vehicle packaging and vehicle performance

Steer by wire

Enhanced performance; improved packaging; improved passive and active safety

Electromagnetic valve control

Lower emissions; optimal power; individual cylinder control; lower cost

Integrated starter/generator

Faster starts; quicker charging; design flexibility; low noise and vibration; improved fuel economy

New Alternator Designs

The very high electrical power demand of current cars is also resulting in the development of more efficient alternator designs. One approach is to water-cool the alternator, circulating engine coolant through passages cast in the alternator housing. In some cases, the alternator is entirely surrounded by a water jacket.

A liquid-cooled alternator design was first introduced (in very small numbers) in passenger cars in 1995. That design used two conventional alternators mounted on the one shaft, and developed 14V/220A with low noise levels. BMW has since introduced (on cars such as the 750iL mentioned above) a water-cooled alternator that uses a single brushless design developing 14V/150A. The BMW alternator uses liquid cooling for two major reasons: to reduce by up to 3dB the alternator noise associated with normal fan-cooling; and to increase electrical performance. Other advantages of the design include:

  • Rapid engine warm-up due to the utilisation of alternator waste heat;
  • packaging advantages due to the absence of an alternator aircooling duct;
  • a longer alternator life;
  • good fording ability for the car.

However, probably the most dramatic development in alternator technology is the Integrated Starter Alternator Damper (ISAD) being developed by German company Continental ISAD Electronic Systems GmbH & Co. The ISAD combines the function of a starter motor and alternator into one assembly that is located between the engine and gearbox. ISAD is able to generate output voltages of 12, 24, or - significantly - 42 volts. The device eliminates the:

  • conventional starter motor and solenoid;
  • flywheel;
  • conventional alternator;
  • alternator pulley and belt drive system;
  • and in some cases, the harmonic balancer.

Both BMW and Citroen have shown vehicle prototypes using 42 volt ISAD systems. In a car equipped with a 42 volt ISAD system, each normally belt-driven device could be replaced with an electric motor. In some cases this would have significant advantages - the airconditioning compressor could be located close to the cabin instead of at the front of the engine, for example.


Cars featuring water-cooled alternators or combined starter/alternators, 42-volt wiring and much higher electrical loads are likely to be appear in the next few years. No longer will "12 volts" necessarily mean car voltages....

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