Technical File

42 Volt Systems

For almost 50 years cars have used the 12-volt electrical system. This system took over from the earlier 6-volt system because the extra voltage was required to power equipment such as radios and electric windscreen wipers.

Modern cars have electrical equipment undreamt of in the 1950s-and its becoming a problem for the 12-volt electrical architecture. The current systems must have high amperage to provide sufficient power for modern electrical accessories. High amperage requires heavy duty electrical cables which add weight and bulk.

The car industry is now looking at adopting a 42-volt system to remove these problems. This system would use a 36-volt battery but charge it at a slightly higher voltage, in the same way that 12-volt systems usually run at 14 volts. Because voltages of 50 or higher can stop the human heart, 42 volts has been chosen as the maximum safe operating voltage. A significant advantage of 42-volt architecture is that it would allow many accessories, such as air-conditioning and power steering, to be electrically powered. These systems are currently belt driven in 12-volt cars and consume engine power even when not being used. The result should be better fuel economy.

The 42-volt standard will also allow the use of starter-alternators, a single device that would replace the starter motor and the alternator in current cars, thus saving weight. A 42-volt starter-alternator is powerful enough to be used as a drive motor. That means that cars equipped with them could be driven as mild hybrids, which use electrical power to assist the main engine when moving from standstill.

The spread of 42-volt systems has been slower than expected mainly due to the initial cost being greater than the savings they promise.

Editors Note: A 42-volt system would also allow the cheaper realisation of an effective Xenon headlight system since the electronics required for the voltage conversion would be smaller and more cost effective .Also the thermostatic fans used on most modern cars would require smaller motors when operating on 42 volts thus saving weight. Even the prospect of an electrically driven water pump may be possible thus eliminating all belt driven accessories from the vehicle.