Whatever Happened to the 42-Volt Car?

It's been just about 10 years since the auto industry was abuzz with the impending transition of automotive electrical systems from 12 volts to 42 volts. Didn't happen. Isn't gonna. Stop holding your breath.

In the first place, why on earth would the car industry make such a global change, obsoleting trillions of dollars in 12-volt infrastructure and aftermarket accessories? Simple: Twelve wasn't cutting it anymore. Wiring harnesses were getting heavier and bulkier to run all the power accessories on board, such as heated seats and power windows. With 42 volts in the system, vehicles could use thinner-gauge wires and smaller motors because the higher voltage would mean reduced amperage--and amperage (or current) is what dictates the wire size, not the voltage.

The bundle of wires that go from the car into the driver's door is a good example: Some luxury cars had more than 60 wires in a bundle as thick as your wrist, making the bundle unwieldy to install and prone to damage. Switching to 42 volts, however, could shrink the bulk of the bundle by half.

So what went wrong here? The automotive industry found that switching off 42 volts wasn't that easy. Huh? A mechanical switch for, say, the window motor, has two metal contacts (well, a bunch, actually). When you press a switch, the two move into contact with each other and current flows to the motor. When you let go, they snap apart, interrupting the flow of electricity. But this doesn't happen instantaneously--the surface area of the metal contacts that touch doesn't go from something like 10 square millimeters to nothing in zero time. As the contacts move apart, the current density goes up in inverse proportion to the remaining area. And at the last instant of contact, the current density is high enough to melt and then vaporize the surface of the metal as a short electrical arc bridges the widening gap. Eventually, the contacts wear to the point that they don't work anymore. In a 12-volt system, this phenomenon was controlled by careful contact design and attention to the metallurgy of the contacts themselves. Forty-two volts draws a much longer, hotter arc, and vaporizes more metal.