Nuts and volts


According to presenters at the Technology and Maintenance Council’s fall meeting, 42-volt electrical systems will eventually replace current vehicles’ 12-volt systems. It’s an idea that’s already generating some, uh, resistance among fleet operators.

Confusion, too. To begin with, a 42-volt system uses a battery or batteries that produce 36 volts, not 42 – that’s the system’s charging voltage.

Today, we use 12-volt batteries, which typically charge at 14 volts. But we still refer to the system as being 12-volt. Using a different nomenclature for the new system was simply a mean trick, if you ask me.

If you connect three 12-volt batteries in series – which is how some of the first new systems will be configured – you get 36 volts. But you would charge the batteries at 3 x 14, or 42 volts, hence the name.

So why do we need more voltage? In many ways, “It’s emissions-driven,” Delphi Automotive Systems’ Bob Galyen told the TMC audience. “It will allow mechanical components (like power steering pumps and air conditioning compressors) to be replaced with electrical ones.”

Ideally, the results would be reduced component mass and less drag on the engine – both of which are good for reducing exhaust emissions.

Another potential advantage could be the use of thinner wiring, making for smaller, lighter harnesses. I’ll explain: The work done by an electrical load – a starter, for example – is measured in watts, which is the product of volts and amps. If you increase voltage, you reduce the amperage needed to produce the same wattage. And, since wire thickness is determined by the amperage it must carry, a thinner wire will do the job.

As appealing as these prospects are, however, changing over to a 42-volt system will not be all sweetness and light. With an eventual mix of old and new systems in the field, one concern that had to be addressed right away is the possibility of an unwitting 42-12-volt jump-start, which could be harmful to equipment and humans. “The batteries will have to connect with blades that are recessed inside a shroud,” noted Tyco Electronics’ Garold Yurko.

The batteries will also need to be internally de-powered before connecting or disconnecting them, due to the dangerous arcing that 42 volts can produce, even at low amperage. He added that technicians would have to take extra care when working on 42-volt vehicles, since the danger of electrical shock is much greater – especially if a technician has perspiration on his hands.

Bruce Purkey of Purkey’s Electrical Consulting cited other service concerns, noting that much of today’s electrical diagnostic and charging equipment will be useless on the new systems. In addition, “42-volt circuits, fuses, bulbs and other components will need to be well identified to prevent mistakes.”

Then there’s the cost. “Today’s switches and relays will arc and burn at 42 volts,” says Carl Smith of Sure Power Industries. “They’ll have to be redesigned and identified. Same thing for fuses and circuit breakers. All of this will cost money.

“We might be able to use lighter wiring and connectors,” he continued. “That will save weight, but they’re likely to be less durable.”

Good point. No one wants a broken wire creating a 42-volt short circuit. The potential for fire is just too great.

Finally, forward/backward tractor-trailer compatibility is likely to require the use of 12/24-volt relays. In fact, the consensus among TMC presenters is that, at least in the near term, we’ll start seeing vehicles with mixed voltages, with low-current loads at 12 volts, and high-current loads at 48 volts.

Despite a lack of standardization, a few manufacturers, reportedly including BMW, Ford, Daimler-Chrysler and Volvo, will offer some iteration of a 42-volt system on their passenger cars within the next couple of years. Hopefully, that market will be a worthy proving ground for long enough that the new systems won’t be too big a jolt by the time they hit heavy trucks.

Paul Richards is editor of Commercial Carrier Journal. E-mail or call (610) 993-9430.