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JOHNSON CONTROLS, a major automaker supplier, has announced it’s developing a 48-volt lithium-ion battery for automobile applications. According to Automotive News, January 21, 2013, an unnamed European automaker (spelled Mercedes-Benz?) will introduce the higher-voltage battery as early as 2015.
Initially, the cars are expected to have two batteries, a lead-acid 12-volt unit supplying juice to conventional hardware and the lithium-ion 48-volt battery handling things like Start/Stop, electrically boosted steering and other subsystems profiting from higher voltage.
Hybrids, of course, already have dual batteries, 12-volters for door locks, lights and other “hotel loads,” higher-voltage battery packs for electric propulsion. A Toyota Prius’s battery pack is a nominal 273 v; a Chevrolet Volt’s, 300 v.
Battery electric vehicles run higher voltages. Typical of today’s BEVs, a Mitsubishi i’s battery pack operates at a nominal 330 v; a Nissan Leaf’s, 400 v.
In time, automakers will consolidate things in conventional cars into a single 48-volt battery. One reason for the higher voltage is an ever increasing electrification of automotive functions. Another is enhanced efficiency—gained by lighter wiring harnesses.
The reason for the latter traces back to Ohm’s Law, V = I x R, where V is the difference in potential measure in volts, I is current measured in amperes and R is resistance measured in ohms. (At least Georg Simon Ohm got that much for discovering the law.)
One implication of Ohm’s Law is that higher voltage pushes more current through a given wire. A hydraulic analogy helps explain this: Think of voltage as water pressure and current as its volume of flow.
Or, engineers can work the other side of things: Employ thinner—and hence lighter—wiring harnesses.
But wait, haven’t we heard this song before?
Indeed, way back in 1998, there was talk of replacing the conventional automotive battery of 12-14 volts with one of nominally 36-42 volts. It was all going to happen in a few years.
What went wrong?
First, back in (pre-hybrid) 1998, automotive electrical loads were growing—but nothing like today. Back then, the first priority was thinner wiring for lighter weight, not the need for higher voltage per se.
Second, by 2002, automakers got cold feet, partly because of the costs of reengineering components and also on safety considerations. Harmless sparks of a 12-volt system could become sustained high-voltage arcs. Connectors, plugs—and shop practices—would have to be significantly upgraded.
Aircraft already use higher voltage, but this is with another dimension of inspection and maintenance. And not without its own issues, eh, Boeing?
With regard to lithium-ion technology (see also www.wp.me/p2ETap-I2), I can think of another reason for considering 42 volts now and not in 1998: toting around the weight of three conventional lead-acid batteries back then!
Technology and times evolve, however. Maybe this time around, 42 volts will be the optimal automotive answer. ds
© Dennis Simanaitis, SimanaitisSays.com, 2013