If you want to help jump start someone else's car, then be prepared to
fork out for a new battery. I jumped started my friend's shogun with
my nissan micra and my battery stopped charging shortly afterwards.
I don't think that's true. There are at least a couple of easy ways to ruin
a battery during jumpstarting...hooking it up the wrong way round, and
connecting cars that use different voltage standards are two that come to
mind. The first is possible if you're careless about hooking up the cables.
Both the first and second are more a problem when jumping older cars, which
sometimes used either a + ground, or a completely different voltage (I think
I've heard of 6V systems). As far as I know, all modern cars have 12V,
negative ground systems, so the main problem is hooking up the cables wrong.
Thanks for the correction--I thought the auto industry had finally
standardized. I guess I can see an advantage to a higher voltage in
starting the (usually) bigger engines in trucks, but I wonder why BMW
chooses to use such a non-standard voltage for one of their line of cars.
New one on me (not surprising), but that still doesn't answer the question,
why was a new standard necessary? What does 42 V do (besides answer the
Ultimate Question of Life, the Universe and Everything) that 12 V doesn't?
Was it simply to reduce starter current? Why is that an advantage--I'd
think you would still have the same power losses in wiring, etc. Certainly
42 V batteries are more expensive than 12 V, so there must have been a good
reason for the added expense.
Yes, but most of it is electronics, which runs perfectly well on +12V/-5V,
as long as the current capability is there. I can understand the higher
voltage being required in hybrids, because there, you actually need the
higher voltage for both the electric motors and the quick-start of the
gasoline engine. It just wasn't clear to me what "electrickery" in
non-hybrids needed the higher voltage.
Interesting, that makes sense. You do, of course, have to balance the
weight saved in thinner wires and smaller relays, against the extra weight
of 42V batteries vs. 12 V batteries. But then again, that extra battery
weight would only be about 3.5x the 12V battery weight, or something like 30
lbs. I suppose the weight saved in wiring could easily equal that. It just
seems like more chance for mistakes--a la the confusion that existed in the
60s over + ground vs. - ground. The electronics industry has come to
understand that standardization is good for business, and that switching
standards really does require a compelling benefit. It was unclear what
that was with a 42 V standard.
I'm not sure how many BMW 7-series drivers have tailgate motors....
Thanks again for the reasonable and informative response.
I see some real benefits in establishing a new voltage standard for cars.
Until it becomes common, aftermarket radios, headlights, and a large
host of other products won't work in the new cars, so dealers can charge
what they want for parts.
As it becomes more common and the parts market increases, all auto parts
dealers will have to buy a whole new inventory of higher voltage parts,
from radios to headlights, other lights, and probably even fuses, while
retaining their current inventory of 12 volt parts. So it's a great deal
for the auto manufacturers and a great deal for the parts manufacturers.
I don't see any real benefit to the consumer, but hey, since when was
that a concern?
As I understand the plan there will be two voltages used at least
initially. !2 volts will be available for
those after market accessories and 42 volts for the built-in loads.
Suppliers are designing (cheap)
converters to get 12 volts from the 42 volts
There is consideration of using electrical motors for such things a
breaks and steering. And of
course as some one mentioned there is the hybrid car and it's power
All of the big trucks that I know only uses 24v just for starting. As
everything else is still 12v. Like lights, radios, fans, etc. That is
why they use batteries in multiples of 2 (12v batteries) i.e. 2, 4, or 6
batteries. They change the battery configuration from parallel to in
series by using simple starter relays.
And why pick 42v if you are going to change from 12v? As in series that
would take three 12v and one 6v batteries in series to produce that. Why
not use 48v or 36v? That way you could use 4 or 3 12v batteries.
The big advantage of 12v is that you normally can't get shocked. Well
sticking your tongue on it you can, but normally no. Now using 42v, this
is now a shock hazard. You can feel that most of the time. So if you are
going to raise it that high, why not go to 120/240AC? This makes more
sense running electric motors off of this vs. 42vdc. And you can plug in
your laptops, TVs, stereos, etc. off of this as well.
How much watts does one need to push an electric car at highway speeds
anyway? 1200 watts? 2400 watts? 12vdc to 120vac inverters should work
nicely if it isn't very much.
Good question, maybe it's seven 6V batteries--or a single battery with 28
1.5V cells. On the other hand, maybe it's based on non-lead/acid battery
technology, with a different cell voltage. Seems clear to me they're
completely getting away from 12V batteries, though. Doesn't seem like it
makes for a convenient source of 12V for standard electronics, although
perhaps OEM and AM suppliers have begun making 42V automobile electronics.
There's a big difference between feeling it and being killed by it--although
you may be right, 42VDC and 120VAC may be equally dangerous. A few thoughts
about "why not AC?" As far as I know, there's no such thing as an AC
storage device (battery), so I guess you'd have to use a battery with an
inverter, which would add complexity and losses. That's OK for things like
110V outlets in current SUVs, where its intended to be a small fraction of
the total power consumption of the vehicle, but I don't think you'd want to
do that with all the electrical applications in the vehicle. Plus, most of
the electronics are intrinsically DC devices anyway, so you'd have to add a
rectifier--more complexity, more loss. Finally, how would an AC ignition
system work? What would replace the condenser to give the high voltage
needed for a spark--capacitors don't behave the same way for AC as they do
for DC. What is the time constant for the spark--if it's longer than 17 ms
(60 Hz), can you even generate a spark with 60 Hz AC? Many of these are
honest questions that I don't know the answer to.
I don't know about cars now, but it didn't used to be very much. I remember
one road trip when I was in grad school in the 80s, in my first-generation
Pontiac Sunbird. Started in Boston early in the morning, heard a faint
"clunk" from the front end of the car, but everything was working fine so I
just kept going. I had the radio on the whole trip, and along about
Pittsburgh, it got dark and I turned the lights on. When I pulled into my
parents' driveway outside Lancaster, Ohio about 4 hours later, the lights
were so dim I could barely see the road, but the radio was still working. I
was pretty certain by then that the alternator belt was the "clunk" I heard
in Boston (it was). Just for kicks, I turned the car off and tried the
starter. The battery was so dead that it wouldn't even click the solenoid.
The spark took less energy than it takes to click a solenoid! I had gone
the whole 500 miles with no alternator, and all the electricity in the car
coming from only the charge that was in the battery when I left. I had even
stopped a couple times along the way to get lunch and dinner. I was really
amazed. I'm sure cars now can't do that, though--that car was pretty
bare-bones by comparison--6 cylinder, had a distributor, AM radio with
aftermarket FM converter, and I think might even have been carburetted,
although I'm not certain I remember that correctly.
You could do it with a flywheel but the output frequency would droop as
it slowed down - unless you got all clever with synthetic AC which
rather defeats the object. It'd be nice to have regenerative braking
Pay no attention to that man behind the curtain.
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