Current reversal in 2 connected car batteries?

Alex Coleman wrote:


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.
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computar2006 ( snipped-for-privacy@ntlworld.com) gurgled happily, sounding much like they were saying :

Complete crap.
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We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "computar2006"

Your battery was on its last legs anyway. No way could jumpstarting knacker your battery.
--

Dave
SE6a

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saying something like:

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.
Eric Lucas
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( snipped-for-privacy@sbcglobal.net) gurgled happily, sounding much like they were saying :

Some use 42v now, notably 7-series BMWs, and - of course - most trucks are 24v
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gurgled happily, sounding much like they were

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.
Eric Lucas
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snipped-for-privacy@sbcglobal.net wrote:

Because it's the NEW standard. All (or most) of the auto manufactures are on board this new standard.
Bill K7NOM
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gurgled happily, sounding much like they were

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.
Eric Lucas
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( snipped-for-privacy@sbcglobal.net) gurgled happily, sounding much like they were saying :

There's rather a lot more electrickery in your average new 7-series than just the starter motor...
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Here is an article on it: http://www.findarticles.com/p/articles/mi_m0EKF/is_48_48/ai_94767338
gurgled happily, sounding much like they were

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gurgled happily, sounding much like they were

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.
Eric Lucas
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It doesn't need the higher voltage, but the higher voltage lets you get away with thinner cable & smaller relays, which is cheaper & lighter. & makes things like tailgate motors much more practical.
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wrote:

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.
Eric Lucas
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There's not actually any extra battery weight, as you need less current for the same power you end up with only a fractionally larger battery for the same power to the starter motor.
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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?
Roy Lewallen
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Right. I can also see an opportunity for manufacturers of 24 (or whatever they go to) vdc to 12 vdc converters. But they will probably be overly complicated to allow a larger price tag..

-
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Roy Lewallen wrote:

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 requirement.
Bill K7NOM

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gurgled happily, sounding much like they were

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.
--
Bill



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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.
Eric Lucas
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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 though.
--
Skipweasel
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