Current reversal in 2 connected car batteries?

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember saying something like:

For this particular episode, 12V neg ground.

Car to car, both fairly ordinary vehicles. At this time in the UK, the only other common higher-voltage use is 24V on heavy trucks. 42/48/50V vehicles are extremely rare. Of course, he could have attempted it from a milk float or forktruck - it wouldn't surprise me.

As for hooking it up wrong; well, that's a possibilty, but he'll be more careful next time.

That's not a good assumption, considering the new standards that are coming along, plus all of the classic cars that are around from the 1940s - 1960s. My MGB has a 12V-neg gnd system, but through my participation on Usenet rec.auto groups, I've seen warnings about similar sports cars from a similar time period having postive ground, and I think I've even heard of 6V and/or

18V systems, but I can't be certain about that. The driver of the old car with the odd electrical system *might* be aware of what his vehicle has (although, unless he's one who works on his own car, he may not), but it is more likely that the driver of the other car might have no idea that he's dealing with other voltage standards...or even that such things exist. Most people are not very educated about the vehicle they drive. They just know that they feed it gasoline, they get in, they turn the key, they press the pedal and they go. Most know that they also need to have somebody replace the brakes and oil once in a while. Beyond that, it's a black box.

Well, see comments above about the fact that "ordinary vehicles" has a statistically finite chance of not applying.

That's my point. To blindly say "No way could jumpstarting knacker your battery", a statement which, taken on its own at face value is incorrect, does nobody any service, and possibly prevents him from learning the lesson, if indeed it's what needs to happen. Even just adding the condition "if you're talking about two vehicles with healthy 6V-neg gnd systems" at least tells him that there is something more he may need to look into and learn about.

Eric Lucas

The number of Positive gnd , 6v or 24V Mitsubishi Shoguns is pretty negligible. You're more likely to encounter a dead short than an non 12V neg gnd car which isn't covered in warning signs or driven by someone who knows it.

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember saying something like:

The whole comment I made was predicated on the assumption that the OP wasn't a total idiot and knew that both cars were 12V neg ground. Recall that there was another party involved in the jumpstarting and he, at least, probably had more clue than the OP.

When I was younger and sometimes today, we still jump 6V (either positive or negative ground) with 12V negative ground equipment. Although all of this equipment uses generators instead of alternators. I would not try this with something that uses an alternator or all. And gosh does a 6V starter turn over an engine really fast with 12V connected to it. Even hard to start engines generally pop off really quickly. :D

Hahahaha... I knackered my car battery trying to start a semi truck in cold weather. Just simply jumping, my car battery didn't have enough current to turn over the diesel engine. So it had taken me 20 minutes using my car's alternator to charge up four of the huge 12V batteries enough to give it enough boost to finally start the truck.

And I let my car run another 5 minutes after disconnecting to charge my car battery up enough so I could start it later. Well 12 hours later, I went to start it and my battery was dead. Used the truck to now start me up. All was well after that. What a PIA! lol

Oh but I do have a story about jumping someone else that could cost you or someone plenty. My dad and his friend jumped a vehicle and the battery blew up in my dad's friend's face. They had a garden hose right there and thus hosed him down with water and he turned up okay afterwards. Thank goodness. Just some cuts on his face.

I have jumped hundreds of vehicles and I have never seen this ever happen. I have heard these stories though for decades. This was the first time my dad has seen this happen too from hundreds of jumps.

The safe idea though is to connect up the last connection to ground and far enough away from the battery so that the spark doesn't ignite the battery vapors. Although with most things made of plastic and having plastic shell covers on everything. And everything metal is also painted if you can find the real metal, this is getting harder and harder to do on some makes and models. :(

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.

Eric Lucas

AIUI, they settled on 42V for safety reasons. Anything below 48V is considered "low voltage". A 42V battery will nominally be just under 48V.

Standards. See above.

1HP is about 746 watts. Figure out how many you need.

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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.

I don't know? :(

As far as the ignition, starter, instruments and other normal things found in cars for decades could still use 12v battery. But things like lights would last longer on AC vs. DC. And I would think electric motors for the electric car side is more efficient being run from AC vs. DC.

Yes I too have ran cars from a faulty charging system. You can usually run the engine for many hours off of a good car battery. :D

One horsepower is 746 watts, very nearly 3/4 kilowatt.

Maybe some of the readers recall the little 50 cc Honda motorcycles at the tip of the Japanese bike invasion. They were alleged to develop 5 HP, or about 3700 watts. With a single rider they could do about 40 mph wide open on flat ground (although their speedometer would indicate about 60).

All the hybrid vehicles I've seen use voltages of 100 volts or more -- a new one reviewed in Saturday's paper used 300. This makes sense for the power levels involved for propulsion. But it makes no sense to run the remainder of the electrical system at that level. As far as I know, all the hybrids use 12 volts for the electrical systems.

Roy Lewallen

Grrrr! There ain't much of a difference between 42v and 48v to really matter. Yet someone controlling the standards says there is. :(

[snip]

Isn't every 50cc of a gasoline engine roughly 1HP? I know 49cc can travel at speeds of 30MPG (mopeds). Thus my '72 Honda 350cc motorcycle must be like 7HP and I know I can exceed 70MPG on that thing. And my '95 Honda 1100cc motorcycle must be like 22HP. And I don't know what the top speed of it yet. lol

Now the old VW Beetles were around 1200cc and usually traveled at a top speed of 83MPG. So that would be like 24HP to do the same. And if 746 watts is correct, pushing the old VW would be roughly 17,904 watts. Wow!

*gulp*

Anyway I forgot how they vary the speed of AC motors? Doesn't it require different windings for every RPM? If so, that wouldn't work anyway. :( Although just thinking about adding a torque converter and we are back in business! :D

Ah... another one confirms this. :)

Umm.. I believe every 50cc equals to 1HP. And mopeds in the states anyway has to be under 50cc. So they are 49cc of course and can't exceed

30MPG by law I believe. Although overriding the governor, one could exceed up to 40MPG I would think.

I owned a moped (Honda I think) when I was living in Hawaii back in the

70's for a short time. Later I bought a real bike. ;) It was only 49cc and it would run easily 30MPG, but it was held back by a governor. Once I climbed a mountain at about 5MPG for hours and I finally got to the top. Then I came down and just let it run wide open. It hit over 50MPG and that thing suddenly became very unstable and scary. I used the brakes to slow down and I thought the brakes were going to fail. As full brakes almost didn't stop the speed from increasing at all. But it barely did and it had taken like a minute to get it just to slow down to under 30MPG.

That all makes sense to me. :D

It's the same as saying a metre is 100cm. It doesn't need 'confirming', it just *is*.

You believe wrong. Totally wrong.

FWIW the continuous power limit for a 50cc moped in the UK is 5bhp. Racing

50cc bikes (yes they do exist) will get a lot more than that.

Other engines will be a lot less.

MPG is fuel economy (miles per gallon) you are talking about miles per hour, i.e. speed

It's designed to do less than 30mph. Of course it had trouble.

No comment...

Well you know, you can't trust everything you hear. lol

Oh ok... I was thinking otherwise. I stand corrected. :D

You know my brain was thinking MPH while my fingers were typing MPG, sorry!

lol It operated fine at 30MPH or less... but once it hit over 50MPH and the front wheel was all over the place... I thought I was going to learn what sliding on pavement was at 50+MPH on the seat of my pants was like. But luckily that never happened. :)

You know, I believe that moped was designed for only up to 150lbs. and I was 6'1" @ about 210lbs. at the time. Exceeding the maximum limit in speed and weight and combining coming down a mountain (making things really worse)... I am so glad the brakes finally still won. It's not something I want to repeat ever again. :D

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "BillW50" saying something like:

Kind of wildly different capacities, doncha think? The damage may have been caused by your initial attempt at a straight jump.

I saw it happen to a workshop manager who was in the habit of jumpstarting a knackered old forktruck every morning with an arc welding set. The silly sod was lucky not to lose his eyesight and part of his face.

That particular morning, he'd asked me to do it - no way, I said, so he set to with a will and it happened. Funnily enough, he never did that again.

| Grrrr! There ain't much of a difference between 42v and 48v to really | matter. Yet someone controlling the standards says there is. :(

They are always making new and different standards. If you don't like this one, there will be another coming along shortly. Or just make your own and get everyone to go along with you. I'd rather see 48 or

24 or 12 as the case may be. Where people come up with all these odd voltages ... oh wait ... probably a fist full of dice :)

Back when TVs had tubes, each year the TV manufacturers would come out with their new models. And every year, the tube lineup was entirely different. The innards of the tubes were pretty much the same, but each year they had different numbers of pins and pin layouts, often different filament voltages, and came with different combinations of tubes in the same envelope. So each year every service shop would have to buy one or more sets of the new tubes so they'd have them on hand. Shops ended up with a huge inventory of different tube types which could have been replaced by no more than a couple of dozen standard tubes.

Looks like the auto manufacturers have caught on.

Roy Lewallen