48 volt car elect not going to happen

re. 48 volts being dangerous I've worked around 48V systems for ~30 years. These systems actually tend to run more 52-54 volts due to a higher voltage being required to keep the batteries float charged.

I've never ever felt even the slightest tingle by touching the + & - leads at the same time, for most people you have to get up above 60 before they feel anything. I have however felt a mild tingle one time, I was touching the hot lead while my arm was touching some grounded metal, I'm guessing the skin on my arm is a bit thinner than on my hands.

The major danger with 48 volts, is, as it is with any system capable of supplying more than a few dozen amps, is a short across the leads. I've known people who have dropped wrenches across the buss bars in telecom sites where the power plant is rated in the thousands of amps on a continuous basis, generally the wrench flash vapourizes with a

*VERY* inpressive dissplay of light & sound. The person who does it is invariably *VERY* carefull never to do it again :).

That being said I'd expect in automotive systems an increase in battery voltage would require less current from the battery. Automotive companies being what they are (cheap) this most likely will mean a battery with a corresponding decrease in plate area per cell, with a higher resultant internal cell resistance, thereby actually decreasing the battery's maximum current output to levels (1/4?) of an equivalent 12 volt system (less sizzle).

H.

H.

[ ... ]

No -- the human *skin* is a fairly high resistance object, though it varies with perspiration, and emotional stress (among other things). Remember that skin resistance is one of the parameters charted by lie detectors.

If you have two small cuts or punctures (try pins in one finger of each hand) as little as 6V is sufficient to stop the heart. This bypasses the skin. And the needed current is not amps, but mA.

It may not kill immediately, but it can hold muscles contracted causing you to hold onto the source of the current, so the heart will fail in not too long a time. I know that I had a voltage of about 48 VDC with some AC on it get hold of me through a sharp corner of a heat sink. I could not control the muscles of that arm to pull it clear. The only way I got clear was to grasp the wrist with the other hand, and yank it with the muscles of that hand.

Try between wet sweaty palms and wet sweaty feet on concrete. And DC is worse than AC, because it never reaches zero to let you get control of the muscles for even a millisecond.

Enjoy, DoN.

I accidentally fully grabbed a flyback transformer. Working on televisions at the age of 15 back in the 50's. I stared at that hand for about 4-5 seconds totally unable to uncurl my fingers. Then I remembered why my older mentor had told me to keep one hand in my pocket. Used the free hand to unplug the cheater cord from the back of the telly. BTW, I just now realized why they must have designed those cords to plug into the set instead of hard-wiring them with strain reliefs etc.

Ivan Vegvary

Sometimes it takes a while...

You need to catch up- the 48 volt system would drive ALL the accessories (wipers, blower motor, ect) and not just the hybrid drive. The reasoning is that higher voltage would mean smaller, lighter motors and lighter gauge copper wiring.

-Carl

PS We have two Prius in the family.

It is 'only' twenty four volts. ;)

Wes

On Sun, 03 Feb 2008 22:18:59 -0600, with neither quill nor qualm, "Martin H. Eastburn" quickly quoth:

As an ex-wrench, I'd agree from my own history, but I have not seen a mechanic without gloves on his hands (cloth, leather, latex, or nitrile) in nearly a decade. Yes, I've been shocked by a battery. Usually the back of my arm across the terminal. It can be interesting.

With 42v systems, I'm sure that battery disconnection would become standard in many more shops. Smart mfgrs would install cutoff switches for that purpose; cheaper than all that warranty time.

You worked one handed, eh? A union shop, I presume.

-- Our greatest glory is not in never falling, but in getting up every time we do. -- Confucius

Lighter gage wire yes but less iron NO. For a constant amount of power (HP) you will need same size iron core FOR DC. If you go to AC sure as in 400 cps . Which brings up a good point why not use an inverter and make the accessories AC? With the current technology of VFD s it should do wonders for reducing weight, wire, etc. ...lew...

I don't think so, Lew. I think the same copper, current, iron, flux density etc will deliver 4X the power at 4X the voltage because it will provide same torque at 4X the speed -- if it can mechanically handle the speed. Speed requires precision => cost.

I agree with you that motors probably wouldn't shrink, because the loads dictate the torque and speed requirements. These could be met with smaller faster motors and more gear reduction -- or similar motors just wound with finer wire to operate at higher voltage and lower current to perform the same as before. The faster motor with more gears gears would weigh less, but cost considerably more.

DC motors are really AC motors already. That's what the brushes and commutator do, or elex in the case of brushless DC motors. DC is applied, but the armature windings see AC as the motor spins.

On Mon, 04 Feb 2008 07:47:16 -0500, with neither quill nor qualm, Wes quickly quoth:

Suit yourself. And judging by your answer, you should see

...if you're not already listed there.

P.S: For the record, the difference would be a/c vs. d/c and 96/72 volts, ya literalist.

-- Our greatest glory is not in never falling, but in getting up every time we do. -- Confucius

Once again, you've earned your name.

If that had been DC, you may have died. Either from being unable to let go, or from being pushed away by your muscles, into something else that could kill you.

For symmetry, it ought to be gasoline powered. Best regards, Spehro Pefhany

That was a joke question...

i

I think the electrocution hazards are minimal, but the arc-fault hazards are substantial.

Dammit Don -- just when I thought I understood DC machinery you hadda go and say they are really AC !!!

I don't think so, Lew. I think the same copper, current, iron, flux density etc will deliver 4X the power at 4X the voltage because it will provide same torque at 4X the speed -- if it can mechanically handle the speed. Speed requires precision => cost.

I agree with you that motors probably wouldn't shrink, because the loads dictate the torque and speed requirements. These could be met with smaller faster motors and more gear reduction -- or similar motors just wound with finer wire to operate at higher voltage and lower current to perform the same as before. The faster motor with more gears gears would weigh less, but cost considerably more.

DC motors are really AC motors already. That's what the brushes and commutator do, or elex in the case of brushless DC motors. DC is applied, but the armature windings see AC as the motor spins.

say they are really AC

Oboy. Got my foot in a bucket again. I suppose a motor that runs on DC power is properly referred to as a DC motor. A key difference is that the field in most DC motors is stationary, often supplied by a permanent magnet. The armature, however (and therefore) must see AC excitation and exhibit a rotating field. That's why the rotors in DC motors are made up of laminations rather than just blocks or bars of iron or steel. In the familiar brush-type motor, the commutator makes this happen; the excitation polarity on each pair of commutator bars reverses twice per revolution. Frequency increases with speed until the back EMF balances applied voltage minus I*R lossses.

You never got bit by the -48 DC itself, it was always from the inductive bounce from the line relays, or other Reverse EMF effects. Or from the 130V 20Hz ring voltage on top of the -48DC.

The offices ran at 52V to 54V nominal float-charge voltages, but the

50V limit in the NEC was and still is the defacto dividing line between Class II and Class I wiring. We had 130 VDC and 180 VDC carrier repeater power circuits too, but it was treated the same as other telephone circuits - though we did put special red ID sleeves over binding posts to warn people not to mess with them.

Wrench? Feh, nothing! I was at the other end of the building, but I sure knew something happened - Try an aluminum level, straight across the Main Bussbars right over the batteries. (Name of the guilty party omitted to protect the clueless, even though the clock has long since run out.)

Sylmar CA, Stepper, around 1983-84. Five 24-cell 2400AH strings and five 400A rectifiers online at the time, IIRC.

Impressive arc light wasn't the half of it - he dropped every call in the building, and when the short cleared the voltage bounce blew every 30A voice-battery filter capacitor fuse in the place. And all the alarm fuses in parallel.

Whole lot of hummy calls till they all got changed, which took hours. Had to call all the other switchrooms to send over extra 30A cartridge fuses and bodies to change them - you had to use a 48V work light as a mini load bank to charge the capacitors, then you could put the fuse in.

They can cut the battery CCA requirements down a bit because of the higher voltage, but they can't really reduce the overall battery reserve capacity by much. You still have requirements on running tail lights and/or emergency flashers for a set amount of time, to provide emergency lighting.

I still think if they jump at all, 24V or 48V would be the more logical points to pick, because the equipment already exists, motors and lamps are readily available, etc. 42V is a bastardized level, unless they're talking a 36V nominal battery and 42V is the float voltage level.

-->--

On Mon, 04 Feb 2008 23:41:05 -0800, with neither quill nor qualm, Bruce L. Bergman quickly quoth:

--big snip of funny/scary story

Is that what triggered the Sylmar quake way back when? I don't recall their covering any light show at the time. ;)

I'll bet they are.

Say, Sparky, what's a good lubricant for light bulb interfaces? (500w

120v Chiwanese photographic continuous-light stand) I have a dual porcelain/aluminum connection which is stubborn to screw together and the salesman says "use any oil or grease sparingly". What's the typical electrical grease which won't be a fire hazard? TIA

-- Our greatest glory is not in never falling, but in getting up every time we do. -- Confucius