42V Car Batteries

Tom, I had a similiar story. I was just coming in to the Palm Desert CO when my boss calls me over, hands me a box, and says to take it over the the

When I get to the 1000 Palms office, my second line manager RUNS TO THE TRUCK and gets the box. Now, this gets my attention, since I had hardly ever seen him, and never moving faster than a slow walk. So I park the truck and go inside to find out what happened...

What happened was, the box contained fuses. They were for the rectifiers for the office power supply. A contractor had been working on tightening the bolts on the power busses with an untaped wrench, when he dropped it. It landed square across the +48 and ground busses, and had blown every fuse in the power supplies. Unfortunately, in a normal office you don't stock enough fuses to replace them all at once, so they had called over to our larger office to get enough spares to get the office back on line.

The contractor was lucky he was working ABOVE the busses when he did this. They were sweeping up ball bearings for weeks after this.

Charlie

I've never understood why high-current bus bars aren't insulated with a bit of plastic, etc. And especially high-voltage bus bars. Opening the cabinet and looking at the metal bus bar for our Institute building's AC- power from the transformer gives me the heebie-jeebies. And that's below 1kV. I work with my own design stuff to 20kV, but I'm sorry, it still gives me the willies. This summer I designed a 2MW (2000kW) pulser and put it into service, but it had a sub-kJ fuse, a microscopic fraction of our building's circuit-breaker rating. So those are clearly different beasts. And you know what, all the dangerous wires in my machine were insulated.

I would be more concerned about the contact resistance, as well as what the terminals are made of. Lead isn't that great a conductor, and the junk they toss in doesn't make it any better a conductor nor keep it from melting.

Offhand, the resistivity of iron is about 100 nohm.m, so for a 12" bar of about 1/4 x 1/2" = 1/8 in^2 dimension, you get 0.8 cm^2, or 8 x 10^-6 m^2 cross section, and about 0.3m length. 100 nohm.m * 0.3 m / 8 x 10^-6 m^2 =

12V / 0.00372 ohm = 3225 amperes, so contact and internal resistance are going to be a big factor, if not the limiting factor.

On the upside, if contact resistance is good, much of the power will in fact go into heating the battery itself.

On the downside, iron's resistivity increases rapidly with temperature. Once the iron hits 100C, resistance is up by 60%, and power is up by 60% or more.

Iron's specific heat capacity is 0.45 J/g.K, or 450 J/kg.K. The 12 x 0.25 x 0.5" bar weighs about 4.5 pounds or 2 kilos, so the energy for that 100C temp rise is 90kJ. If the short-circuit current is 500A, the power dissipation is 930W, which will heat up the bar by 100C in somewhat less than 100 seconds, since the resistivity and power dissipation increase with temp rise. Well, that's good news for the wrench-holder, ignoring contact resistance anyway. In the mean time, the other 10V dropped across the battery will have heated it a little bit, but considering the weight and water content, I couldn't say if it's enough. But it doesn't need to get as hot, either.

Tim

-- Deep Fryer: A very philosophical monk. Website @

I love your analysis, but if you can get 3225 amps the battery is in good shape to crank a starter.

Regards Ken

I've often wondered the same thing. (Well, not often these days... haven't worked around that stuff for years. But I used to wonder.)

My guess is that it's tradition: "We've always done it that way. Our fathere did it that way. Their fathers did it that way..."

Personally, I'd prefer to see some common sense prevail.

If you have wrenches anywhere near several thousand amp circuits, sooner or later Mr. Murphy will ensure that one of those wrenches will meet one of those several thosand amp circuits.

Tom

Speaking of heebie-jeebies, in my youth I worked for a time doing cargo ship upgrades. One task that still makes my skin crawl to think of was to sit underneath switchgear that had bare 480VAC 3 phase busbars carrying about 1000 amperes. My job was to catch the end of bare braided/shielded cables being pushed up from the deck below before they hit the busbars.

And my boss loved to complain about how high his worker's comp insurance was....

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LOL, I did allow for internal resistance to be a factor. A big factor in this case! ;-)

Tim

-- Deep Fryer: A very philosophical monk. Website @

Roger Hamlett snipped-for-privacy@ttelmah.demon.co.uk posted to sci.electronics.design:

Telephone companies have used 48V DC systems for many decades, it has even become a standard industrial power voltage. These are often backed by lead-acid or NiCad batteries to handle the high currents involved. 36V DC (the proposed 42 V system) is new, not yet standard. Both 24V DC and 28V DC are standard and have been around for decades as well. Calling extreme rated system voltages useful for differently rated systems is inappropriate.

At my day job, the third rail has 800V on it with breakers limiting current to just 3000 to 5000 Amps.

The crowbar glowed red, and then orange, but did not draw enough current to trip the breakers.

Tim.