Here's an interesting question:
Is it possible to design a ground-fault interrupter which can carry --
and safely break -- a 1000 amp current going into a 1000 volt load?
OrionWorks wrote:
From: John Coviello
...
> That miracle battery is on it's way finally! Lithium ion
> batteries have sufficient power densities to deliver 300
> mile per charge and can actually recharge in 5 to 10 minutes.
> You know what that means? People can pull in and recharge
> their EVs on the go, just like filling up the old gas tank.
> That day is coming and it will kill oil when people realize
> how cheap electricity is in comparisson.
While I also look forward to the day when EV or equivalent non-petroleum
based vehicles dominate our hi-ways I seem to recall that pushing that
much juice through electrical cable to recharge car batteries may turn
out to be hazardous to one's health! I don't know how much actual
concentrated amperage would be involved to charge a battery within 5 -
10 minutes, but I'm sure it's substantial. I'm sure there are a few EEs
in this group who are more than capable of doing the math. I seem to
recall Mike Carrell once warning the readership that there is the danger
of "vaporizing" the battery or nearby components.
I'm no EE but maybe we can estimate it anyway.
Say that 300 mile range involves an average output of 10 HP. (That's
probably within a factor of 2 of the real number, which I seem to recall
is ~ 15 to 20 HP for current cars without regenerative braking; the real
range for an efficient electric car is probably somewhere between 5 HP
and 20 HP.)
300 miles at 40 MPH = 7.5 hours
10 HP ~ 8 kW => 60 kW hours for 7.5 hours
5 minutes is about 0.1 hours.
To push 60 kWh into the batteries in 0.1 hour, we need to run about 600
kW into the batteries. That's as much as a small generating station
produces -- you don't pull that out of an ordinary outlet. (I suppose
the charging station must be using batteries or supercaps to store the
charge, and they fill them up during the night.)
Now, if the battery pack puts out 400 volts (wild guess), then 600 kW
would go in at a current of 1,500 amps.
That's a lot of current, but it's not an impossible amount. Just the
same it probably indicates that the battery pack should be more like a
kilovolt than 400 volts; that would bring the current down to a far more
manageable 600 amps. At that point, we're looking at a number which is
in the same ballpark as the starting current for cars today -- and it's
most likely _lower_ than the starting current for the old 6 volt cars
from the days of my youth. It's clearly do-able, at least from a
current standpoint.
But, in any case, you'll want to use very hefty cables, maybe some
silver, and you'll want to be very, very careful of the connector
design, to avoid having the plug melt or burn. And, if the wires are
long enough to have noticeable inductance, you don't want to be standing
next to them if one of them breaks (or the plug pops out).
I can believe this. I purchased one of those 15 minute rechargers at a
local battery store last year. It's a marvel. Works as advertised. Of
course, you have to buy THEIR special brand of batteries in order to
take advantage of the quick charge. Fortunately the device will recharge
regular rechargeable batteries as well, but within a more traditional
length of time: 4 - 6 hours. When the recharger is performing a 15
minute charge a very noisy fan turns on to keep the electronic
components from melting down. Almost sounds like a mini-turbine turning
on at full blast. Without a doubt, it's the loudest recharger I've ever
heard. I'm surprised I don't smell ozone pouring out of the thing. And
now, they can do this in less than 5 minutes? That means the amperage
would have to be three times the volume than my already fast 15 minute
charger. The device would be screaming!
Maybe, maybe not. Just because it handles more current doesn't mean the
electronics must dump more heat.
Classic example is a linear power supply versus a switching power
supply. A well-regulated high-current high-voltage linear supply
typically needs substantial heat sinks; a well-regulated high-current
high-voltage switching supply typically doesn't. The difference is that
the linear supply runs the electricity through what is, essentialy, a
variable resistor in order to drop the voltage to the regulated value.
The resistor (actually a power transistor) basically just throws away a
fraction of the output power equal to the difference between the bulk
supply voltage and the regulated voltage, divided by the bulk voltage.
The switcher, on the other hand, uses a switching transistor which is
always either fully on or fully off, and in principle, it doesn't need
to throw away any of the input power as heat.
So, the point is just that a better/fancier/more-expensive design could
potentially produce more current to the battery without producing more
heat in the charger.
Still, I love watching the contraption. Not sure I would say the same
thing if I was attempting to recharge my EV with equivalent technology.
You want ME to to connect the cable to that anode??? All the more reason
to hope that Mark Goldes' room temperature superconductive cable may
make it to market within the near future.
You still have the problem of the connector, which may be the biggest
issue to start with -- hard to make it low enough resistance to keep it
from burning up.
And you have the problem of broken wires in spades, if his
ultraconductors are anything like other HT superconductors: they're a
lot more fragile than a stranded copper or silver cable, which you
couldn't break without using an axe on it.
Otherwise, wear rubber boots. ...and stop sweating.
That's going to be an issue no matter what's inside the cable. Any time
you've got hundreds of volts at essentially unlimited potential current,
you've got a major hazard.
One nice thing about DC (assuming the charger is DC!) is that it doesn't
tend to permanently stop your heart, short of cooking it.
Your skin resistance starts out pretty high. However, once you punch
through the skin and run some current through a person, that resistance
drops a lot; I don't know how far, unfortunately. At 1 kOhm (wild
guess), and 1 kV, a person would be absorbing 1 kW, which is about
what's inside a microwave oven. You'd most likely get severe burns on
your hands and feet but wouldn't be instantly killed.
Electric chairs, in contrast, are specially designed to cause
unconsciousness, stop the heart, and then dump an incredible amount of
energy into the body just to be sure the effect is permanent, by using
carefully placed electrodes and a sequence of at least two different
voltages.
Regards,
Steven Vincent Johnson
www.OrionWorks.com
