FW: FW: Fuse Amperage??? for a fuse between the battery and the c ar.

[Livolsi, Stephane]

I am quite certain that the starter motor does not present close to a short
to the battery.  The starter motor will restrict the amount of current
allowed through that circuit.  Consider that you can take a relatively small
wire, hook it up to a small 12 volt light bulb then hook that up to your car
battery and to ground. (a good example of this is one of those small test
lights that look like a screwdriver with a wire coming out of it)  Is there
600 amps going through the bulb? No, and the wire isn't melting either.
That's because the bulb will not allow that much amperage through (unless,
of course, it's a really powerful light bulb).  Now take that same small
wire, touch it to the positive terminal on your battery and then to ground
without the bulb in the circuit and watch the insulation melt.  That's
because the full amperage of the battery is trying to get through that small
wire and there is nothing there to restrict the flow.  The starter motor
will act in the same way as the bulb and will only draw enough current to
allow it to run, which should be well under 175 amps.

Now if the circuit is fused, if ever there was a short to ground, the full
600 amps would try to get through and the fuse would melt before any of the
surrounding wire melted.

How much damage will you prevent?  Well, my back seat wouldn't have caught
fire if there had been a fuse protecting the main cable coming off the
battery.  Granted, I did do something very stupid to cause it but still it
happened. My battery shorted to ground right under the seat and there were
sparks and flame like you wouldn't believe.  I have since relocated the
battery to the trunk and ran a new #1 gauge wire from the battery all the
way to the starter solenoid.

The resistance of a fault must be very close to zero, but because you have
so much potential energy in the battery trying to get through at the same
time you still get a tremendous amount of heat.

This is just my interpretation of auto electronics theory.  YMMV
Stephane
> ----------
> From: 	Richard J Lebens[SMTP:rick-l at rocketmail.com]
> Sent: 	July 30, 2002 2:07 PM
> To: 	Audi Quattro List
> Subject: 	Re: FW: Fuse Amperage??? for a fuse between the battery and
> the car.
>
> This is an interesting question.
>
> Doesn't a starter motor present pretty close to a short to the battery
> (600 amps limited by the source impedance of the battery)?
>
> Won't a fuze this size let 3-4 times it's current rating through for
> several seconds before it melts?  Does this mean with a 175 amp fuze
> you can crank your car for 5 seconds at 600 amps but it blows after
> that?  Will it interrupt the fault before the energy in the battery is
> depleted?
>
> What is the resistance of your rear seat springs and associated
> fittings?  How much damage will your prevent with a fuze in series?
> Will a fault blow the fuze(what is the resistance of the fault)?
>
> In our litigious society why aren't they standard equipment on all
> cars?
>
> --- "Livolsi, Stephane" <Stephane.Livolsi at investorsgroup.com> wrote:
> > I have to agree with Marc.  When your battery shorts, I think that
> > all 400
> > or 600 or whatever amps available try to go through, which would
> > definitely
> > blow a 175 amp fuse.  Whatever current goes through the starting
> > circuit is
> > limited by the starting motor itself and that is well under 175 amps.
> >
> > I think that you can almost weld sheet metal with the arc created by
> > a
> > shorted battery .....(let's not ask how I know this, ok folks  ;)
> >
> > .02
> >
> > Stephane
> >
> > > ----------
> > > From: 	Marc Swanson[SMTP:mswanson at sonitrol.net]
> > > Sent: 	July 30, 2002 10:11 AM
> > > To: 	Richard J Lebens
> > > Cc: 	quattro at audifans.com
> > > Subject: 	Re: Fuse Amperage??? for a fuse beween the battery and
> > the
> > > car.
> > >
> > > > Are you asking about a fuze on the battery?  When you are
> > starting your
> > > > car the battery is delivering all the power it has for as long as
> > the
> > > > switch is on. The starting circuitry is a very low impedence,
> > similar
> > > > in magnitude to your seat springs connected to ground.
> > >
> > > agreed that it is a high current draw, but not that high!
> > > Under full load of starting the 175 amp fuse won't blow, but.. say
> > if
> > > your main battery feed line chafes against the body and shorts to
> > > ground.  That would start a fire without a fuse in place.  That
> > would
> > > certainly be a significant enough draw to trip the 175 amp fuse.
> > >
> > >
> > > Later
> > >
> > >
> > >
> > > --
> > > ----------------------------------
> > > Marc Swanson, Software Engineer
> > > Sonitrol Communications Corp.
> > > Hartford, CT
> > >
> > > Email: mswanson at sonitrol.net
> > > Phone: (860) 616-7036
> > > Pager: (860) 948-6713
> > >  Cell: (603) 512-1267
> > >   Fax: (860) 616-7589
> > > ----------------------------------
> > >
>
>
> __________________________________________________
> Do You Yahoo!?
> Yahoo! Health - Feel better, live better
> http://health.yahoo.com
>