The Penny Drops

[Richard J Lebens]

--- "Mark L. Chang" <mchang at ee.washington.edu> wrote:
> On Fri, Sep 13, 2002 at 03:52:55PM -0400 or thereabouts, Marc Swanson
> wrote:
> > The battery is a big fat capacitor.  Many charging systems rely
> somewhat
> > on the battery to smooth out the voltage for the devices connected
> to
> > the system.  Plus some of the engine electronics may not work at
> all at
> > anything other than a narrow band of say 10-13 volts.  Go outside
> of
> > that even for an instant and it may shut down.  Driving a car
> without a
> > battery is just a really bad idea in general as it can damage the
> > electronics for this reason, especially in newer cars.
>
>
> I get that. But if I had clean power from the regulator, I can't see
> any
> reason why I would need a battery once it was putting out power. Just
> from a feeding power standpoint.
>
> --
> www.mchang.org | www.acmelab.org | decss.zoy.org


Oh crap   here goes

One way to generate an EMF (electro motive force or voltage) is to
rotate a coil through a magnetic field which is the same as rotating a
magnetic field around a coil like in the alternator.  The voltage from
the coil is proportional to the number of turns in the coil and the
angular velocity of the coil (or here the field).  To get current to
flow from the alternator to the battery this voltage (potential) has to
be higher than the battery.  You would like it to be higher than the
battery at idle which will result in it being >> at high RPM.

The battery serves as a big filter (like a chemical capacitor) to
convert the energy from the alternator to stored energy over a very
narrow voltage range (12-14 volts).

Without the battery to stabilize everything the regulator will switch
wildly and chaos will reign.  The system voltage will vary wildly.
Something will limit the upper boundary and you can only hope it isn't
limited by something like an injector driver, ignition primary driver,
radio or CPU.

The voltage regulator does regulate the voltage but it does so by
switching current through a coil of wire, the field winding.  This a
large inductance and a way to store energy.  When you turn it off the
magnetic field does not immediatly collapse and the alternator
continues to convert mechanical energy into electrical energy for a
short time.  This dosent matter with the battery there because the
current produced by the alternator and decaying magnetic field is
trivial compared to the stored energy present, but with a light load
like just the Engine Control Unit the voltage overshoot is significant.


I don't doubt the evidence that the car will run without a battery but
I wouldn't try it.

__________________________________________________
Do you Yahoo!?
Yahoo! News - Today's headlines
http://news.yahoo.com