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From: mag@whuxlm.UUCP (Gray Michael A)
Newsgroups: net.auto
Subject: Re: Alternators
Message-ID: <668@whuxlm.UUCP>
Date: Sun, 3-Feb-85 14:25:47 EST
Article-I.D.: whuxlm.668
Posted: Sun Feb  3 14:25:47 1985
Date-Received: Mon, 4-Feb-85 04:48:25 EST
References: <776@sdcsla.UUCP>
Distribution: net
Organization: AT&T Bell Laboratories, Whippany
Lines: 47

> 
> I read an article about how some guy invented a box that cuts out 
> his alternator most of the time thus saving gas by not having the
> device under a load when not needed.
> 
> The article goes on to say that the alternator only comes on when
> either the break lights come on or the batt. level is low.
> 
> There is only a minimal level of friction from the now free spinning
> alternators.
> 
> A couple of questions.  First would this work?  How would one go about
> making the box "know" when the batt level is low and turning on the
> juice?  
> 
> Any ideas?
> 
> Dave Wargo
> U.C.S.D.

Sure it would work!  It called a voltage regulator and every car
I ever heard of has one.  Some are better than others, but they all
sense battery level and only charge when it is low.  If you charged
when the battery is not low, you'd blow it up.

Incidentally, are several ways to know when the battery level is low.
A hard way is to measure the specific gravity of the electrolyte.
An easy way is to apply a controlled charging voltage briefly and
see how much current it draws.  A fully charged battery will draw 
about an amp at 14 volts.  Another way is to draw a controlled current
out of the battery and check the voltage at its terminals.  The
voltage at current X will decline as the charge of the battery does.
(Both of these methods are basically measuring the internal resistance
of the battery, which, for lead-acid batteries, increases as the
charge is lowered.)

Alternator output is controlled by the level of current in the field coils.
As the field coil current(a small current) goes up, so does alternator
output.  Also, so does the mechanical load on the engine.  Besides
preventing the battery from blowing up,  accurate control of alternator
output saves a little bit of gas, and a fair amount of wear on the alternator
bearings.

This guy may have invented an improved voltage regulator, but he hasn't
exactly done anything earthshaking.

Mike Gray, BTL WH