Date:         Wed, 12 Nov 2008 23:25:03 -0500
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: alternator--voltage vs amps
Comments: To: Richard A Jones <jones@COLORADO.EDU>
In-Reply-To:  <491B9BDE.7050505@colorado.edu>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 10:15 PM 11/12/2008, Richard A Jones wrote:
>There are lots of web pages that explain how/why the
>voltage is adjusted with the alternator.  What I cannot
>find is an explanation of amperage.  Amps is what charges
>the battery--at the correct voltage.  I do not understand
>how the amperage output of the alternator is varied.

For simple resistive loads in a DC circuit, the following rule
applies: E(volts) = I(amps) * R(ohms), therefor I = E/R and R =
E/I.  But a lead-acid battery is not a resistive load.

So the amperage is limited by a) what the alternator can put out at
given rpm and temperature, and b) what the battery will accept (its
effective resistance, if you will) at a given temperature, voltage
and state of charge.

A three-stage "smart" regulator will use the full output of the
alternator which causes the battery voltage to gradually rise.  Once
it rises to a certain point, high enough that the cells are
gassing,  the regulator shifts modes and maintains a constant voltage
for a period of time while the battery gradually accepts less and
less current.  When this current becomes small (or after a timed
period) the regulator then reduces the voltage to below gassing level
and "floats" the battery at a lower voltage.

An automotive regulator simply regulates to a constant voltage
(usually less than the gassing voltage) and supplies whatever current
the battery will accept at that voltage.  The big compromise is that
if the voltage is less than gassing voltage, the battery will charge
slowly and never fully charge; but if it's more than gassing voltage
the battery will overcharge and "boil" dry.  Since in normal
automotive use a battery is never discharged very far, this
far-from-ideal system works adequately; however as soon as you start
using it to supply non-trivial amounts of energy so that it
discharges, the shortcomings become apparent.

Lots of info available at http://www.amplepower.com .

Does that help?

--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage," '85 GL "Poor Relation"

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