John,

You are absolutely right if discussing a generator. The field is composed of
2 or 4 electro-magnets mounted to the outside shell and the magnets will
hold enough magnetism once magnetized that you don't need an external
battery for the generator to re-start.

But a alternator rotor retains so little magnetism that it usually takes a
small amount of current to start one up. That initial current is supplied
through your GEN dash light. Then, once the alternator starts up a smaller
of two diode bridges inside the alternator provides current to the
regulator. The larger diode bridge provides current to the car's electrical
system.

BTW, this is why your GEN light stays on when your alternator or regulator
fails. The alternator rotor is no longer receiving excite current from the
regulator and it continues to draw the small amount of current through your
GEN light. But the current is not enough to drive the alternator to put out
any real power.

Tom
www.towercooler.com

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@GERRY.VANAGON.COM] On Behalf Of
John Rodgers
Sent: Saturday, April 24, 2010 12:39 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: alternator installation

It's been a long time since I have thought on this - but I seem to recall
that a strong charge from the battery(or jumper battery) is required to
activate the field of the Alternator or the thing won't charge, even if it
is turning. If your battery is strong enough to turn your starter motor -
which takes a whopping amount of amps to turn against the load of the engine
- it should be charged sufficiently to activate the field.

John Rodgers
Clayartist and Moldmaker
88'GL VW Bus Driver
Chelsea, AL
Http://www.moldhaus.com

On 4/24/2010 11:41 AM, Bill MacLachlan wrote:
> Hi all
> List wisdom&  experience needed:
> I'm installing a new alternator.
> Label on the new alternator says have the battery fully charged before
> hooking up the alternator.
> My battery is low.
> How critical is the warning?
> Do I have to dig out the trickle charger and recharge the battery first?
>
> thanks
>
> Bill M
> 82 GL I4
>
>
>