Alternator ailments resolved

alancordeiro at alancordeiro at
Tue Jul 3 17:30:27 EDT 2007

The optimum battery charging voltage depends on the temperature
the BATTERY electrolyte is at (affects the sepcific gravity) Hence on cold
the abttery can be charged at a higher voltage, while on warm days, the
needs to backoff a bit to prevent gassing or overcharging.

It also means that the alternator needs to slowly reduce the charging
as the battery warms up (from the charging current as the car runs)

Not being able to physically monitor the battery electrolyte temperature,
the altenator uses its own case temperature as a surrogate, there it uses
principle that the forward voltage drop across a diode fals as the junction
temperature rises to create a reduction in output voltage as it warms up.

But it also drops its voltage if it runs hot due to heavy auxilary loads
(fuel pump,
ac fan, rear defroster, headlights...). It also drops if the turbo warms its
case up....

I have usually found that the charging voltage from Bosch regulators built
Audis is 0.3 to 0.5 volts lower than Bosch alternators built for other OEM
Aud alternators give out 13.8 to 14.0 volts, whicle a Bosch in another
vehicle may
give out 14.2 to 14.5 volts. I suspect thsi is because they are built to the
requested specs. Using an external voltage regulator usually allows higher
voltages in general, giving brighter lights. It also decouples the charging
voltage from
the alternator case temperature. YMMV.


----- Original Message ----- 
From: "Kenneth Keith" <auditude at>
To: "200q20V mailing list" <200q20v at>
Sent: Friday, June 29, 2007 2:57 PM
Subject: Re: Alternator ailments resolved

>I have read the claim that the upper limit of voltage on the Bosch
> regulator is too low to allow it to catch up or keep up with periods
> of high demand (such as the radiator fan on full blast forever here in
> the AZ summer heat, with other accessories on).  One such claim was
> made on a website offering the external regulator conversion, so that
> source is obviously not unbiased.  Here's one that isn't a commercial
> page:
> I have noticed how warm the alternator and air shroud area can get,
> which is quite warm given it's proximity to the turbo.  Maybe the
> claim that high temperature adversely affects output is baseless.  I
> haven't tried the external conversion yet, but I still find it
> interesting.
> Ken
> On 6/29/07, Bernie Benz <b.benz at> wrote:
>> The regulator is mounted on the cold, intake air end bell of the
>> alternator, no heat problem with the stock input air shroud in place.
>> The Bosch regulator does an optimum job, within the constraints of
>> possible poor connections and grounds.
>> Bernie
>> On Jun 29, 2007, at 10:18 AM, Kenneth Keith wrote:
>> > Or alternator brushes on the voltage regulator, since it is also the
>> > brush holder in these particular ones, as opposed to other apps or
>> > conversions that have an external VR and separate brush holder.
>> >
>> > Anyone do the external VR conversion, by the way? Seems like it might
>> > help to get it away from the hot alt, and also supposedly provides for
>> > a higher voltage to catch up better than the stock Bosch one.
>> >
>> > Ken
>> >
>> > Kneale Brownson <knealeski at>
>> >>
>> >> Voltage regulator brushes on the alternator, Grant.  I guess those
>> >> could be called the alternator brushes.
>> >>
>> >> Grant Lenahan <glenahan at> wrote:  Regulator brushes?
>> >> I'm lost. You mena alternator brushes??
>> >>
>> >> Grant
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