RE: How much electric current?

["Gerry Reichelt" <greichelt@mmcable.com>]

Excellent reply/answer! I'd never heard it explained that well before.
Thanks

Gerry Reichelt

-----Original Message-----
From: owner-scirocco-l@scirocco.org
[mailto:owner-scirocco-l@scirocco.org]On Behalf Of John Erickson
Sent: Sunday, June 24, 2001 11:04 PM
To: scirocco.org
Subject: Re: How much electric current?


"Patrick Bureau"  wrote:

> Well if you have a 90A Alternator, then use a 90 amps gauge, then you
> will have an accurate reading of what it produces.
> I found that a Voltmeter was more tale-tell on what is going on with
> your alternator/Battery than just amperage.
> but that is just my personal (probably going to get flamed for it)
> opinion.

And:

"Gerry Reichelt"  wrote:
> Seems like a voltmeter would be more realistically useful. Unless you
> already have one.

To quote one Ron Kwas (very knowledgeable about fixing up classic Volvos):

"The ammeter shows the mean battery current. It is therefore an excellent
indicator of the state of health of the charging system of the vehicle. It's
too bad but many cars just aren't outfitted with amp-meters anymore. Wendy
and I have just become parents so we replaced our Merkur XR4Ti with a Mommy
Wagon 745 Turbo (once you go turbo, you never go back). On Volvo turbos such
as this, a voltmeter is standard equipment. Quote from the manual: "The
voltmeter indicates the voltage in the electrical system and thereby also
the state of the battery." Did you notice the difference? Ammeter monitors
charge system. Voltmeter monitors battery.

The information supplied by the voltmeter, however, is only marginally
useful since (see chart, special thanks to Mr. Yoder of Interstate Battery)
the 100 to 0 percent charge condition happens within a mere 8 tenths of a
volt span! Even with an expanded scale voltage meter (many automotive
voltmeters are) by the time we notice a low voltage condition we are already
in trouble. This, as opposed to a current meter which as soon as it shows
discharge while the motor is running signals a charging system
problem--thankfully while we still have an almost fully charged battery to
get home or to safety on. Furthermore, the only absolute voltage reading
that is truly indicative of the "state of the battery" is the "open circuit
voltage," that is, before the alternator is charging or the loads are
discharging the battery. When the motor is running, the voltmeter indicates
the vehicle's system voltage which will be pulled up or down as a function
of mean battery current.

Open Circuit Voltage    Percent Charge
12.68                        100
12.45                         75
12.24                         50
12.06                         25
11.89                          0

(70°F) Source: Battery Service Manual, Copyright 1987 Courtesy Battery
Council International

I believe the reasoning behind the trend of installing voltmeters instead of
current meters is (without getting too technical, I guess it's too late
already?) because voltmeters are (slightly) simpler to incorporate into the
automotive electrical system. I suggest, therefore, for those drivers cursed
with a voltmeter (It does beat an idiot light only*) to make this simple
modification which will make the instrument more useful: with a fully
charged battery and the motor not running, observe and mark the "low normal"
voltage with a small dot of white paint on the meter glass. Start the motor
and after a minute or so, after the voltage settles out, mark the "high
normal" voltage in the same way. Later, when the indication deviates from
these levels it will stand out, clueing you into a possible abnormal
condition. By making this simple modification, you have essentially turned
the voltmeter into a 3 zone instrument (Under, Normal, Over), and like the 2
zone ammeter (Discharge, Charge) have made it, I believe, somewhat more
informative."

I, too, think the ammeter is the best instrument to monitor the health of
your electrical system.

--
John
'00 Jetta GLS VR6
'87 Scirocco 16v
 + El Paso, TX +



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