Tracking down a battery drain

Wed Sep 29 21:40:58 PDT 2010

. yes, if you made sure everything was off 450mA is quite high . but make
sure that everything was off, I'd bet there was a dome light, fan or
something like that which was causing the high reading.  Don't go too crazy
until you took care of the obvious.  

Based on the analysis it sounded like your battery was fine . but the thing
that still bothered me with your initial report was that the battery was
sitting over 12V and the car still would not start.  

The proof of the pudding will be to live with the car for a couple weeks to
see if it is starting reliably.  

Audi voltmeters are notoriously susceptible to errors created by interior
fans getting ground via the same wire used to ground the voltmeter.  This
can usually be confirmed via the car's wiring diagram.  I'm pretty sure the
V8 is the only Audi I've dealt with which did not exhibit this effect.  I
suppose if the voltmeter is in the instrument cluster unit it will likely
have a better ground. Bottom line, don't trust the IC voltmeter as a
precision instrument . if you really want to know what the voltages and
drops are use a hand held multimeter.

Steve Buchholz

450 mA standby current draw is too high, you should be seeing well under 100
mA, preferably less than 30 mA.

A typical car battery has 40-60 Amp hours of capacity. At this rate, the
battery will be fully discharged in:

50/.450 hours = 111 hours = 4.6 days. It'll probably be discharged enough to
make it hard to start in about half that time. If you don't drive long
enough each time you're out and about to re-charge the battery fully, after
a few days to a week, you'll have a dead battery.

500A is fine. I wouldn't have replaced that battery unless it didn't crank
well at cold temperatures.

You're going to need to do the test with the ammeter or test light as others
have recommended. An important thing to remember is that if you've got the
door open (to access the fuse box) you'll need to tape down the door switch
so the dome lights don't come on. 

If you don't do this testing and fix the cause of the current draw, you'll
be on your way to wrecking this new battery within a few weeks.

On Wed, Sep 29, 2010 at 11:22 AM, <TooManyAudis at aol.com> wrote:

Took the car to Advance Auto last night, had them do a charge on  the
battery, then a system test.

Existing battery (4 years old -- Huw incorrectly guessed at its age being 6
 years, resulting in 4 demerits) was putting out 500 +/- amps, with it
being  rated for 700 amps new.

Put in a new battery for $100.  Rated at 850 amps with 700 cold  cranking
amps.  Started strong this morning.

The salesman at Advance did a charging system test after the install of the
 new battery.  Results are:

1)  Battery:  12.56v; measured at 817 CCA, rated at 700  CCA;  Temp:  79
degrees f.;  Good Battery

2)  Starter Test:  Voltage:  10.99v;  Amps:   215.8A;  Time: 1.03 seconds;
Cranking Normal.

3)  Charging System Test:  No load:  14.00v, 4.4A;   Loaded:  13.47v 2.4A:
No Problems

4)  Drain Test:  0.45A:  Pass

With # 3, my Instrument cluster voltmeter showed right around 12v with  the
high beams on and AC on max during this test.  Is there an explanation  for
the difference.  Is the IC voltmeter picking up voltage after the  radiator
fan, blower fan and compressor clutch?

Also, with #4, is 0.45A too high?

Thanks for the help.

Tom
Charleston, SC

1995 A6q  5-speed (up for interesting offers) 249k
1988 80q (sold, and still going  strong!) 250k+
1991 200q20v towed away in the middle of the night by a VW  diesel pickup
275k+
1985 4kq a freakin' beast of a car! 275k+ and presumably  still going
1987 4k died in a hit and run 250k+
1983 4k Very first Audi 35  to 38 mpg.  Victim of crank key failure 300k+

In a message dated 9/29/2010 9:29:32 A.M. Eastern Daylight Time,
glenahan at vfemail.net writes:

so why  guess? Measure the idle draw,and load test the battery. 20:1 its
the battery.  6 yrs is 2 beyond its shelf life.

Grant
On Sep 28, 2010, at 4:14 PM,  Steve Buchholz wrote:

> I'm with Huw ... the voltage you're seeing  should be enough to crank ...
likely high internal resistance in the battery  is the problem.
>
> I'd also allow a bit higher battery draw with  everything off ... 10mA is
too low IMO, I wouldn't worry until the current got  up into th 100mA range
...
>
> Steven Buchholz (mobile)
>

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