Is it the battery or the alternator that's at fault?

[Avi Meron]

Robert,
If the battery was bad, you will not have a problem while driving, since the
alternator is supplying the voltage!
You would notice it when you tried to start the car!
All your indications show that it is a bad alternator not a battery!

If you doubt any of the above comments, take a voltmeter across the battery
while the engine is running and you should have OVER 13Volts! If the
alternator is bad you will have less.  The alternator can still be bad (even
if you are reading 13Volts+), you need to load the battery (at 1,200+RPM)
with the fan or the lights on and it still should hold more or less ABOVE
battery voltage!

Good luck,
Take care,
Avi

-----Original Message-----
From: quattro-admin at audifans.com [mailto:quattro-admin at audifans.com]On
Behalf Of Robert Stirrat
Sent: Sunday, January 27, 2002 11:28 PM
To: 'quattro at audifans.com'
Subject: Is it the battery or the alternator that's at fault?

Fellow Listers,

I got an interesting fault on my typ 89 coupe on Saturday evening.  I was
driving with dipped headlights when suddenly my stereo and dash lights lost
power.  About twenty minutes later the power resumed and the autocheck
computer returned the OK message.

As soon as I used the indicator stalk I lost power again.  When I got home I
turned off the ignition and cranked the engine.  The engine started without
any problems.  The autocheck computer displayed the amber battery symbol.

I switched on the the dips again without any decrease in brightness.  The
car's voltmeter was reading just over 12 volts.  I increased the electrical
load with the heater in position 1, I noticed the dash lights go dim, and
dimmer still with position 2 until final losing the dash lights power at
position 3.

So It's fairly evident that I've got a problem with either my battery or
charging circuit.  I removed the battery for charging only to find that the
electrolyte was a murky brown colour and all levels where low in each cell.


I topped up the levels and charged the battery for 8 hours at 2 amps.  I put
the battery back in the car not before removing all the white crud from the
battery tray. Exactly the same symptoms occurred, again with the voltmeter
displaying a little over 12 volts.

It looks like my battery is on it's last legs, I take it the discoloured
electrolyte is internal corrosion of the plates?  AM I right?  I've still to
try measuring the voltage drop across the battery terminals to check if the
alternators is providing a charge.  Though wouldn't this give the same
reading as the cars voltmeter?

Any help, hints, tips or advice in solving this would be much appreciated.

Robert Stirrat.