Hi - I have recently replaced my car battery and want to know if I need to code it. It's a 2002 A6 so I don't know if coding is for newer cars/batteries. 

The battery is also discharging so I think there may be a parasitic draw although multimeter readings were within tolerance. 

Any help much appreciated.

Thanks

Welcome Hector,

I would be surprised if the battery needs to be coded on your car.

Parasitic drain ? What are you terming ‘within tolerance’ ?

Regards,

Gareth.

Hi Gareth

Thanks for the reply.  I fully charged the battery and then plugged a multimeter between the car's negative cable and the battery's negative post. The draw was 40 milliamps. I believe that over 50 milliamps may be a problem.

I recently went on holiday and the car started ok after nearly two weeks so I thought all was ok. However I went to start the car today and the battery was flat even though I drove it five days ago.

It's on a charger now and I'll have to investigate once it's fully charged.

Regards

Hector

Thanks Hector,

A 40 mA draw after the system has ‘gone to sleep’ is not a problem. 
Obviously, a situation where a battery runs flat in 5 days certainly confirm a problem.

Before moving on, could you let us know what make of battery you have just fitted?

Regards,

Gareth. 

As Magnet said, 40 mA is perfectly normal. Over five days that equates to about 5Ah drain which is only a fraction of the capacity of a battery. That suggests to me that either the battery is defective or something such as the starter solenoid is not unlatching after use.

An auto electrician may be your best source of help.

With your multimeter, check the voltage on the battery if it fails to start. Should be 12.4-12.7v. A new 12v battery should be trickle charged before installation. It’s advisable to code it to the vehicle with VCDS for battery management reasons, but that won’t explain a non starter. Was new battery like for like cold cranking capacity, AGM version?

Edited November 6, 2025Nov 6 by spartacus 68

Thanks Richard, but I’d be very surprised if a replaced battery on a 2002 car needs to be coded - but I could be wrong I guess. 
Regards,

Gareth. 

Thanks for all the comments so far. Much appreciated. Just to summarise:

• New battery replaced recently. It's a Varta Dynamic SLI 85Ah (F19). No mention of recoding in the workshop manual, other than keys or radio, but I've never even had to recode any of these and had the car 23 years.
• Recently replaced alternator with refurbished one.  With engine running the multimeter reads about 14 volts, so I guess it's charging OK.
• Replaced rear cigarette lighter unit as that was draining the battery very quickly, but that seemed to fix the problem.
• Took daily readings and battery was losing voltage but only a little and after sitting for about 12 days it was over 12v and started.
• Charged up the battery yesterday and will have a look at it tomorrow (it's raining here). The battery is disconnected so will take a reading tomorrow.
• I also need to change the drivers window regulator (and check motor) as it stuck halfway up and took a while to get back to closed position. That will have caused a significant drain but not sure if that's the root cause. 

Will get back to you with my findings in a few days. Thanks for all the comments and advice.

Regards ...

Thanks for coming back on this Hector.

Taking you points, I think it’s fair to agree that the battery doesn’t need to be coded. 
Varta would have been the OE. battery, and should rule out that being the offender, although I have had a couple of issues of them failing in recent years, and now favour Exide. 
Does the car have an alarm and possibly an immobiliser? 
If so, you would need to measure the drain on the battery after a couple of minutes -when the car has fully shut down. 

12 hours ago, Magnet said:

Thanks Richard, but I’d be very surprised if a replaced battery on a 2002 car needs to be coded - but I could be wrong I guess. 
Regards,

Gareth. 

You're right, apparently started around 2007/2008 on B8 A4 model. With that in mind - just plug and play!

 

Edited November 6, 2025Nov 6 by spartacus 68