A8Pete

I have a 2004 A8 4.0 Diesel. I have a battery warning light on the dash board.

Before I change the battery, is there any special procedure I need to follow to fit a new battery? Do I have to do anything with the VIAG diagnostics, or maintain a 12v supply while changing etc.

Many thanks

MP4.2+6.0

No, I don't generally try to maintain power in the system. It remembers most everything, other than maybe the time of day you easily reset through MMI. It may also lose the "pinch settings" with the power window switches that tie in to one-touch up and down. All you need to do there is run each window down and up once by holding each switch on the driver's door switch set.

You should recode the power management module for the new battery with the serial number. If it isn't Audi supplied, what everyone does is just change the last digit of the prior serial number by 1. If you have no access to VAG COM/VCDS, eventually the system seems to figure out you have a new battery. The new serial number coding seems to force it back to its defaults instead of using its recent learning about battery capacity and the like.

BTW, when my battery failed on my 2006 A8 W12, I really had no special warning light. When I have seen that light on some of my older Audis, it usually had more to do with an alternator issue. Are you sure it is the battery?

A8Pete

Thank you for that, much appreciated. Im not 100% sure its the battery yet. I get a yellow battery warning light in the center of the dash. I have been playing with the EPB with function and adaptation tests. I had a charger connected but it seems to have died anyway.

I'll run a charger on it for a few hours and see if it come back up. I just wanted to check out the procedure in case I have to change the battery.

I'll put a meter on the battery tomorrow and see what volts I'm getting. I assume you put it on the positive and negative terminals to perform a volts test, or do I use the negative charging post?

MP4.2+6.0

You should be able to test via either the terminals directly, or with the charging post for the negative side. Charging post should always be used for any charging to avoid confusing power management system.

VAGCOM/VCDS is of course better for codes, diagnostics, etc., but there are some "shadetree" mechanic workarounds that often can help diagnose or at least get you more confidence in what is likely the real issue.

An indirect basic way to get at what may be going on w/ your alternator is to disconnect your battery completely from the car. Then charge it directly at the terminals for a while. Now check voltage. If you see 13- 14 volts, so far so good. If you then let it sit a day and still see 14 volts, still good. If you are down at or sub 12V, it probably lost a cell and you are done. If you happen to have a load tester (basically a big 12V heater with some battery clamps), even better to check it. You can also open up the cells via those screw lids (if it is the Audi/German OE type) on top of each cell; cut the label with a knife if needed to get at them. Loss of water in a cell, especially if the pates are uncovered, or any weird looking chemical reaction stuff down down inside are also pretty strong pointers the battery is probably gone. But, if basically the battery instead shows and maintains full voltage with a charge up aand sitting a day or so, and visually looks okay inside the cells, something else is likely wrong, such as a failed alternator.

Dimmed headlights when running used to be another alternator clue, but at least with HID's and LED's in general, I doubt that is still reliable. The interior overhead light can be another kind of similar simple clue--full brightness vs. kind of dim even with car running, whether the battery is marginal or not.

A8Pete

Thank you for those tips. I checked again and the dash reports "Engine Management Low Battery Level".

I'll give it a full charge all day tomorrow and report back. I'll measure the voltage afterwards and see what she's got.

I do have a VIAG/VCDs. Only just got it so I dont know my way around it yet. Where do I check for battery related fault codes?

Brozee

Search is your friend...

Use a trickle charger to save settings, clock settings etc, or just pull the battery. Really doesn't matter. I've done both. If you don't have VCDS to recover settings or tweaks you've applied then use a trickle charger to keep the cars systems online while you change the battery.

A8Pete

OK, so I charged the battery for 10 hours and it had 12.87 volts after charging. Car as always starts on the button. But I still get the Power management battery level warning on the dash. When I look on the MMI the battery level is shown as 10% or less.

I read somewhere that the powere management module can get confused sometimes.

Is there a way of resetting everything? I have Viag/VCDS, but am unsure of how do that.

mishar

Did you connect negative cable to the hex post or directly to the negative pole of battery?

12.87 V is low after charging. That battery is probably bad and should be replaced. You may check it under load at an electrician, but I think it would be just waste of a time and money.

A8Pete

Update 2:

I used a ctek trickle charger today for 9 hours. Before putting the charger on I looked at the MMI and it showed 10%, plus I had the normal yellow battery level warning on the dash cluster. After charging (using negative post) the MMI reports 60% and the yellow warning is gone. The voltage measured is still 12.87v.

What is a good voltage for a healthy battery. I always thought a car battery cell was 2.1volts and it had 6 cells so 12.6v would be optimum. Am I wrong on this one?

mishar

When charging battery negative wire should be attached to the shiny hexagonal pole just above the battery. Charging it directly to the negative post will confuse the battery management and you will have faulty reading in MMI. If everything is OK it will find it's way soon and show the right information.

You are right about voltages. I started that reply with more information and than cut it short. Not a good cut obviously.

What I meant to say is that voltage is not good indicator of a battery health. Battery that have lost most of it's capacity can show good voltage after charging. Test under load is much better indicator, but only really good is discharging it and measuring energy it held.