stephenscaturro

My S8 battery dies after 5 to 7 days of non use. Audi has looked at it and can't find what is draining the battery. my Audi dealer changed the battery twice in the last 3 years. Now it is dead after i got back from vacation. I usually put a trickle charger on when i know i am not going to use it for a while. This time I forgot since being retired i use it more often and have had no issues for a long time. Should I use a different battery, has anyone fixed the drain?

MP4.2+6.0

Welcome BTW!

Not enough info. I sympathize with the problem, but your answer goes back to the insufficient diagnostics so far. Many things can be the underlying problem form D3 experience. It is typically a component that does not "go to sleep" and continues power draw. Stepping back, that's what is important to understand on many modern cars, whether Audis, Ford or whatever--things that draw power have to shut down electronically, but many either have a "soft" shut down because owners want to use them after turning the key off, or they are systems used periodically with key off (alarms, door entry, lighting circuits, etc.) Variously, things like the the MMI system, bluetooth (how you keep doing calls post shutdown), the keyless door handles, an alarm horn back up battery that corrodes the innards, and lots of others.

They can look at current draw post shut down, and then status of devices that should have gone to sleep. There are lists of those in Audi service documentation. But takes time and effort, which costs $$$. If you want to do it the old "cheap" way btw, you pull fuses on likely culprits and see if that affects things over time or with a power draw tester. But, likewise takes time and successive steps.

No reason a different (brand) battery would help. One thing I can note though is the S8 is way "underbatteried" as Audi's go. When I saw mine, I was pretty shocked it is only a 75AH battery. That's more like a 4 banger stripper A4 size wise, not a luxo barge w/ lots of power suck options. I can tell you from the parts listings that at least for my 2015, it is the SMALLEST capacity battery used for any D4--not even the 3.0T size IIRC. Odd. I think it was probably some misguided attempt to take a little weight out in back on a porker vehicle in general. It probably also has to do with the micro hybrid type set up they use as part of 4.0T cylinder on demand (COD) to backfeed extra charging to the battery under certain conditions. What you could do as a practical matter is ask them to go through the D4 battery sizes and fit the biggest capacity stock one. Might be the W12, or maybe the 4.2 pre-the COD but just guessing. They all go in the same place under the spare, and presumably all fit physically. Why we all used to do on D3's, where there was the "big boy" size and the "wimp" size, and one in between. Then have them re code it for that bigger capacity choice--or DIY with VCDS if you have it. When mine gives up the ghost, what I intend to do. I already notice that on very hot days at idle the sat radio drops out and display says it isn't receiving signal, even though sky is clear. When I get moving again, signal returns. A sign to me power is dropping when under strain of idle, a hot day with heavy fans running, and not a lot of alternator charging. Thus, more capacity would likely help even my "healthy" situation. (Might be my antenna getting hot too, but that's off topic.)

stephenscaturro

MP4.2+6.0
Wow I am not that mechanical. I guess asking the dealer to deal with this isn't quite enough. They checked the drain and told me the draw was what it should be for the clock and computer and other things that continue to draw current when the car is off. I did see the video of how to find parasitic drain. Now that i have more time i will go through each circuit to see what is the draw when asleep. then i should have the info. you spoke about. I did not state that this car has an APR stage one tune and stage 2 down pipes which i love. Not sure if the tune matters. Thanks for the most complete answer and my new battery will now be the big boy as you mentioned.

MP4.2+6.0

Tune shouldn't make any difference. If you have any kind of aftermarket wiring though--for things like radar detectors, cameras, stereo mods, cellular WiFi fi set ups or other electronic gizmos, etc.--be suspicious of those.

Below is a screen shot for the 2013 D4 batteries. Seems like same list as my 2015. As I mentioned, my 2015 indeed has the smallest 75Ah one. They change part #'s on these from time to time, but the list shoes both a 105 and 110Ah version.

stephenscaturro

MP4.2+6.0
You are again right on the money. I did install built in radar, laser jammers and dash cameras both front and rear. That car loves to be driven fast and I do not like tickets. Finally I got it started by using 2 jumpers from 2 separate cars. Ran the car for a few hours both driving and idling. The battery read 3.3 volts when it was dead and over 14 when it started. Last night with everything off read 12.44 and this morning 12.06. It is dropping quickly and why it is going dead in a few days. I will disconnect the after market stuff 1 at a time and see what the drop is overnight. In my other Audi I used another installer who convinced me to include a separate battery not to drain the main battery with this equipment. My next battery will be the 110ah for sure. thanks for your advise and i am glad i joined this forum.

MP4.2+6.0

Your battery may be shot again. Try disconnecting cables so battery sits alone without powering car. Have it charged and then watch it for voltage drops. My experience is often when a battery goes way down like that (3V) it doesn't come back well.

Try disconnecting your add on equipment as another test and then see how battery behaves. I only run my radar detector through ignition switched power so it otherwise just stays "on" but clearly draws no power off. Be suspicious of the cameras since those may be wired to stay on to monitor parked car.

ajspaz

Any thought on if it should be an AGM battery? My 3.0T has an AGM (with the specs of the 105CE battery from the above list). My understanding of the 110 Ah battery listed as 105DL above is that it is not an AGM battery. Or at least doesn't have AGM printed on it like the others.

MP4.2+6.0

D4's in general went to AGM IIRC, while D3's and older are conventional wet cell. My 2015 S8 is the 75AH and marked as AGM. I just changed my 2013 Q5 main battery and it was marked AGM too. With a lot more older ones out there, in going the other direction D3 owners typically report no issues.

If you notice in the list, the crank amps and the capacity specs are not consistent lower to higher. It made me wonder actually whether some of those on the list might be wet cell while others are AGM.

What would be most important to me is knowing you can get the given battery input into VCDS or its dealer or other equivalent. That what tells the car what battery you have, its capacity, etc.