ltooz_a6_a8_q7

First time the car couldn't start yesterday, in my daughter's school parking lot. The battery was full when I drove it there, after waiting for about 20 minutes, the car wouldn't crank, all the indicators were flashing although the MMI AMI was still playing music. The battery seemed dead, no DVM in the car so I couldn't measure the battery voltage. Checked the MMI battery indicator and it's 0%, there it is, the first time ever that's NOT 100%.
Got a jump and it started up right away, drove home, check the battery - 14.1V while the engine's running and 12.3V when the engine is off.
Check the MMI battery indicator - Still 0%. So it doesn't seem the battery indicator measured from the battery terminal, where does it measure from? It's time for me to read up about the the battery charging system? Was too lazy to connect VCDS to get the codes.
I think it's time for me to get a new battery since this one is original (11 years old). Walmart has an H8 that consider to be for 2006 A8L for $123. It does seem a little narrower and shorter.
Any experiences you want to share before I get the new battery and get my battery indicator to 100%. When my battery was 12.1V, it's still 100%, and now with 12.3V, it's 0%?
When I replace the battery, I'll turn off the MMI screen motor as well.


Thanks,

Louis

plat27265

When a battery goes flat, its internal electrical/electronic values change. These values can be capacitance and resistance. This is what the MMI is monitoring. A voltage meter won't tell you whether the battery is holding a charge or not. It may show you 12.1V or 14.1V but if its storing capability is gone, all that charge will be gone in a jiffy.

Nowadays, it is rare for a battery to go 11 years in between failures. I went as far as 10 years in my 1998 Nissan 200SX. However, the battery in my 2011 VW GTI only went as far as 6 years.

MP4.2+6.0

Basically, battery meter is more or less good to tell you...your battery is gone. I think it is using an algorithm that looks at aging of battery and theoretical capacity to hold charge against recent power drain and then throwing out a dumbed down display. Thus, looks at recent power use and general age, but not really useful for when battery takes a pretty typical dump and cells/plates/ supporting structure just give out, often just like yours. My W12 went from solid to flaky in a week and then took its final dump where it would no start even one short drive and 15 minutes after charging.

There are perhaps three sizes of battery for A8, and more or less two you find listed commonly. They all fit. Length is what should vary and in turn their overall capacity. I get the big boy. More power, though oddly in my year listings commonly show 4.2 gets big boy and W12 gets a smaller one.

Big boy is Group 95R/H9. Among the smaller boys are the common Audi Group 49/H8. Wallyworld battery is likely either a Johnson Controls or an Exide, two of the three major players in batteries. Audi/VARTA ones are Johnson Controls in recent years. Cold cranking and reserve capacity are what matter given basically only three true players control manufacturing, with some differing flavors of construction and warranty, and putting aside the AGM ones. This one will likely last balance of life of car anyway, so I would just get a well priced big boy.

ltooz_a6_a8_q7

Thanks all, here is what I found in sizes H8 vs H9

Bosch @ Pepboys H9
https://www.pepboys.com/product/details/672425/00840

Duralast @ Autozone H9
Duralast Gold Battery H9-DLG - Read Reviews on Duralast Gold #H9-DLG

Everstart AGM @ Walmart H8
https://www.walmart.com/ip/40647529

https://www.walmart.com/ip/16782659

H8/h9 is only 1-1.5" different in length - Everstart has 900 CCA.

Do I really need to use VCDS to change the Battery? RE-CODE the battery?

Cheers,

Louis

richard-tx

Yes.....or take it to the dealer.

ltooz_a6_a8_q7

The car won't run, won't charge? Since I've been reading about all these assumptions what need to be done seem to be "too over engineering or REALLY? even with Audi", I may have to try to see if the "Audi followers are just religiously follow" or "NEVER FOLLOW" like Audi used to be. Just the same as the air strut - the unit must be replaced as a whole and the sport and non-sport versions are different - I proved it wrong. My car will be dedicated for experimenting stuff - especially members only think it's worth less than $5k LOL. Let's see this weekend, I'll replace it with a new battery, without VCDS to see if it works. At least, give hope to people who don't have VCDS with all these VINs options. The Ross-tech marketing people will price themselves out of the market in a few years with all these complicated options.

Cheers,

Louis

Jack88

You can't be the guinea pig, Louis. It's already been done. If I recall, the car figures out it has a new battery on its own after a time. Replace the battery, give it a bit of time, and if issues persist, go from there.

uberwgn

Lou, if the battery has been permitted to go virtually dead, it's best to re-code, even if it's a phony code where you only change one digit from the original.


You should also go in an clear faults and warnings.

awdinut

It does not matter how much charge your battery has; it's how much of a charge the Battery Management System THINKS the battery has that matters. And when the BMS thinks your battery is low the BMS starts turning non-essential systems off. I think the BMS monitors drain rate and, based on re-coding date, decides how much drain your battery can handle. So when I did not re-code for new battery the first time I put one in, I had those PITA low-battery-level-eppisodes. But when I would check actual charge with multi-meter the battery had normal voltage level.

I coded the battery and that ended the "low battery" incidents. Also, with the uncoded battery, when I would have the low battery level reading and would go for a drive to charge back to 100% on the MMI reading, it would take 20 min ! After re-coding, on the ocaision that I did in fact run the battery down watching an in-drive movie with my girlfriend for instance,, it would take much less driving to restore to 100% reading. I guess maybe when the BMS thought the battery was old it charged at a lower rate...all I know is it charged faster.

So, ever after I have coded new batteries.

dvs_dave

Just swapping the battery for a new one will be fine, and the car will work. The point of recoding for a new battery is that it resets the battery management system which has gotten used to a battery of a certain performance, and hence shutting down various systems sooner to maintain sufficient cranking power, and adjusting the optimum charge current to suit, etc.

Without doing the recode, the system will eventually figure out that the battery is performing better so will adjust to suit.

There's a whole load of other electrical system performances that it influences also, explained in detail here, SSP 287 pages 17-36. Worth a read to understand the what, how, and whys of the system.
http://www.vaglinks.com/vaglinks_com...Components.pdf

In short, no recode it'll be fine, just it'll take a while for normal electrical system performance to resume. The recode resets it immediately back to full performance.

Up to you try it out for yourself, but little point in this instance not resetting as you have vcds, and we already know what happens.