Battery information for V8 cars

[phaphaphooey]

My car just threw a spoiler malfunction error when I turned off the car. I attempted to restart the battery was so low that the engine wouldn't turn over. It would explain some electrical weirdness that I have experienced lately. However, I am having difficulty determining the specs of the stock battery and am across the country on business currently and am formulating a plan of attack when I get back this weekend so I can quickly get the car back on the road for the work week. I have searched here and have looked on Autozone/Advance Auto Parts websites and in both places it has been discussed/matches listed are H6-H9 sized with some saying AGM is a must and others saying don't bother. So, I would be most appreciative if another owner of a V8 car would enlighten me as to what the specs are on the OEM battery (both size and CCA) as well as if it is the size that fits the best or if other sizes fit just fine. Also, does an AGM battery really matter?

Thank you to the community as always.

[kebabman]

I haven’t done a battery on my RS7 but I replaced the battery on my R8, I just looked at the Ah and the CCA on the factory battery label (after taking half the damn car apart to get to it) and then bought an equivalent AGM battery from Autozone. I personally think AGM is worth the extra money but that’s up to you, either will work, plenty of info on t’internet about the pros/cons of AGM vs wet cell.

The battery was a different shape but it still fit, measure the OEM battery and the space before buying so you can be sure the new one will fit.

You’ll also need a VCDS cable to reset the battery serial number so the car knows how to charge it appropriate, apparently especially important for AGM batteries. I have a Ross-Tech cable so even though I was unconvinced of how important it was I did it. Again there’s a bunch of threads on t’internet with instructions just do a google you’ll find what you need.

[MacFady]

I know your just asking what the correct battery is, but curious, did you check the battery and diagnose that as the problem or is this just the first dart?

[phaphaphooey]

Thanks for the posts. I do have VCDS and have already read on how to do the reset procedure and plan on doing that. Unless the stock is an AGM battery I am leaning against it. I usually use wet cell batteries but did put an AGM battery in my 911 and did not feel that the battery's performance justified the extra expense over the traditional wet cell. I will report back on the stock specs when I get the chance to pull the battery tomorrow and hopefully that will help individuals in the future.

I have not had the battery tested as pulling it on a 4.0 car is quite difficult. It is a diagnosis of exclusion: it has run low fairly quickly if not on a tender recently while I have sat in the car doing coding work, as the weather has gotten colder at my home it has taken longer and longer to crank in the mornings, it barely turns over the engine after a short trip during which the alternator did not have enough time to charge the battery, etc. It does seem on the early side for a battery failure but I guess it is not completely out of the norm since the battery is a little over 3 years old at this point.

[Vinng86]

If you have a multimeter, do a quick check of the voltage across the terminals. It should be around 12.6 V, and should not drop lower than 10V when starting the car.

[wwhan]

Here is an old thread that is fairly informative on the subject, even though it is about the B8 S4: Replacing OEM battery S4 B8 (https://www.audizine.com/forum/showthread.php/671808-Replacing-OEM-battery-S4-B8?p=11090923&viewfull=1#post11090923)

[Delfin]

Battery is in the passenger compartment...

https://www.nydailynews.com/news/national/woman-girl-died-inhaling-hydrogen-sulfide-coroners-article-1.2817657

[phaphaphooey]

Thanks for the helpful reply wwhan. I didn't find that thread since I was looking for C7 platform specific threads but I ended up taking the exact same path as MerkurRS in that thread and coming to the same conclusion.

Specifically, my RS7 had an OEM Varta H9 that was labeled as an AGM battery that was rated at 950 amps on the battery. Whether that is CA or CCA I don't know. Regardless, I couldn't find an AGM H9 in stock locally (Autozone/Advance Auto Parts/O'Reilly's/Sam's Club) that had 950 CCA and certainly did not want to replace with a weaker battery. Thus, just like MerkurRS in the thread that you quoted, I ended up with an Autocraft Gold battery from Advance. It has the same specs (950 CCA) as the OEM battery with the only difference being it is a traditional wet cell battery as opposed to the AGM build of the OEM Varta battery. Also interesting was that, again as MerkurRS noted in that thread, the battery from Advance that I purchased was essentially identical to the OEM battery and was obviously a rebranded Varta. Made in Germany, same type of date code and other markings on the battery, etc. I installed the battery, coded it with VCDS, and we will see how it goes. Specifically, I am interested to see if the replacement with a non AGM battery will impact battery performance/longevity. The battery I purchased has a 3 year replacement guarantee so I am covered for at least that long. Also, the good thing about Advance is that there are always some good coupon codes floating around. I used a 25% off which made for an affordable battery considering the specs and size of the battery. I will update this thread if I have any problems.

Delfin: don't really see the point of your post in this context but you are right, the battery is in the passenger compartment, hence the imperative of matching the OEM specs as closely as possible. Regardless of what battery is chosen, OEM or aftermarket, making sure the battery is connected to the vent tube that dumps noxious fumes/overruns to the outside of the car is paramount.