mishar

Sure! We share your membership fee.

SS_RTR

1: Using VCDS - Audi A8 (4E) Battery Replacement - Ross-Tech Wiki
Most usually use increment the serial # by one if you aren't using an OEM battery.

2: no, I think you basically want to minimize the draw from the battery when you initially connect, but from my experience, the car will basically "wake up" once you connect a good battery, so expect a slight spark.

3: I don't have a vent hose on my battery

4: If you have to charge, remember to either charge the battery out of the vehicle or make sure to use the negative post above battery instead of the post on the battery.

mishar

Charging the battery out of the vehicle is same as connecting wires directly to the battery posts. You are bypassing Power Management so it won't "know" about that charging. It will take some time for it to catch up with the battery state.

The best charging is by connecting to the plus terminal of the battery and to the shiny post above the negative terminal. Unfortunately in the most cases charging the battery won't help on the long run. Regular led/acid batteries (not a deep cycle ones) don't like to be completely discharged. Once that happens they are permanently damaged and it is just a matter of time to the complete failure.

Measuring tension (voltage) means almost nothing. Damaged battery can get above 12V very fast but still has very low capacity. Basically just like a way smaller battery.

MP4.2+6.0

If you used an AGM battery, those typically have no vent by design. If a conventional flooded starter battery--what comes originally in the D3's and is normally cheaper replacement--the vent is very important. It will be there on the one that comes out if it was OE or most replacements. A similar fitting will be on one that goes in if it is the proper size (two different ones/options--both fit) in conventional flooded construction.

If the flooded/non-AGM type, letting acid vapors float around in a quasi sealed space back there--or if acid ever runs out with overcharge and high water level--is not good for the metal to say the least. BTDT on older Audis where they were still under the back seat. Same drill on old Bugs. Far rear of trunk has same basic issues conceptually. Even when they were underhood in the plenum area like my 2000 C5 4.2, they had a vent hose, as does my under plenum Mini. All were std. flooded type. Basically no worries on the typical AGM since those are sealed; on boats sometimes even mounted non-vertically.