You're right about the recoding voltage, BMW specifies a minimum in all their related SIB's.

As far as the old wives tale thing, I believe what you heard, I just don't know if it's completely accurate. All the chargers seem to have their own proprietary formula for optimal charging. Unlike what you heard it's my understanding that AGM batteries apparently prefer higher voltate charge so an option like the Battery Tender Plus which BMW itself sells, and outputs at 12 volts/1.25A, really isn't such a great choice compared to a CTEK charger that outputs 14.7 volts/3.3A in its specific setting for AGM batteries. And hogwash or not, it's interesting to note that the Porsche rebranded CTEK charger does not have a AGM setting since Porsche didn't use AGM batteries and so they didn't want someone to charge at 14.7 volts (I spoke to CTEK about this). Now will both get the job done? Probably. But unless someone did a head-to-head test over the lifespan of the batteries I think it would be impossible to know for sure if one is better than the other. In BMW's SIB on charging they specifically call for the Deutronic charger to be set at 14.8 volts constant voltage while they don't specify the output current. So that's interesting to note as well.

My point was the differences between the two are inconsequential. You can use either the CTEK or any other quality charger and the performance of the battery will be the exact same. The key here being quality, so long as that exists you absolutely DO NOT need a special charger for an AGM battery. Would it be nice to have, sure...but its not necessary in any capacity.

WRT Porsche, BMW AFAIK does not use AGM batteries either. At least none that I've personally laid eyes on. The 1.25A tender they sell is a joke, as the amperage really isnt sufficient to prevent Sulphation. So far as I know, 2A is the minimum to reverse the sulphation process. Also the Deutronic chargers are designed to maintain a relatively high specific voltage, and vary amperage as needed for coding/diag. Black and Decker also makes units that do this. Does that mean they're any better for simply charging/maintaining a battery on a car that sits in storage? No, not really.

Battery Vent
 

I had to purchase a new battery yesterday (3rd since car was new)

I had the original BMW Battery then an aftermarket, and now a new BMW battery (-982 Excide). Does anyone have a picture of the vent elbow that comes out of the battery for venting? The one the parts dept. gave me does not fit. Is it OK to drive around for a couple of days without it venting?

how long does it take to recharge your battery if its completely drain, with a 12volt battery charger

The battery will vent with or without that tube. The problem is it might vent all over the inside of the battery compartment. The exide battery should have come with a battery vent hose, if not you can sometimes find them at a battery store.

At how many amps? Also, how badly discharged/how old is the battery? If the battery is too old, or too badly discharged it may never recharge properly again.

12/30/11. Baltimore, MD. Just replaced original (7 yr) battery on my 2004 X5 3.0. Dealer (Stealer) wanted $180 + tax for OEM 640CCA 3 yr waranty made by DEKA + $60 installation. Shopped around, found a cheap $109 + tax. 3 yr. waranty Everstart Maxx 94R 765CCA made by Johnson Controls, self installed (not too difficult), works fine so far. All my Walmrt batteries lasted at least 4 yrs. Saved $135.

Everstart Specifications Sheet for Maxx
 

I went down to the local Everstart (WalMart) Dealer and found these (attached) specs on the Maxx 3 including CCA and Dimensions...still at $109 and I had to self install ... 6 months works great in a 2003 E53.

The previous owner of my X replaced the battery last year before I bought it, with a Bosch battery from Pep Boys I think, OEM specs but he got the one with 850 CCA spec'ed for the 4.8is, he told me with the additional electronics of the EHC2 air suspension that he thought the truck needed more CCA to deal with.

Been a year and my battery works great, highly recommend Bosch as well.

I find that battery choices can almost be like what type of oil to use. :)

But for the most part...any 49/H8 or 95/H9 battery class size will fit/work. And since there's only a few battery manufacturers in the USA that make most all of the battery brands...most of the specs are very similar...with only the battery case's color and brand label separates them from all looking the same.

You can see on the list below that Johnson Controls & Exide manufacture a lot of the popular brands that can be found locally where ever you live in the USA. The list may have changed by now...but for the most part...these 2 mfgs make the majority of the batteries sold here in the states:


The 49/H8 batteries tend to cost the less (respectively)...with the 95/H9 batteries costing the most. One may also want to consider warranty length and the ability for the warranty to be honored. For example, many people select one of the Autozone "Duralast" batteries because they are almost nationwide, and your name goes into their database...so if you were out of town and needed a new battery...your info could be called up regardless of you being from out of town...and if your battery was still under the free replacement or extended pro-rated part of the warranty...it would be honored.

Another example (using Autozone)...they sell the Duralast 49 DL, Duralast Gold H8, and teh Duralast Gold 95R...again, any of these batteries will work in an e53 X5...their specs are almost similar...their warranties are only different by one year...they all vent (some from the top center & some from the side)...and they range from around $100- $135-$190. Interestingly, the 49 DL is the least expensive but has the highest RC (reserve capacity):