Since I learned a lot from this forum. I figure I give back something. The message was to answer another post over the M5 forum but I think it would be helpful for the M6 forum, too.
I got an AMG charger and charged it overnight. However, I still got the increased discharge battery after 2-3 days of moderate driving. It was annoying so I decided to replace it before I get stuck in subzero temperatures. After one month with the new battery, everything is good and my electronic still operates 100% correctly. I left town for 9 days, and left the car in subzero temperatures without the trickle charger connected. It started right up when I got home and no increased discharge battery message either.
BMW m5/6 use AGM batteries. In the US, there are 3 manufactures that make majority/all AGM batteries or car batteries in general. Those are East Penn behind the well known Deka brand, Exide and Johnson Controls. All/most battery brands are a relabel from one of those manufactures.
From 2012-2013 M5s were using a 92 AH battery but couldn't supply enough power to last so BMW replaced with a 105 AH to resolve the issue. After 2013, I believe m5/m6 all come with 105 AH batteries.
The link below is from BMW of South Atlanta. It shows replacement batteries are either from East Penn or Exide
https://parts.bmwofsouthatlanta.com/...rimLevel=11862
So with all the facts presented, let’s get to the next step. Where to buy it?
Like I said earlier most battery brands are just a relabel from one of those manufactures. Example, Super Start Platinum from Orielys autoparts is actually East Penn battery. I gave that example because this auto store is widely available in most states and the cheapest in prices. batteriesplus.com resales East Penn AGM batteries, too. If you look up a battery brand from your local places and search it online. Most likely it's from one of those manufactures. It looks like BMW uses East Penn and Exide for supplying batteries in the US. Furthermore, you can go to the website of those two companies and see the specs. you’ll see it matches exactly from CCA to the physical measurements and weights.
I have seen a lot of posts about how you need to use only BMW batteries or else electronic will fail. Not sure where people get that information or just their opinions without facts. A battery is a battery, as long as you have the correct specs, and from a quality manufacture, it should be the same. In addition, BMW isn’t manufacturing their own batteries. They have suppliers providing them with batteries. More than likely East Penn and Exide are their suppliers in the U.S.
I typed this long message because I saw so many misinformation out there, and I hope this will help someone without getting ripped off from BMW dealers. I don’t hate BMW dealers, I use their services from time to time, but the battery service is just a rip off. They want 500+ to replace a battery that cost less than 200 bucks from a retail store. I bet they bought it under 130 bucks a piece (guessing/opinion here).
Finally, how to replace it? Just replace it like any other cars with the exception you need to code it afterwards. It seems like it’s easier to stick with the same AH your car currently using so you don’t have to reprogram to a new AH.