There have been a great deal of discussions about 12 Volt battery replacement, which kind to use, when to do it and proactive or not. Here is my most recent experiences and what I have learned.
First, when my model 3 was new in 2020 I replaced my 12 volt AtlasBX with an Ohmmu. The idea of having a lithium battery that could run for years vs. a basic lead acid seemed an easy decision. The Ohmmu worked flawlessly for months until Tesla started messing with the charging software.
Ohmmu developed a fix and replaced batteries. Well done.
Then Tesla hired a 12 volt battery engineer (Ohmmu knows the name of the engineer) who decided to add a battery over charge test to the software. The Ohmmu's Battery Management Computer (BMC) seeing the over voltage during the test would disconnect the Ohmmu to protect it (as it should) for about 5 minutes. But this caused multiple error messages. Back to the drawing board.
Ohmmu issued version or gen 3 of the Ohmmu. Many have stated worked great than about a month (including myself) then suddenly all kinds of messages again. Ohmmu is now working on Gen 4. My understanding is preliminary tests good expanding test group.
In the mean time, since I still had my factory AtlasBX on the shelf (now 2 years old but only cumulatively 10 days worth of usage) I charged it up (still had 95%) put a tester on it, it tested good and reinstalled while waiting for an Ohmmu fix. Messages gone and no warnings.
About 2 weeks after the install of the AtlasBX, I was having work done (seasonal brake maintenance) and was told by Tesla some battery messages were noted and they were going to "Proactvely" replace my AtlasBX. None noted by me either as warnings or notifications but maybe there is something they see that I can't. Battery replaced - no charge.
Which now raises the question:" How reliable is the AtlasBX". Now before everyone piles on with success stories there have been too many that have posted that warning to failure was either minutes to a day or so. That got me wondering if while waiting for a Ohmmu solution (I still think Lithium is the best way to go - so did Tesla on later 3/Y builds) what would be the best battery choice should it take months for an Ohmmu solution? AGM, EFB, or regular lead acid battery? AGM's are built for cycling (they are used on cars for example the have start/stop features even some have alternates that go offline until a certain voltage drop then re-engage to charge things back up to help mileage). EFB batteries are like a hybrid - they are a lead acid but designed for greater cycling. Then standard lead acid batteries (size 51R is what is needed). If one looks at the owners manual under specifications it will say the 12 volt must be 33 Amp/Hr rating or greater. The AtlasBX is 45 (and 625 CCA although CCA is not needed).
I was at Electrified Garage yesterday in NH (knowledgable good folks there). I asked which alternative battery to Ohmmu (which they used to install). I discussed the EFB (Enhanced Flooded Battery - see AutoZone for an example). AGM and regular. I pointed out that Orielly's for example published on the AGM it needs 13.8-14.6 volts to charge and 13.4 to 13.6 volts to float voltage. Otherwise with testing or monitors on the 12 volt have reported the charge voltage is 14.55 and the float is 13.1-13.2. The EFB had no such limits. So I asked the question to Electrified Garage "If a customer came in right now what battery would you put in?" Without hesitation the answer was "AGM". But I said "What about the charging requirements?" There answer: "the AGM is designed to cycle and all that will happen is the total charge will drop and at some point the cDC-DC charger will step up and recharge it". They had done some Tesla retrofits for a police department and got errors with the AltlasBX was loaded with accessories so they swapped out to AGM and no more warnings. I asked if others that had the AGM installed had any warnings and the answer was "no". They also said the Model 3 float charges to about 13.5v and Tesla has been trying to get the software right for charging since the Model S.
Another poster in a different thread stated he had gone to Orielly's because it was a weekend and put in an AGM a while ago. After something like 3 years 50,000 miles finally replaced it as a matter of routine maintenance. I think he replaced with the AtlasBX.
So yesterday I decided to put the AtlasBX on the shelf (as a backup) and install an AGM. I chose Orielly's but any of them meet spec of exceeding 33 Amp/Hrs. So far, no warnings (other than the ones that occur during removal process). Checked today, nothing overnight either. Prior to install, it was a 95% as delivered so I charged to 100% and tested. Tested good. We shall see but I have more confidence in the AGM than the AtlasBX while waiting for the Ohmmu solution.
So in conclusion I am running an AGM while waiting for the Ohmmu solution. As I has said earlier, I still think a lithium like Ohmmu is the best solution but at least I have more confidence in the AGM than the AtlasBX. I would caution that if Ohmmu had BMC issues, other lithiums SHOULD have the same problem as they all use a BMC. Some are claiming "no warnings" with their Lithium batteries but I will wait for an Ohmmu solution.