Ok, so after two years of sitting in the garage (on-road insurance expired August 31st, 2021; so last time the car was driven was before that) finally got around to installing a LifePo4 battery. I was lucky to find no corrosion in the battery bay had formed, from acid battery off-gassing and it will stay corrosion free now since it will never see a lead acid battery again. Of reference, you can only carefully open the driver's door with a dead battery, but you can NOT close it again without power or risk damage to the glass/frame.
Battery is exact OEM size, with OEM like appearance. This is a LifePo4 or Lithium-iron battery, NOT a lithium-ion battery. This kind of battery chemistry is much more stable, almost no risk of thermal runaway or fires. Downside is it is a bit heavier than a lithium-ion battery, but that's fine, still a huge weight savings and no more battery bay corrosion risk. I didn't weigh it, but it is less than half the weight of the factory battery, maybe even approaching 2/5ths the weight from my rough guesstimate. At least for what you can see with your eyes, the case and terminals are good build quality:
Battery is shipped from the factory in a discharged state, it is recommended that you fully charge the battery with a dedicated lithium charger before installation. There is a huge warning sticker on top, to not use a normal battery charger. Luckily I already have a Ctek Lithium charger, which is probably the best on the market. Ctek is OEM for BMW, Corvette and many other brands.
Installation is perfect OEM fitment. I also installed the Ctek permanent wiring harness, for direct connection with the aforementioned Lithium charger without needing to use the clamps. The in-cabin voltage meter reads about 13 volts. Feels like it cranks over the engine faster and with more power/less strain than the factory battery.
Note: this doesn't affect me because I don't drive in the winter, but there is one down side to all lithium batteries; these batteries are seasonal like summer tires; they are not to be charged in freezing temperatures. The same goes for your cell phone or your laptop, never charge them if they are freezing. EV's like Tesla's have heaters for the batteries for winter charging.
Now obviously, a battery like this does not have a heating system, and in theory, nor does it need one, because if mounted in the engine bay, the engine is its heater.
The question is (and I have not been able to get an answer for this), is the BMS (battery management system) smart enough in a winter use case scenario to NOT charge the battery while the temperature is freezing, but as you drive around and the engine bay warms up, bringing the battery itself above freezing, only then allow charging to commence?