I was doing some reading about lead acid battery charging again and forgot where I read it, but apparently at cold temperatures, more modern cars with AGM batteries will increase the float charge voltage for some reason. I have no clue why this is the case, but apparently the maximum target voltage can be as high as 15.6V?
A paper I read said LTO-NMC cells will suffer permanent damage if charged over 3.0V (that's 15.0V for a 5S pack) because chemistry, so using this on a newer car with a "smart charger" on a cold day would either destroy the battery (if it has a BMS with high voltage cutoff) or trigger the disconnect and probably make the ECU go nuts. All the cars with dumb charging I've seen don't really go over 14.4V ish, which is safe.
For a newer car or a car with a larger engine, I'm starting to think a heated and low temperature cutoff LiFePO4 battery would make more sense, perhaps with a supercapacitor bank near the starter to help with cranking, as 50Ah of LTO starts to get somewhat bulky and heavy. The higher voltage would help with starting as well.
ReLion has a 50Ah heated battery available off the shelf, but its 675 dollar asking price is very steep, and the low current limit means you definitely need the supercapacitors
https://relionbattery.com/products/lithium/rb50-lt
There are also these "Grepow" cells that can charge at 0.2C and -20C with reasonable capacity degradation, which is better than the 0.1C max rate that a normal battery can handle at just 0C:
https://www.grepow.com/page/low-temp...e-battery.html