Do not use LiFePo4 as a car battery, at or around freezing you will start doing permanent damage as the alternator tries to charge it. The solution is Lithium Titanate. you can purchase them from AliExpress (Alibaba is for businesses, they will sell it to you but use AliExpress first). Titanate can handle the extreme temperatures and has an insane charging/ discharging rate. the only problem is they are large cylindrical cells, you will need to relocate them through the firewall and maybe install it under a seat or build a box so someone doesn't kick it. for a 12v system usually 6 cells minimum are required, but since it's for a car you MIGHT need 1 extra cell because the alternator pumps out 14.4 volts I think. You will want to add balance wires to connect your battery bank to the BMS and you will want insanely large cables, I would try 4/0 first WITH a fuse and if it's not enough you will need THHN cable, it starts at 250 MCM and goes up really high. THNN is for buildings. for the proper fuse you will probably need a Class T fuse rated for the wire size. Consult a professional, you are playing with a battery chemistry more powerful than lifepo4 and larger capacity than most lithium ion except for vehicles. Almost forgot Titanate has an extreme cycle life, like 20,000, it will outlast your car. for comparison lifepo4 has an insane life cycle compared to Pb lead at 2,000 to 4,000 cycles. that translates to about 10+ years so do the math for titanate you can see what I mean
-an addendum: for lifepo4 you do not want to keep the cells at 100% SOC state of charge for a long time. it is harmful long term, and while it can survive going down to 0% SOC it is also not advised. it is usually set in the BMS battery management system as 2.5v for the cut-off voltage and if you limit the max capacity to around 90% or less your cells will last much longer. so the lower limit is about 10% SOC or more. again that's for lifepo4, TITANATE probably has another set of rules to follow, firstly follow the manufacturer specifications, it will be listed on the aliexpresss product page. also do a cross reference match for specs of similar cells. I have seen one manufacturer list lifepo4 as being able to tolerate temperatures well below freezing, that is an outlier and wrong, lithium is in it's infancy, even many manufacturers do not know what they are doing and their recommendations change like the weather. use common sense, cross reference specifications and read what other people on forums do, many are knowledgeable! also I suppose this is a "fun fact" (I hate that meme) lithium titanate is used in car sound competitions because it can handle the abuse, they also fill the vehicle walls with concrete for the sub-woofers
2nd addendum: forgot to mention you will burn out your alternator unless you introduce a charge controller of some sort. what you must do is find a device to limit the amps coming from the alternator because lithium batteries will attempt to pull as much current as the alternator can supply until it burns out unless the battery is already nearly full which than the BMS will cut off the charge according to your upper voltage limit that you chose as per the SOC chart for titanate. remember titanate has completely different voltages so do not attempt to use the same lifepo4 voltages examples I gave
