what does it take to build a 12v lithium replacement for a lead acid car battery, & have it charge with the altornator?

I'm sure it's been asked before but it is difficult to google search this without getting a bunch of dumb results and ads for "lithium" portable jump starters.

Would it be possible to drop a ~12v LiFePO4 pack into the car's battery tray and hook it up to the oem wires?

Of course the answer is no because the altornator will keep feeding a charge to the battery and will never cut it off. And that would destroy the lithium cells. Right?

But is there a BMS or an altornator-to-lithium-battery product out there that could solve this problem? Reason I ask is because my Miata needs a new battery soon and I would love it replace it with something less than half the weight.

Thanks!

Yes and no.
Yes it is possible, but no, you probably shouldn't attempt it the DIY way. By that I mean do not build your own battery for this purpose.
Reason being that unless you know what you're doing, it is likely to end badly.

While car batteries do not need a lot of capacity or voltage, they do need to deliver a huge amount of current. So you need cells designed for that kind of purpose, or you need a lot of paralleled cells to reach it. You also need some monitoring circuitry capable of delivering really high currents, and while these do exist, they usually aren't exactly cheap.

Good news is that lithium batteries for cars do actually exist and aren't particularly hard to find (at least where I'm living, I don't know if that's the case for you). Those are plug and play, so you do not need to mod anything on your car. Just type "car lithium battery" on Google and you'll find many. They look just like regular lead acids.

Thats a bummer to hear but thank you for the help!

There are actually some of those available on batteryhookup, IIRC, from BMW.
https://batteryhookup.com/products/bmw-oem-3s-12v-85ah-869wh-battery-module
https://batteryhookup.com/products/bmw-oem-4s-14-8v-75ah-1-11kwh-battery-module
https://batteryhookup.com/products/bmw-oem-5s-19-2v-75ah-1-44kwh-out-of-case
That's jus tthe cells, and doesn't include the protection/etc stuff, AFAIK.

Recommended Max continuous discharge: 150a
Recommended Max pulse discharge (10 seconds): 600a
Recommended Max continuous charge: 100a

Click to expand...

Also there *are* some threads about DIY starter batteries, some of theem are in this list
https://endless-sphere.com/forums/search.php?keywords=starter&terms=all&author=&sc=1&sf=titleonly&sr=topics&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
but there's irrelevant stuff there, too, and not all of them are listed.