There is a lot of fear and misinformation circulated about LiFePO4 batteries in general and direct charging from the alternator in particular.

The discussions get a little weird. If they were about coffee they'd sound like this:

Yeti aficionado: What you need is a $40 Yeti mug to put your coffee in.
Me: Yeti mugs are nice,¹⁾ but most people don't need one unless [insert niche use cases here]. People have been drinking coffee out of regular coffee mugs way before Yeti was founded.

Newbie: yeah, I heard if you don't use a Yeti mug your coffee maker will be damaged by brewing too much coffee!
Me: It is possible to brew too much coffee to put ion any mug.

Yeti aficionado: If you make a really big batch of coffee and don't have a 40oz Yeti to put it in you could spill some.
Me: Really, 'most any 40oz container will hold 40oz of coffee.

Yeti aficionado: If the Yeti isn't big enough you could buy three of them for only $120 and drink them in parallel
Me: Or use a big regular mug?

Yeti aficionado: Yes, Yeti is expensive, but buy once cry once.
Me: Rtic and even Walmart's Ozark Trail stainless mugs test comparably. There's usually little reason to cry at all.

Yeti aficionado: Coffee is hot. You need a Yeti mug to protect yourself from burns.
Me: Yes, hot things can burn us. We have to pay attention. This is why we put coffee in mugs instead drinking straight from the carafe. Drinking coffee isn't rocket surgery.

Yeti aficionado: If you don't use a Yeti the coffee will pour at up to 1000 degrees!!!!!
Me: Ok, that's not how the physics of water works. At normal atmospheric presssures…..

Yeti aficionado: What is all this mumbo-jumbo about atmospheres? Buy a Yeti and be done with it. Me: You don't have to memorize the minutiae. The larger point is water boils at about 212 degrees F under normal conditions. And you can force that temp up or down, if desired, by adjusting the air pressure.

Yeti aficionado: Whatever. If you don't get a Yeti mug your coffee will get cold and you will have to throw that coffee away!
Me: We put coffee into insulated containers when we want to keep it hot (or cold, for that matter) Heat transfer and insulation are well-understood. It's not a big deal. I mean, styrofoam cups are a thing.

Yeti aficionado: Styrofoam! What a joke. You can't even drive over a styrofoam cup with your truck without it crushing and spilling all your coffee.
Me: Why would you…. nevermind.

Yeti aficionado: But Yeti made a Youtube video showing that if you pour hot coffee on your head it could hurt!
Me: <sigh>

Here are the problems DC-DC are designed to solve. If you have these problems you probably need DC-DC. If you don't have these problems you probably don't need DC-DC.

An alternator has an Amp rating and only so much power it can donate to house battery charging under given conditions. In the absence of better information we can assume:

• the alternator can output about half of its rating continuously while driving at normal speeds.
• both the vehicle's own power requirements (lights, engine computer, fans, etc) also have to come from available power