This is an old revision of the document!
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: But coffee is hot. You need a Yeti mug to protect yourself from burns.
Me: Yes, hot things can burn you. 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: