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 drinking coffee instead of charging batteries they'd sound something 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]. If they already have a mug on the desk they can probably just use that one; no need to buy something fancy. 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 in 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. Here are some actual performance measurements from a few dozen people using mugs other than Yeti…
Yeti aficionado: Well, maybe you're some kind of tech dweeb that can get away with it, but everyone else should use Yeti so they don't get 3rd-degree burns on their face.
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: 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. We can measure this temperature with a thermometer.

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>

Ok, enough silliness and satire. Let's get serious.

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 official information we can ballpark:

• 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
• very conservative use of the alternator might be 1/4th of its rated capacity²⁾, moderate use might be 1/3rd of rated, and aggressive use might be 1/2 of rated.
• if the alternator is overtasked (especially when airflow is impaired) it can overheat and fail.
• DC-DC chargers have stable current outputs; direct charging current tapers as the bank charges.

Let's assume we have a 200Ah bank and a 150A alternator. Let us further assume we have decided that 1/3rd of rated alternator output (50A) continuous is acceptable while driving. We can handle brief excursions to 75A (1/2 of rated) so we have a 75A fuse on the POS wire.

First things first - how much current would the bank pull if you connected a wire from the starter battery POS to the house bank POS?

• theory tells us that I=V/R. This means that current will be the ([alternator voltage] - [resting voltage of the battery] / total resistance in the circuit. We almost always know the voltage part but few know the resistance of their charging circuit. After an observation we can back into that value.
• an often-repeated (and wrong) answer is 200A, because the BMS cuts off at 1C.
• testing tells us³⁾
  • current at middling states of charge averages ~0.2C, or 40A.
  • current at high states of charge⁴⁾ averages ~0.1C, or 20A.
  • current at low states of charge⁵⁾ averages ~0.32C, or 64A. ← we need to think about this

64A is clearly above our 50A target for continuous current but below our 75A max. There are two factors in play that make this Generally Not A Big deal:

1. direct charging current starts tapering immediately after charging starts (the V in I=V/R is ever-decreasing); and
2. LFP voltage pops out of the lower knee quite quickly after charging starts (the V in I=V/R is suddenly reduced).

Related idea: - how much current would the bank pull if you connected 50A DC-DC between the starter battery a wire from the starter battery POS to the house bank POS?

The obvious answer is 50A, but there's an asterisk. Any time the DC-DC is boosting voltage⁶⁾ it will pull more current from the alternator than it sends to the house bank. So we would probably size a 40-45A DC-DC to make sure we don't have >50A continuous.

Let's do it again with the same setup but a 100Ah bank.

• the DC-DC will charge at 50A ← 0.5C is a little high, we should think about that
• direct charging current at middling states of charge averages ~0.2C, or 20A.
• direct charging current at high states of charge⁷⁾ averages ~0.1C, or 10A.
• direct charging current at low states of charge⁸⁾ averages ~0.32C, or 32A.

Now which one is harder on the alternator?

Once again, 300Ah bank

• the DC-DC will charge at 50A
• direct charging current at middling states of charge averages ~0.2C, or 60A. ←- not good, as this will be continuous
• direct charging current at high states of charge⁹⁾ averages ~0.1C, or 30A.
• direct charging current at low states of charge¹⁰⁾ averages ~0.32C, or 96A. ←- clearly unacceptable

There is a good chance DC-DC will be required here to stay at our 50A continuous max. But there is little harm in turning the key to take quick measurements since our wiring is fused at 75A. If current is as high as predicted the fuse pops and we start thinking about a DC-DC.