charging LiFePO4 from the alternator: FUD vs reality

This morning I got a FUD-filled reply from a youtube “influencer” about why relays aren’t “appropriate” for charging LFP. It involved batteries “expecting” steady voltges/currents, etc. I swear people are making stuff up. It’s ok to be confused. It’s not ok to confuse others.

Then in the afternoon I read a post from a user whose bank obeys physics rather than the fearmongering of influencers.

800Ah bank vs 130A alternator

Surely this will suck all 130A out of the alt, right? Right?

OP is actually worred about low charging rate at idle:

  • 300w at idle (let’s assume 13.4v, so ~22A
  • 1000w at cruise, so ~75A

I’m not sure what numbers he was expecting from a 130A. He’s considering a high-output replacement optimiszed for idle-speed charging and that would certainly get him more.

I wouldn’t charge at idle or try to run a huge bank off a 130A alt, but his needs aren’t my needs. He’s using a Li-Bim, a device I find to be of dubious value1, but it’s not my build. And in any case the BiM is effectively a relay while connected so throughput will be the same as a relay.

He’s also use a chassis NEG return which is probably the secret sauce that keeps charging acceptance under control. Compare this video where 100Ah and 280Ah banks are connected using direct NEG return. The 280Ah pulls 39A at idle (160A alternator) and 72A at cruising RPM, largely because there is a dedicated NEG return.

an unfortunate update

I think our friend is in for heartache:

300 ish Amps at idle will maybe let me almost hold my own during work hours if I’m running the A/C in eco mode. If I’m going to run the engine, I want to gain an advantage so I went ahead and ordered the high output alternator. Idle output is 160A and full 320A output at 1200 RMP.

A 320A alt won’t/can’t put out 320A hot/continuous. I’m predicting:

  • ≤100A at idle, hot/continuous
  • ≤220A at cruise, hot/continuous

    And even those numbers might require a dedicated NEG return and doing the Big Three upgrade. I’d like to be wrong about this.

  1. I’ll eventually write up something about my issues with the BiM and how I’d like to see it improved.