question

sweyn avatar image
sweyn asked

Charging a LiFePO4 battery from an alternator without a DC DC charger

On various online media, I’m seeing people promote the idea that instead of charging the lithium battery via a DC DC charger off a starter battery connected to an alternator, all you need is a “long lead” that will “add some resistance” and the lithiums will charge just fine directly.


Is this true?


If not, why not?


Detailed answers that I can forward would be good!

orion-tr smartalternator
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1 Answer
Nev avatar image
Nev answered ·

There are a lot of naysayers out there on the interweb but we have successfully charged our Sinopoly 300Ah 4 cell LiFePO4 battery perhaps thousands of times via our Canter 100A rated alternator at 70-85A until full over the last 8 years of full-time travel.

The single battery both starts the truck often multiple times daily and powers our house behind.

Yes I have seen that Victron YouTube video where they show smoke with a low revving alternator but ours spins >3000 rpm even at fast idle.

No smoke yet or at least there is still a lot remaining 8 years later.

Direct alternator charging was originally nearer 95A and inserting a short length of 4 gauge cable that only ever was warm to the touch lowered the charge current to nearer 80A but the end 100% SOC 14.5V was too high for my liking so I removed the link and added a Victron Argodiode battery isolator in series with the alternator. That reduces the end voltage to c14.1V and lowered the current by about 10 amps.

All still in use and performing perfectly.

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sweyn avatar image sweyn commented ·
So that’s one vote for this practice.


Anyone against it?

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dc4me avatar image dc4me sweyn commented ·
I "personally" wouldn't go that route but that is just me. Saying it can do it is one thing. Results could be different depending on equipment used. My thought is why push an alternator close to it's limit (as in scubadoo) when you can use the Orion. 75-80 amps the alternator is working hard. Yes he said he was seeing 95A but how long would that last?
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kevgermany avatar image kevgermany ♦♦ sweyn commented ·
You may or may not get ok results. If you get it wrong, it will be expensive. There's a big difference between a bodge and doing things properly. And the long cable will only limit current, not final voltage once charge rate drops. So you will be relying on the BMS, which I assume you're using, to cut charge, even though its cut off is a protection, not a charge controller
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sweyn avatar image sweyn kevgermany ♦♦ commented ·
To be clear, this is not for me.

I have all the recommended systems.

I’m trying to establish if this theory is sensible, and if not, why not?

Is there a particular case that makes the idea bad?

Is there something that is missing?

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Alexandra avatar image Alexandra ♦ sweyn commented ·

There is no current limiting at all except through the cable - hopefully..... So left to chance really, if there is no correctly sized DC to DC installed.

Hence labelled as bad practice.

Also good reliable results rely on all the variables being just perfect for getting away with it.

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sweyn avatar image sweyn Alexandra ♦ commented ·

That’s what I thought. Realistically, whatever resistance you add with a long cable will only produce the correct current and voltage at one moment in time.

As the charge progresses and the potential difference between the battery and the alternator changes, the current will also change.

This will change the warmth of the cable being used as an energy dump which will alter the resistance compounding the variability of the voltage and current received by the battery.

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kevgermany avatar image kevgermany ♦♦ sweyn commented ·
Point being that an Orion controls the charge by monitoring/adjusting voltage. An alternator just outputs a fixed voltage, (although 'smart' alternators are different and controlled by the vehicle ECU).
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