LiFePo4 Refresh

[Johann]

Interesting, indeed. I'm new to the topic of LFPs, but I have some thoughts. I thought one of the great benefits of LFP was that it could take advantage of all of the output of high-output alternators, and in so doing charge up very quickly, reducing engine run time for charging. If you're charging the AGMs with the alternator you lose this advantage. If you're charging LFPs with AGMs, what advantage is there at all? You're just shuffling energy from one store to another with the consequent transfer inefficiency.

Well, yes and no… I don’t think that an LFP battery will take more than say AGM during bulk. But the AGM will quickly begin to limit the current when it hits absorption, and this is why high output alternators are much better with LFP. It’s not that the LFP can take higher current, it’s that it can take whatever current until it’s full, and not start limiting current at 65-85% SOC and make the alternator run for hours at low output. So if the B2B is sized right, you still gets this advantage with LFP. The B2B only charges when the alternator is supplying sufficient voltage, so the lead doesn’t charge the LFP really, it all comes from the alternator. Now of course the best setup is an LFP compatible smart regulator, but if you keep the lead start battery and a dumb alternator the B2B from lead works well if it’s sized to about 50% of the dumb alternators output (that’s all it can reasonably do anyway).

Well, the system should be designed to protect against alternator overheating, especially with a high-output alternator; the smart regulator should do this; and, the BMS will protect against LFP overcharging, which is where the BMS disconnect comes from, no? In fact, I'm told some BMS's, or systems built with them, can avoid the disconnect by reducing or ending the alternator output (via field current control, presumably).

Not so?

If you have a high output alternator with a smart regulator then yes, your alternator should be fine and you could use the B2B from the LFP to the AGM. Dumb alternator not so much…

BUT one very common misconception is that the BMS will protect against overcharging. The VAST majority just protect against overvoltage NOT overcharging. Protecting against overcharging is up to you in your system setup. Unless you have a tricked out BMS setup like a fully integrated Victron system. And yes SOME BMSs have a Field Control Circuit so protect the alternator before disconnect, but that is not for normal control… more like an airbag… the alternator regulator should handle charge control in most setups.

 

[jviss]

It’s not that the LFP can take higher current, it’s that it can take whatever current until it’s full,

According to Victron the maximum recommended charge rate for AGM is 0.2C, and for LFP 0.5C. Other vendors quote other numbers, it varies. But there is no doubt that you can hit LFP harder in charging than the equivalent capacity LA, as well as the other charging advantages you mention.

And, who has a 125A alternator and is using a built-in, dumb regulator?

 

[LeslieTroyer]

Interesting. Unless you have external regulation I think typically you charge the AGMs with the alternator and use the B2B for the LFP (mine is set up that way). The APD should protect against a BMS disconnect, but won’t protect against overheating the Alternator or overcharging the LFP.

I have external regulation with the MC614.

 

[Maine Sail]

n.

And, who has a 125A alternator and is using a built-in, dumb regulator?

Every boat owner with a newish Yanmar...

 

[Hayden Watson]

Every boat owner with a newish Yanmar...

Until they wise up and replace it with external regulation.

 

[Maine Sail]

Until they wise up and replace it with external regulation.

Absolutely and Balmar actually makes a conversion kit.....If you don't "wise up" this can be the result when charging LFP.....

 

[jviss]

What a waste, for the lack of a $5 fusible link. Why don't they protect these things?

 

[LeslieTroyer]

What a waste, for the lack of a $5 fusible link. Why don't they protect these things?

Fuseable link won’t prevent a meltdown. Only a regulator with good alternator thermal protection.

 

[jviss]

Fuseable link won’t prevent a meltdown. Only a regulator with good alternator thermal protection.

How's that? It's my understanding that excess current due to a lower load impedance creates more heat than the alternator case can dissipate. So, if you fuse a 125A alternator with a 125A fusible link, any more current than that is going to pop the link, and the alternator won't overheat. You might fry the diodes unless you have a zap stopper installed, but even that is better than melting the stator.

Do you have a different analysis?

 

[Hayden Watson]

How's that? It's my understanding that excess current due to a lower load impedance creates more heat than the alternator case can dissipate. So, if you fuse a 125A alternator with a 125A fusible link, any more current than that is going to pop the link, and the alternator won't overheat. You might fry the diodes unless you have a zap stopper installed, but even that is better than melting the stator.

Do you have a different analysis?

The problem isn't that the battery CAUSES the alternator to put out more than 125A because it does not. The problem is that the LFP's will take that 125a from empty to full and that long time at max output causes the alternator to overheat. What is needed is a way to slow down the alternator's output when its temperature gets too high.

 

[jviss]

The problem isn't that the battery CAUSES the alternator to put out more than 125A because it does not. The problem is that the LFP's will take that 125a from empty to full and that long time at max output causes the alternator to overheat. What is needed is a way to slow down the alternator's output when its temperature gets too high.

Hayden, I believe you are incorrect. The alternator/regulator combination will certainly put out more current than the alternator is rated for if presented with a low enough impedance. There is no current limiting on an alternator, the alternator doesn't "know" it's a 125A alternator. If it's rated a 125A alternator that means that in nominal ambient temp conditions and with a specified airflow it will be capable of sustaining 125A output without melting. You can wreck an alternator quickly with sufficiently low impedance loads, and you can do it with Lead-acid, you don't need LFP to do this. Put a discharged 400Ah FLA bank in front of a 60A alternator and see how long it lasts.

The way an alternator output is slowed down by temp is by having a typically externally regulated system that has a temp sensor on the alternator and the batteries, and that will reduce the output, via the field current, if the alternator or battery temps get to the set threshold. These are not uncommon.

 

[jviss]

Just to be clear, an alternator Amperage rating doesn't mean that's the maximum current it will source, it means that for a specified environment that's the most it can source without damage. The actual current is a function of the load and the regulator.

 

[LeslieTroyer]

I have to agree with Hayden.

 

[jviss]

I have to agree with Hayden.

And to you care to elaborate on that, explain why you think he's correct, or is this just a popularity contest?

 

[jviss]

What is an alternator rating, after all? What does it mean to have a 125A alternator? Where does it say "125A for only one hour," or some such thing?

Why do you think the load has nothing to do with this? How about trying a short circuit, and seeing how long the alternator lasts? Will it only source 125A into a dead short? If not, why would it only source 125A into a low impedance bank? What's controlling the maximum current output? Is the regulator measuring current? Not the typical one, the typical one is measuring voltage, as in "voltage regulator." Smarter ones detect current through a shunt in the alternator output, and can limit output, as in my Freedom 20/Link-2000R system. But the typical simple setup doesn't. That's why the size of the bank and the alternator have to be matched properly, to avoid burning the alternator.

 

[Hayden Watson]

Actually, it is North American nomenclatural laziness that even calls it a 125A alternator because what they put out is Watts not amps just like any other charge source. We get it right with solar and wind generators but drop the ball on alternators and shore power chargers.
It will probably put out 1500W (125A x 12v = 1500W). As the voltage goes lower, the amps go higher so a very discharged battery will take more than 125A but the alternator can still put out a number of watts at a given rpm's with full field current. The problem is that most of them are not rated to put out rated watts at 100% duty cycle in a hot engine space. In most FLA setups, the internal resistance of the battery comes up quickly to throttle down the alternator and it doesn't run hard enough for long enough to get into that overheated situation. LFP banks are often much larger and have much lower IR so the alternator runs at max for too long and looses the battle with heat.
The best setup is one that monitors the alternator's stator temperature and reduces or cuts off the field current if a set temperature is exceeded.

 

[LeslieTroyer]

And to you care to elaborate on that, explain why you think he's correct, or is this just a popularity contest?

Chill JVISS. I say that because what I read and understand indicates that LiFePO4 batteries can accept high current for much longer periods than lead acid. Alternators are rated for lead acid, where the put out full amps for a short (<30minutes) period. And no I don’t see duty cycle on the label. Feel free to disregard and see how long your battery lasts.

 

[jviss]

Actually, it is North American nomenclatural laziness that even calls it a 125A alternator because what they put out is Watts not amps just like any other charge source.

Alternators have been rated in current output for over a hundred years. It makes little difference what the output voltage may be, it's the current through the stator that makes the heat.

As the voltage goes lower, the amps go higher so a very discharged battery will take more than 125A but the alternator can still put out a number of watts at a given rpm's with full field current.

Are you saying the alternator is really rated in Watts, not Amps?

LFP banks are often much larger and have much lower IR so the alternator runs at max for too long and looses the battle with heat.

A continuous rating at a particular ambient temp and airflow are just that, and the alternator so rated should run like that forever.

 

[jviss]

Alternators are rated for lead acid

That is utter nonsense. Show me such a spec, please, if you insist on continuing to assert this.

 

[jviss]

A 125A alternator, in its specified environment (mostly thermal) will put out 125A continuously. Unless there's a duty cycle caveat attached to the spec - which I have never encountered.