Why will my alternator burn out, it never did before

[Cheechako]

Alternators generally get too hot after 15-20 minutes of full charging. IME a smart regulated 100 amp alternator puts out 80-85 amps for 15 minutes or so initially then gets too hot and backs off fast.
Of course some of this is due to battery type and how well it will accept the high rate of charge also.

[Fuss]

Originally Posted by jordanbigel

Sigh.

"Do you think that the alternator only charges batteries? That’s how everyone seems to think."

No. No one thinks this.

And no, it's not the same thing. Even an electric oven is not going to pull the amps from the alternator that a (large) LifePo4 bank will. An electric oven cycles on and off, it draws power, heats up, reaches target temp, then the oven cycles on and off over the cooking period to keep the temp at (close to) the target. Did you observe this with an ammeter while it was running?

By contrast, a large LifePo4 bank (mine is 1200ah) if it is down 50% will draw 120amps from my large frame alternator for 6 hours straight. 100amps go to the bank, the other 20 goes to feeding loads connected to the house bank. Six hours. Have you run your alt at full capacity for 6 hours straight ever? Even a turkey will cook in 2 or 3 hours, not 6, and like I said, an electric oven does not draw the full amount of amps continuously, and during the off cycles when the oven is not drawing full power, the alternator gets to cool down.

Not. The. Same. Thing.

Really, so a 600ah 24v bank at 50% and cooking a chicken for an hour is not going to used the full capacity of a 150ah alternator.
Do you really want to make this statement….

[jordanbigel]

Originally Posted by Fuss

Really, so a 600ah 24v bank at 50% and cooking a chicken for an hour is not going to used the full capacity of a 150ah alternator.
Do you really want to make this statement….

Not only do I really want to make that statement - I DID make that statement, and I will go further. You are not listening.

Before I had my 1200ah 12v LifePo4 system, I had a set of 4 8D 12v Gel cells with a total capacity of 1000ah, very similar to your 600ah 24v bank.

I know from experience, with my large frame alternator, that bank at 50% capacity would complete bulk charging in less than an hour and then switch to Absorption phase. At that point, the batteries resistance throttles down the acceptance rate dramatically while the bank continues in absorption phase for several more hours. During that time, the alternator was barely breaking a sweat, as yours would be barely breaking a sweat in absorption phase.

Add to that again what I said about the electric stove cooking a chicken, for an hour, and cycling on and off during that hour, it is NOT EVEN CLOSE to what my LifePo4 bank draws. Not even close.

Anyone else want to debunk what I said other than Fuss? Anyone?

[Fuss]

Oh, I’m excluded… ok, then let’s remove the complication of the chicken, I don’t really like chicken anyway…so really, at 600ah 24v battery at 50% is not going to use the full capacity of a 150ah alternator.
Do you really want to make that statement.

[Lepke]

Keep an eye on the alternator bearings. If its run hot many times the bearing lube will bake off and leave the bearings dry. It's not hard to lube or change the bearings, but you have to open up the alt to do it.

[sailingharry]

Yup! I have 270A between my two alternators on my 740Ah AGM bank. If down near/below 50%, I start off at about 200A, maybe a bit more (of course, there's 20A of loads that I can't suss out of the charge info). 20-30 minutes later (I won't disagree if you say it's 15.... LOL) it has stabilized at about 140 or so (ie, 50% of rated capacity), and it will do that for at least an hour or two.

And when I convert to LFP (this winter? Maybe next) I expect to see the exact same charge profile for the first two hours.

[Statistical]

Originally Posted by Fuss

Oh, I’m excluded… ok, then let’s remove the complication of the chicken, I don’t really like chicken anyway…so really, at 600ah 24v battery at 50% is not going to use the full capacity of a 150ah alternator.
Do you really want to make that statement.

It will use the full capacity of the alternator initially but not for hours (plural). As SoC rises charge acceptance will decline. However since you can't be convinced run a small frame "dumb" internally regulated alternator for hours at peak output charging a large LFP pack. We curiously await your results.

[Fuss]

True, not for hours but at 50% for at least 30-45 mins…oh dear, here comes the chicken again…and with this extra cooking, for 1.5hours at 150ah round about.

[Statistical]

Originally Posted by Fuss

True, not for hours but at 50% for at least 30-45 mins…oh dear, here comes the chicken again…and with this extra cooking, for 1.5hours at 150ah round about.

Hours is the point. An LFP pack could be 80% discharged and be quite substantial (600 Ah+) so a 150A alternator would need to run 3+ hours to recharge it and trying to do that you will cook it. The exception would be if it is a smart externally regulated alternator and the controller artificially reduces output to less than what the LFP pack can pull.

Even a single hour at full output would be very hard on a small frame alternator and while you may think that you do that frequently with lead acid you probably don't especially if you have solar onboard as well. Multiple hours means a very quick death.


If you are enough to wonder why it would be different. It is internal resistance. Internal resistance of LFP is about 1/20th to 1/100th that of lead acid batteries. Current flow depends on the difference in voltage (battery voltage vs alternator output) and resistance. The high internal resistance of lead acid battery means current flow will slow quite rapidly as the voltage increases. LFP has both a flat voltage profile and very low internal resistance it will soak up everything the alternator can give potentially for 4+ hours and the alternator will croak trying to accomplish that.

[two-rocks]

Fuss: get some lifepo4, install, cook chicken,, charge.

You can follow up here on what went wrong, while eating crow.

[Fuss]

Originally Posted by Statistical

Hours is the point. An LFP pack could be 80% discharged and be quite substantial (600 Ah+) so a 150A alternator would need to run 3+ hours to recharge it and trying to do that you will cook it (unless it is a smart externally regulated one which artificially reduces output to less than what the LFP pack can pull).

Even a single hour at full output would be very hard on a small frame alternator and while you may think that you do that frequently with lead acid you probably don't especially if you have solar onboard as well. Multiple hours means a very quick death.

Hey, look at this, we are now talking hours…we are already onto a win here, the posts are working, earlier on it was videos of alternators burn smelling after 5 mins, and now we are up to whole hours.
Not sure about the chicken convincing anyone but …sometimes getting the point across is not easy.
Anyone not agreeing with statistical here with his hours, maybe you need yourself a more robust alternator.

[Statistical]

Originally Posted by Fuss

Hey, look at this, we are now talking hours…we are already onto a win here, the posts are working, earlier on it was videos of alternators burn smelling after 5 mins, and now we are up to whole hours.
Not sure about the chicken convincing anyone but …sometimes getting the point across is not easy.
Anyone not agreeing with statistical here with his hours, maybe you need yourself a more robust alternator.

I am not saying 100% of alternators will survive even an hour. Just that LFP recharge could soak up the full output of an altnernator for 4-6 hours while motoring and that will be a death blow to any "dumb" alternator. You are going to need a "smart" regulator which can backoff the output or you will cook it in short order.

They can die even sooner that that. Your belief that you get the full output of your alternator for even an hour is unlikely unless deeply discharged, no solar, and the alternator is relatively small compared to pack size.

MainSail used a bench powersupply to provide 0.4C charging to an AGM lead acid battery with 50% SoC. Within an hour it was down to 0.2C. The supply didn't get weaker the charge acceptance of the battery was cut in half in an hour. Pack voltage rises pretty linearly in lead acid batteries. That combined with internal resistance means at higher SoC the lead acid battery simply can't pull that high of a current.

https://marinehowto.com/how-fast-can...ry-be-charged/

[sailorboy1]

The alternator doesn't care what it is supplying. It only cares how much and how long. Why is there so much "discussion" on this? If Fuss says his alternator can supply all his batteries can accept and at same time cook a chicken so what? If he is correct great for him, if not oh well.

Meanwhile you can find lots of stories of burnt up alternators after someone changed to lithium so that is a fact that happens to some. You ever chose to believe that as possible or you decide to ignore it.

[sailingharry]

Originally Posted by Statistical

mainsail used a bench powersupply to provide 0.4C charging to an AGM lead acid battery with 50% SoC. Within an hour it was down to 0.2C. The supply didn't get weaker the charge acceptance of the battery was cut in half in an hour.

Mainsail does incredibly good work and gives us a lot of really useful data. I didn't much like this particular episode of his though.

Starting at .4C as a charge rate was exceptionally unrealistic. It proved his point that large alternators don't gain you much, but forgot the asterisk that says only if you're starting with more than most.

Consider a modest house bank of 400ah. Mine is 740. 0.4C would be 160 amps. An alternator that could produce a real 160 amps for any significant period of time would be more than most people could mount and drive.

Now consider a common AGM cruising battery of 600ah, and an externally regulated 120a alternator that will start off at 100a and drop to 80a within 15 minutes (so basically starting at .16c and dropping to .13 C, or less than what he was providing after an hour into the test). If that battery started at 50% soc, it will take at least an hour, probably 2 hours before you hit acceptance voltage. Not 1/2 an hour as he suggests.

And my quarrel with his argument here is that upgrading from a 60a alternator to a 120a alternator, which for most people would be a significant and expensive upgrade, you will likely cut 30% or more off of your charge time -- and if you only have half an hour of engine run time, you will put twice as many ah in the bank.

[rgleason]

Fla, Agm, Gel Cell, and LFP are all different. It is difficult to make a generic statement about how alternators will/should work with different batteries, except perhaps that generally output will be less than expected.