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Old 05-14-2022, 05:12 PM   #1
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Simple Alternator Question - if there is such a thing.

Should a simple internally regulated alternator put out close to it's rated amperage into a depleted battery bank regardless of the battery bank size?

Example: Assume a 200 amp-hr or a 800 amp-hr battery bank are both depleted 50%. Will a 90 amp internally regulated alternator put close to 90 amps into either bank until its voltage regulator starts tapering off as it nears some threshold of full?

Pretty sure I know the answer. It's the next question I have a problem with.
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Old 05-14-2022, 05:56 PM   #2
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Not usually. Internally regulated alternators are usually set to about 14 volts. At high current draws the voltage drop in the wiring takes it down below that, maybe to 13.5.

That isn’t enough voltage at the batteries to push significant current.

But try it and see. You will need a DC clamp on ammeter to measure the current.

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Old 05-14-2022, 06:45 PM   #3
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Thanks David. I've measured the amps going into my 220 amp-hr bank at 60 amps for over an hr. My real question is, what happens when you exceed the C rating for the battery bank? I've tried 2 different alternators and each did the same thing at 1000 rpm - probably more with more rpm. I've cooked a couple of FLA banks and am thinking the relatively small bank can't handle that many charging amps. But then, I would think every boat with a small bank and every auto would have the same problem with higher output alternators .
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Old 05-14-2022, 07:20 PM   #4
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Thanks David. I've measured the amps going into my 220 amp-hr bank at 60 amps for over an hr. My real question is, what happens when you exceed the C rating for the battery bank? I've tried 2 different alternators and each did the same thing at 1000 rpm - probably more with more rpm. I've cooked a couple of FLA banks and am thinking the relatively small bank can't handle that many charging amps. But then, I would think every boat with a small bank and every auto would have the same problem with higher output alternators .

What voltage are you seeing? If you push too many amps into too small a bank it pushes the voltage up faster than for a larger bank or lower amperage. When the voltage reaches the alternators regulation voltage, it will hold the voltage there and reduce amps as needed to not exceed the target charge voltage.
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Old 05-14-2022, 08:51 PM   #5
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Both my alternators that feed a house bank put out well in excess of 14 volts measured at the batteries.

Never cheked for how long but I know it has been for hours on a 675Ah bank and a 90A alternator. Also fhe same for my 220 amp RV alternator and 225 Ah bank. That one does ckme up faster so the time I would guess is shorter. But it doesnt cook tbe bank
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Old 05-14-2022, 10:02 PM   #6
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I measured voltage across the batteries every 10 minutes along with amperage going into the bank. 60 amps went into batteries until the last 4 minutes. Started at 12.63v and ended at 13.4v. At 36 minutes it measured 13.3v and started to drop amperage. By 40 minutes it was down to 53 amps. AGM batteries felt cool to the touch.

My concern is that I'm exceeding the C rating for the batteries and this is at the dock at 1000 rpm. While running I'd expect even higher amperage making it worse. Turning on the battery charger adds even more amps.
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Old 05-15-2022, 07:23 AM   #7
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That is more amps than I would expect. What is your alternator’s rating.

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Old 05-15-2022, 09:18 AM   #8
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Lemme, at rest, fully charged Amps, CCA and now C.
I am so confused.
I have 4 4D/200 amp (3 for the house) batteries, alternator... without going to the ER, about 140amps
But then we confuse everything by adding the 2 solar panels.
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Old 05-15-2022, 09:35 AM   #9
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your voltage readings are interesting. never got over 14 volts? 13.4 is a good float voltage for agms, but i would expect an internally regulated alternator to run up to 14-plus volts. are there some other high loads on the system while it's operating?
i think i'd be looking at external regulation so you could run a better charge profile.
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Old 05-15-2022, 10:28 AM   #10
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That sounds typical of automotive alternator regulators to me. Many of them are set for 13.4V max, to keep from boiling a flooded battery on a 10 hour drive. Also, many of them will derate significantly when warm. Even a high rated expensive marine alternator will derate to perhaps 60 - 70% of its rating when hot (or else burn up early). 60 A into a 220 AH AGM bank (assuming good quality batteries in good condition) is not too much - they should be able to take 100 or even more when down 50%.

At 13.4V, it will charge the AGMs fully, but it will take a very long time (like 8-10 hours). If you have AGM and want them to last, you really need external regulation set up specifically for an AGM charge profile.
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Old 05-15-2022, 11:21 AM   #11
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Here is one Dara point:

I had an 80A Oem Hitachi intrernally regulated alternator tied to 440 Ahs of FLA batteries. After a night on the hook, the batteries were down to 75% and when I started the engine the current was 30-40 amps , but it dropped to about 20A aftert five minutes and slowly dropped to 10Aover several hours of engine running at 1500 rpm.

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Old 05-15-2022, 03:09 PM   #12
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This will help explain it

h https://marinehowto.com/automotive-alternators-vs-deep-cycle-batteries/

ht
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Old 05-15-2022, 04:01 PM   #13
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DavidM - these are 90 amp Leece Neville alternators found on lots of 3208 Cats. I had nothing else running on the boat and the inverter/charger shut down.

DDM - Where did you get the info that putting over 100 amps into a 220 amp bank is OK. Everything I've read says to keep within the C rating of the batteries which for most AGMs is .2 X the amp-hrs. FLAs are .1C. I think you are right however, because just about every boat out there without an external regulator would be destroying it's batteries if you are not correct.

My problem, if I really have one, is not undercharging my batteries, but exceeding the C rating. When I had FLAs, I boiled them to the point where I almost caught them on fire. I suspect I boiled them due to putting too many amps in at once. If I run my charger at the same time, I'm putting 90 amps in and that's with it turned down to 30 amps and at the dock at 1000 rpms. At 2400 I'd be higher than that.

CMS, thanks for the marine how to articles. I've studied them in the past until I'm in information overload. But everything they say doesn't seem to address exceeding the battery C rating. The only thing I can find is that it's not good for battery life. And I would think that significantly exceeding the C rating for more than 10 minutes or so would overheat the battery. If the battery can't accept the amps, they have to go somewhere and I would think that would be heat which would explain why I boiled the FLAs.
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Old 05-15-2022, 05:14 PM   #14
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Good quality AGMs do not need to be current regulated. From the Lifeline AGM technical manual for exampl:

"The charging current during the Bulk stage should be set as high as practical; higher current levels mean faster recharge time. For repetitive deep cycling, chargers should have an output current of at least 0.2C (20 Amps for a 100 Ah battery). If the output current is less than this value, the cycle life of the battery may be negatively affected. Due to the low impedance design, LifelineŽ batteries can tolerate in-rush current levels as high as 5C (500A for a 100Ah battery)."

All the Lifeline AGM batteries I have run would take at least 0.5C and more when at 50% SOC. In fact they are more concerned with charging them too slowly. Usually boiling FLAs is due to either too high charge voltage or too high float voltage. Most lead acid batteries work pretty well with a voltage regulated charge: the voltage regulation automatically controls the current due to the chemistry.
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Old 05-15-2022, 07:21 PM   #15
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DDW, when you speak of AGm, Lifeline AGm or LA, are you talking about individual batteries or the battery bank?

I am trying to learn more about different type of batteries and charging systems.
I am not about to change the chemistry of my AGm batteries nor my alternator (with an internal current regulator). Just trying to learn.

How does one determine the SOC of AGm batteries? I do know, one can determine the SOC of a LA battery with a hydrometer but how does one determine the SOC for AGm batteries. Then there is my new introduction to the term "C".

The more I learn, the better questions I can ask.
Thank you for yours and others knowledge and understanding.
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Old 05-15-2022, 11:24 PM   #16
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Generally I'm talking about a battery bank. You can determine the *approximate* SOC of an AGM by disconnecting it from any charge/load for a couple of hours, then measure the "resting" voltage. This is not usually a practical method. So the best way is with a properly set up charge counting meter such as the Victron, Link, etc. These work by measuring the current going in or out and adding it up over time. This works well except that small errors can accumulate so that in many days use it can drift off accuracy a small amount. Normally you will fully charge the bank once a week or sooner, and when fully charged it resets any errors to null.

"C" is of course the capacity of the bank in AH. It makes sense to normalize all the math to "C", because then you can generalize for different size installations, so 0.5 X C is 50A for a 100AH bank and 250A for a 500AH bank.
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Old 05-16-2022, 05:33 AM   #17
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A added a Balmar SmartGauge ( earlier version w/o the shunt) but have been pleased with operation and info provided.
RC of CMS and MarineHowTo.com did a thorough article on the Balmar unit when it came out and he was impressed with its capability.
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Old 05-16-2022, 06:23 AM   #18
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Generally I'm talking about a battery bank. You can determine the *approximate* SOC of an AGM by disconnecting it from any charge/load for a couple of hours, then measure the "resting" voltage. This is not usually a practical method. So the best way is with a properly set up charge counting meter such as the Victron, Link, etc. These work by measuring the current going in or out and adding it up over time. This works well except that small errors can accumulate so that in many days use it can drift off accuracy a small amount. Normally you will fully charge the bank once a week or sooner, and when fully charged it resets any errors to null.

"C" is of course the capacity of the bank in AH. It makes sense to normalize all the math to "C", because then you can generalize for different size installations, so 0.5 X C is 50A for a 100AH bank and 250A for a 500AH bank.
I have a Victron displaya. Normally, the battery charger is on and the display is IAF (infinity). Last night, 8pm, I shut off the separate charger (40amp Xantex). I had the initial reading of 13.2vt and 240hrs. About 7am, I had readings of 12.4vt and 24hrs. We know the 240hrs is before everything settles in. The only identifiable loads on the house batteries are the 12vt fridge, a small inverter to power the TV, head pump and maybe cycling on and off one LED in the head. Historically, I have performed this before and although I did not write down the readings, from memory they seem to be about normal.
I have 3X200 amp batteries for the house bank. Soooo, where do I go from here. (it's a 30amp boat) Guess: 0.5X 600amps house bank = 300 amps for a value for C?? How does that differ from the 'old fashion' method of, 'taking the total amps of the battery bank and divide by half for usable amps'? I can only guess the formula is to formalize the method to determine a value of C. I know, I have to and do perform this test at night to avoid the influence of the solar panels. (too lazy to crawl down there and open the breaker)

The inverter is a separate Xantex 1800 watt. I normally keep that shut off when not in use.
If I use the 1200 watt microwave on the inverter, it will suck the voltage down to about 12.4vt in less than a minute but, recovers quickly when the micro wave cycles off (with the 40amp battery charger ON). LOL guess I could call that a battery load test. I guess to get a true battery health measurement, do a closer to true battery load test, dark of night (to avoid the influence of the 2X130 watt solar panels) battery charger off, inverter on and cycle the microwave, determining how long to get to 12.4vts...... then to see the the time to recover to approx 13vts. Oh what a fun project for a dark and stormy night. LOL Of course, to remove the influence of the solar panels, I could just open the breaker too.
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Old 05-16-2022, 08:02 AM   #19
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DDM - Thanks for the info. Sounds like my understanding of a battery charge C rating is backwards. I thought the 0.2C meant the maximum amperage you should charge. The Lifeline info you reference indicates that it is the minimum - at least for their AGMs. I would think other manufactured AGMs would follow the same guidance. Did I interpret that correctly?
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Old 05-16-2022, 12:14 PM   #20
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DDM - Thanks for the info. Sounds like my understanding of a battery charge C rating is backwards. I thought the 0.2C meant the maximum amperage you should charge. The Lifeline info you reference indicates that it is the minimum - at least for their AGMs. I would think other manufactured AGMs would follow the same guidance. Did I interpret that correctly?

Within reason, the acceptance current of a lead/acid battery is self regulating.


With a lower initial current of say .2C, the voltage will rise slowly to the regulator set point, then hold there and current will slowly taper towards zero as the battery reaches full charge.


With a higher initial current of say 1C, the voltage will rise more quickly to the regulator set point, then hold there and current will again slowly taper as the battery reaches full charge.


It sounds like your max possible current will be well less than 1C, so I don't think you have any issues at all.


It seems that your alternator set point is 13.4V which is fine. It won't charge as quickly vs if controlled by an eternal multi-stage regulator, but it also wont overcharge on longer runs.


To me, the only question is why your 90A alternator is only putting out 60A. Possibilities are:


- You have other DC loads of 30A such that the alternator it putting out 90A, but you are only measuring 60A into the batteries. That would be correct operation and not a problem.


- At 1000 rpm, 60A is all the alternator can produce. That too would be normal operation, and could be confirmed by increasing rpm to see if output increases.


- As alternators get hot, their output declines. This too is normal, but I can't say whether 60A is the expected consequence. Also, starting from a cold alternator, I would expect the output to rise for a few minutes, then come down as the alternator heats up, and you didn't report anything like that.



- A more sophisticated internal regulator will monitor temperature and limit output accordingly. Like above, if this were happening I would expect to see an initial higher output as the alternator is heating up.


- Alternator ratings are best-case numbers. Running cool, running at full rpm, and running for an extended period of time. All those things are pretty much never true at the same time.


Bottom line, I think 60A is just fine, and you don't have any problem.
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