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Old 05-02-2016, 11:06   #1
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Alternator sizing - time for each stage

Does anyone know of a way to figure out roughly the charge time for a battery?

I'm looking at two different alternators and trying to determine if the larger one is worth the extra $$ (I suspect not).

Example:
6-T105's wired as 12V = 675Ahr
Alternator 1: 165A
Alternator 2: 120A
Case A: Battery at 50%
Case B: Battery at 70%
No load on the battery, just charging.

How long will it take in each scenario (1A,1B,2A,2B) to go through Bulk, Absorption (assume to 95%) and Float to 100%.

Assumption (which is not correct, but to simplify) that the Alternators are able to output their full rated output.

If it makes a different I'll be using a Balmer MC-614.

thanks,
Geoff.
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Old 05-02-2016, 11:15   #2
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Re: Alternator sizing - time for each stage

I don't have that answer and am not even sure it is answerable, temp for one thing is a variable.
I suspect the only time difference will be the bulk phase and possibly beginning part of the absorption, which is not of course the longest time.
I'd guess the difference will be less than 30 min between the two.

One thing is, I don't think it's common to get full rated output from an alternator, but that is just an opinion of mine.


If you had said I have AGM batteries, then there may be a better case for the bigger alt?
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Old 05-02-2016, 11:33   #3
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Re: Alternator sizing - time for each stage

My experience is that most will back off from near full output in 15 mins or so. So you get maybe one quarter hour of the additional amps of the big alternator. Not huge…. Maybe 10?
However, you don’t say what the $ difference is, but the bigger output alternator may be more robust.
Either will require modern belting or a double belt
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Old 05-02-2016, 11:52   #4
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Re: Alternator sizing - time for each stage

Yeah, don't forget that serpentine belt kit, even my 80 amp ate belts, I can run my 140 on a serpentine belt that has a little slack in it with no slip.

I have a 660 amp AGM bank, and if the Balmar smart gauge can be believed average nightly draw is about 100 amp hours, and even with an AGM bank, the alt amperage is backing down in ten or fifteen minutes, but I get it seems about 110 amps out of my 140 amp alt. I installed an ammeter connected directly to the alt that monitors only the alternator. I had considered getting maybe a 180 or 200 amp myself and relegating the 140 to the spares bin, but I don't think it would make much difference really.
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Old 05-02-2016, 11:55   #5
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Re: Alternator sizing - time for each stage

With 3 different, ( Mando 50 amp, Zena 150 amp and Delco 10SI 105 amp ) alternators connected to an 800 AH bank, none of them backed off from full output, until the batteries reached 70 to 75 % charge.

Wired and regulated properly either one should output near it's rating.

Calculating how long it takes to charge a depleted bank has way to many variables. Engine speed, alternator temputure and even battery temp.

If money is an issue I would use the 120 amp alt with the temp sensor to the regulator. Keep in mind that with a low battery it may de-rate a few times and take a bit longer to charge.
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Old 05-02-2016, 11:56   #6
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Re: Alternator sizing - time for each stage

Quote:
Originally Posted by geoffr View Post
Does anyone know of a way to figure out roughly the charge time for a battery?

I'm looking at two different alternators and trying to determine if the larger one is worth the extra $$ (I suspect not).

Example:
6-T105's wired as 12V = 675Ahr
Alternator 1: 165A
Alternator 2: 120A
Case A: Battery at 50%
Case B: Battery at 70%
No load on the battery, just charging.

How long will it take in each scenario (1A,1B,2A,2B) to go through Bulk, Absorption (assume to 95%) and Float to 100%.

Assumption (which is not correct, but to simplify) that the Alternators are able to output their full rated output.

If it makes a different I'll be using a Balmer MC-614.

thanks,
Geoff.
Geoff,

The two alts are roughly .18C and .25C, (18% and 25% of Ah capacity) but this is at face value. The alts won't run there for very long before they heat up and put out less current or the flooded bank hits absorption voltage and they catch a break..

Below is a Trojan SCS-225 charged at .15C (15% of Ah capacity). This was a pretty healthy battery and it took approx 7 hours to reach 100% SOC. You will shorten the time to full just a bit by running a larger charge source but with flooded batteries not by much.

As you increase charge current you drop the point in the SOC curve where you hit absorption and you won't significantly shorten charge times to 100% SOC.

I recently did some testing on AGM batteries where the difference from 50% SOC to 100% SOC, when comparing a .2C charge rate and .4C charge rate, netted about a 12 minute difference to 100% SOC (5:30 minutes at .4C compared to 5:42 minutes at .2C). At shorter charging duration's the .4C rate certainly attained a higher SOC faster, in an AGM battery, but the time to full/100% SOC was not cut a hell of a lot shorter..





P.S. As the batteries age and sulfate the time to 100% only grows longer....
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Old 05-02-2016, 12:03   #7
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Re: Alternator sizing - time for each stage

a64pilot

Yes I've assume it's the bulk phase which will be different. I don't currently have AGM, but may in the future which I know will may change the answer.

As for the output - this is why I have the assumption of the rated output
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Old 05-02-2016, 12:08   #8
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Re: Alternator sizing - time for each stage

Maine Sail,

Thanks for the example from your experience.. this is confirming my thoughts that it's not much of a difference.
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Old 06-02-2016, 13:57   #9
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Re: Alternator sizing - time for each stage

In practice with flooded lead acids you will get around 80a max during bulk charge from each of these unless you run the engine at it's max revs which you won't do for charging. Your 'ideal charge rate is C10 so 65.7a. C5 is 130a which you could get from the larger alternator but you would need high revs and high charging voltages plus it only gets you to about 75/80% (the higher the charge rate the sooner it shuts down to absorbtion) so while it could be useful for a fast partial charge in terms of time to 100% it is not going to be much faster and could even be slower.
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Old 06-02-2016, 16:59   #10
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Re: Alternator sizing - time for each stage

Quote:
Originally Posted by roland stockham View Post
In practice with flooded lead acids you will get around 80a max during bulk charge from each of these unless you run the engine at it's max revs which you won't do for charging.
That is a very bold statement and is only true in some situations.

It depends on the engine (RPM range) , alternator (output curve and safe maximum operating speed) and pulley ratios.
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Old 06-02-2016, 17:21   #11
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Re: Alternator sizing - time for each stage

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Originally Posted by StuM View Post
That is a very bold statement and is only true in some situations.

It depends on the engine (RPM range) , alternator (output curve and safe maximum operating speed) and pulley ratios.
Correct. Some alternators will put out 80% of their max output at only a bit above idle speed, if they are correctly set up.

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Old 07-02-2016, 16:33   #12
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Re: Alternator sizing - time for each stage

Roland,Stu,Dockhead.

In my example I assume the alternator is putting out it's maximum as it's clear you cannot be sure given the different ratings (hot/cold) and RPMs so in some ways the question is theoretical and only somewhat reflects reality... but it will help once I get the prices
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