# Rate of charge for 70 amp alternator



## TSOJOURNER (Dec 16, 1999)

I just installed a 70 Amp alternator. My fridge draws abot 70 amps in a 24 hour period. I will be on a cruise with grandchildren and will be on the hook in 2 different locations for 2 day periods. During this time we will probably motor a couple of hours each day, if the winds are good, more if not.
This weekend I was daysailing and only used about 20 amps out of the housebank(300 amp capacity). The rate of charge when I motored at 2000 rpm was only 10-11 amps. I think I remember someone telling me that the more amps that I was down the higher the replacement rate would be from the alternator. If that is true What should I expect the charging amp rate to be when I motored and was down about 80-90 amps and motored at about 2000rpm?


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## Idiens (Jan 9, 2007)

Depends on what electronics are controlling your alternator. If its a simple one, as fitted to the alternator, it puts out about 14.4V and the battery takes what it wants, not much and slowly. So 10 to 15 amperes would be about right. If you have a complex controller, it can make use of the amps that the alternator is rated at and pump in much more.


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## camaraderie (May 22, 2002)

A 300 amp house bank will take a maximum bulk charge rate of about 60 amps and ramp down from there. At anything close to 80% full or more...the batts will ramp down the charging rapidly till you are finally at just an amp or two for the last 10% or so and at 13.3V or so.

When you are down 150amps or so...then you will see closer to full output from the alternator WHEN the alternator rpm rate (of the alternator, not the engine which may be different due to sheave size) is per the full output spec. The simple way to find this RPM is to ramp up engine RPM while keeping an eye on alternator output to a 50% or more discharged battery.


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## chucklesR (Sep 17, 2007)

Dgmias,
For the full long version answer to your question you'd have to refer to Nigel Calder or one the authors who really work the issue.
What Cam's alluded to is that if you are down your projected 80-90 amps (and assuming you have a 300 amp bank) your alternator will pump 60 amps in the first hour, about 10 the next, then 5, then 2.5, then 1 etc..
To absolutely top off you'd need 4 or more hours. 
The good news is if you have a 300 amp bank you don't need to top off at 100%.
Battery recharging is more about percentages that ah, recharging is faster the lower the percentage. If on the second day you motor 3 hours to get back to 90%, on day three you'll motor maybe 3 hours and 10 minutes to get right back there again.


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## hellosailor (Apr 11, 2006)

If you have an internal regulator in your alternator--that's no good for boats, they are designed to "not overcharge". An external regulator is designed to "charge quickly and fully". They are different pieces of equipment, the internal one is cheap, the external one costs more and does more.

To amplify what Cam is talking about: Alternators aren't always matched to engines, either. Let's say that your engine idles at 500rpm but you cruise at 2500 rpm with a "battle speed" of 3800rpm. There's a pulley on the engine that you aren't going to change, because it costs a lot. There's another pulley on the alternator. The size ratio between the two of these, determines how fast the alternator will turn at any given engine speed.

So, if the engine pulley is 15" diameter, and the alternator pulley is 5" in diameter, the alternator turns (15/5) 3x faster than the engine does. At engine idle speed of 500 rpm, the alternator turns 1500 rpm. At that speed most alternators put out next to nothing. At 2500 engine rpm, the alternator is turning at 7500rpm, typically a good spot for nearly full output. But, ooops, at 3800 engine rpm the alternator would be running at 11,400rpm which would burn out most alternators after a short while.

So, the maker typically will install an alternator pulley which won't let the alternator burn out--and reduces power at idle and cruising speeds even more!

The solution to this is, first, to get an alternator with a wide RPM range. Some can put out full power at 10,000rpm (alternator rpm) with a working range of 2500-15000rpm or higher. Cheaper alternators usually have less effective range.

Then, second, once you find out the specs on your alternator, consider having a custom pulley made up in a machine shop if the one you have does not properly match the speeds. Boat builders "buy stock" because it is cheap. A custom pulley will run you about $100 from a machine shop, but it can double the output from your alternator if what you have wasn't properly sized to begin with. A VERY effective fix, if that's all it needs. But you've got to run the numbers to find out where the problem really is: pulley, alternator, regulator. They all have to be matched to each other and the battery bank size, to work well with each other.


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## PBzeer (Nov 11, 2002)

I have a 70 amp Balmar Hi-Output alternator that I run through an external regulator, to a 315 ah house bank of AGMs. I get around 45 amps when they are under 25% discharged, dropping to 20 after the first hour. (and this is on a 13hp engine)


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## jgaddis (Sep 25, 2000)

Can I attach an external regulator to the 55amp alternator on my 16hp yanmar?


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## camaraderie (May 22, 2002)

The short answer jgaddis...is no...not without taking your alternator off and to a shop to have rewiring done and then it is a maybe.


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## Idiens (Jan 9, 2007)

Actually, Cam, the answer is yes - if you get a regulator intended for that purpose. Sterling make one, which I use, it works very well.

Sterling Power Products: Alternator to Battery Charger


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## camaraderie (May 22, 2002)

Idiens...thanks for that link...a most interesting product development. Nevertheless...at $577 bucks + shipping for the cheapest one...I don't think there is gonna be a rush of people with 55amp OEM alternators to buy one! 
Why did you buy yours? It seems as.... if you were starting from scratch that it might make some sense given the claimed additional charging efficiency. What exactly do you have hooked up on your engine charging system?


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## Idiens (Jan 9, 2007)

Hi Cam. I was junking an old Volvo with conventional alternator regulator and diode bridge for dual bank charging for a new Yanmar. It came with an 80 amp alternator, but again with conventional regulator. To get it changed, to one that could be externally regulated, they wanted the engine back with no promise of a return date! 

I had just read about the Sterling alternator/charger and since I had no other 3-stage charger electronics, the Sterling unit solved the problem for me. It works very well, but with wet acid batteries, it comes with the warning to check water levels regularly, so I got a water bottle with a clever self regulating spout.

I did have 5 x 110 Ah wet cells to charge, but one just died, so I am down to 440 Ah.


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## camaraderie (May 22, 2002)

Idiens...thanks...glad to know that option exists. One thing on the site bothered me...the writer said it goes at 14.1V for AGM's and 14.4 for gels...hope that was a typo or some VRLA battery owners are gonna have some problems!


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## Idiens (Jan 9, 2007)

camaraderie said:


> ...the writer said it goes at 14.1V for AGM's and 14.4 for gels...hope that was a typo or some VRLA battery owners are gonna have some problems!


It's output can be set to at least three levels. In the handbook, it notes that AGMs are treated differently in the USA from in Europe, so it offers a choice of two AGM voltages. It is very scathing about gels and seems to favour belting volts into wet cells and providing lots of water (which it points out is a cheap commodity). The older version of the handbook was critical of AGMs too, but seems to have mellowed in the later versions.


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## hellosailor (Apr 11, 2006)

"To get it changed, to one that could be externally regulated, they wanted the engine back with no promise of a return date! "
That is SO ridiculous. To replace an alternator, any alternator, one does not need the engine, one only needs the old alternator to ascertain which mounting configuration it is, what pulley size it is, and what harness connections will be needed.

Having the engine only means that you can see if there is room for the new alternator--but that's no advantage since it doesn't tell you if the new alternator can FIT in the engine room, either. A quick question and a conservative choice of size makes that a non-issue. And, a hell of a lot less shipping expense.

Sterling Power Products: Alternator Regulators

If that's the regulator you mean, certainly looks interesting. Maybe a bit overkill (just kimmed the page) but certainly interesting, and friendlier than Balmar's screwdriver tricks.


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## Idiens (Jan 9, 2007)

HS - I thought it ridiculous too, but Yanmar's support in Benelux is not good, spares take an age to order and receive.

The alternator regulator your refer to is the one I wanted to use, but as the alternator had to remain unchanged, I got this alternator to charger unit.

*Sterling Power Products: Alternator to Battery Charger

*Sterling's response to emailed questions was fast but gruff. I guess my questions were too trivial. - They stemmed from differences between the supplied paper handbook and the downloadable version. For my purpose of belting current into the battery bank during the short period of engine running between dock and seaway and back, it works very well.


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## Rockter (Sep 11, 2006)

I wonder when they say "70 amp", do they mean max intermittent, or max continuous. There is a world of difference between those two specs.

I can speak highly of these alternators...

Ample Power Alternators

... and note the mention of continuous running.

Mine, the 100 A (nominal) machine, has been there for 16 years. From the spec, they seem able to tolerate the most frightening air temperatures.... like 93 degC, so a tropical 50 degC should be ok.

On my ship, the belt gearing is such that it spins at about 9000 rpm at motor 1800 rpm, so cooling isn't too bad.

I have no formal interest in Ample Power.


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## knuterikt (Aug 7, 2006)

Hi

One thing I learned recently was that the amps you can push into a battery also depends on the internal resistance in the battery. I replaced my batteries and noted significant increase on my ampere meter even when the engine was idling.

Knut


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## Rockter (Sep 11, 2006)

A discharged battery has a lower internal resistance than one that is better charged. At constant voltage, it will take more current.


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## hellosailor (Apr 11, 2006)

"frightening air temperatures.... like 93 degC,"

93*C* would indeed be a frightening air temperature to me. Do you get a souvenir "I"ve gone sailing in hell!" T-shirt with that trip?[g]


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## sailingdog (Mar 19, 2006)

Hmm... were you sailing in the caldera bowl of an active volcano??? Air temps of 93˚ C is about 200˚ F... and a bit warm for most places on earth.


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