Rate of charge for 70 amp alternator

[dmgias]

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?

[Idiens]

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.

[camaraderie]

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.

[chucklesR]

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.

[hellosailor]

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.

[PBzeer]

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)

[jgaddis]

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

[camaraderie]

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.

[Idiens]

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

[camaraderie]

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?