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Old 09-04-2010, 18:21   #1
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Is Alternator at 17% of Battery Capacity OK ?

Hi,
I need to replace my house bank and starter battery. I am limited to a 70amp rated alternator. I know the rule of thumb is to size it at 25% of capacity, but was wondering if it is not advisable to put in greater overall capacity even if you can't get a bigger alternator?

I considered putting in four 100ah AGMs, instead of 70ah. Hoping that most of the time I'd be discharging them from the 75% to 50% range. I have a 3 step regulator, and when motoring the batteries would get up to a full charge.

Also, I have three house batteries, and one starter. I understand that if I'm charging them in parallel I should use all the same type and rating, so I was assuming I'd get the four AGMs described above. However, I will mostly be discharging from the three house bank batteries while leaving the engine one off while at anchor. Does this cause a problem for charging the system - when individual batteries will different levels of charge when charging starts?

Thanks in advance, despite reading my Nigel Calder and Don Casey books I'm still confused about this point.
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Old 09-04-2010, 18:53   #2
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Should be no problem. I would recommend something like the "echo charger" to keep your start battery charged. Alternator should be no problem if you do enough powering to keep the batteries charged.
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Old 09-04-2010, 18:54   #3
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As far as the bigger batteries go, its OK to use them--if you take out the same number of amp-hours, the bigger bank will last longer, and take about the same time to recharge. If you go weekend sailing from a marina, the bigger bank will give you more capacity, and you can always recharge it from shore power.

To the second point, if you use a battery combiner to only connect the starting and house banks when the voltage rises above 13v, there will be no problem--if they are all connected to the same voltage source, the deeply discharged batteries will take most of the charging amps because their internal voltage is lower.
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Old 09-04-2010, 20:15   #4
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Bigger banks are generally better because:

1. you'll be using less of their capacity than you would with a smaller bank, therefore increasing the available amp-hours and extending their life;

2. you'll be able to replenish them faster IF you have the charging capacity onboard, thereby reducing costs associated with charging (engine time and maintenance costs, etc.); and

3. you'll have considerable reserve capacity in case you want to or need to go longer between charges.

Note that AGMs can absorb a very large charging current, often exceeding their amp-hour rating. A 400AH AGM bank could easily absorb a 400-amp charging current or more. If you're not careful, this could place an unsustainable load on your alternator and burn it out. To avoid this, you need to somehow: (1) limit the output current; and/or (2) reduce alternator heating. There are several strategies to do this, both mechanical (e.g., regulating pulley size) and electrical (e.g., using smart regulators like the Balmar MC-612 to de-rate the alternator, or using external rectifiers).

With an AGM bank the size you contemplate and a 70-amp alternator, I think I'd want to find a way to de-rate it by about 20% in order to avoid overheating and burnout. Temperature sensors on the alternator and batteries would be highly desirable as well.

If you were designing the system from new, you'd want a much larger alternator...perhaps 4 to 5 times the capacity.

Bottom line: the 70A alternator will do, but charging times will be longer than would be ideal, especially since you'll want to derate the alternator somewhat to avoid burnout.

Bill
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Old 09-04-2010, 20:26   #5
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Start battery

Keep in mind that when sizing a start battery one pays no attention to any Amp-hour rating. Pay attention to the CCA rating and size the battery to give a CCA (cold-cranking-Amp) rating to equal or exceed the stall current rating of the starter motor.

The other essential specification is the internal resistance of the battery; the lower the better. The Amp-hour rating is essentially irrelevant.
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Old 10-04-2010, 01:39   #6
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The 17% size just means that you will be slow charging costing you in fuel and engine wear. You've got a 3 step regulator, good, which I assume can be reset for AGM's.

My question is why switch to AGM's, there's a price and longevity cost to them for deep cycle use compared to flooded batteries? Because they are sealed they are considered maintenance-free although really that should be maintenance-impossible. Over time they will out-gas some moisture or the constituant gasses shortening the total life of the battery. Flooded batteries give you the ability (and the resposibility) of topping up the cells regularly with water, and the ability to check cells with a hydrometer so you have advanced warning of problems coming and a better idea of when to equalize the batteries. The only advantages for AGM's I can see are faster charging (which does nothing for you with the undersized alternator) and a slower self-discharge rate so you don't need to come down to the boat as often.

Most of the sources I have been able to find discussing the relative merits of flooded, AGM and Gel seem to have a vested interest in 1 or 2 of the alternatives. The only source I have been able to find that seems to be disinterested and to have actively tried all the alternatives and discusses the final choice at length is Evans Starzinger, Beth Leonard's husband, whose comments can be read at

Systems.
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Old 10-04-2010, 10:55   #7
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Thanks for all the replies and information. I guess I'll go with the bigger batteries, and make sure I have enough CCA for starting.

As it happens I do have an echo charger in addition to the 3 step regulator. TBH I didn't know what this was for. The boat was re-powered and the old charging system is not hooked up currently. It has a stock alternator that is hooked right to the battery bus. I need to get it converted to external regulation and to use the 3 stage regulator. I hope this regulator will prevent overloading of my alternator.



I was assuming I'd go with AGMs partly because the 3 house batteries are located in the bilge (less than ideal I guess) but they're a bit difficult to access so the lower-maintenance option seemed appealing. The starter battery is in the cockpit locker.

I have 100w solar, and I may add wind-gen as well. So hopefully that will reduce engine time a bit. My fridge is also teeny, which may help keeping the draw low

I'm still not sure if I'll get a 'marine' alternator. My initial plan is to convert what I have to external regulation and run with that for a while. If I find the need to get a marinized alternator I'll get one and keep the current one as a spare. I am planning to cruise, so it will be getting heavy usage.

Thanks again, I'm leaving my job and getting ready to cruise. It is very reassuring to know there's wiser/saltier people here who are always willing to offer advice!

-S
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Old 10-04-2010, 11:14   #8
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Quote:
Originally Posted by btrayfors View Post
Note that AGMs can absorb a very large charging current, often exceeding their amp-hour rating. A 400AH AGM bank could easily absorb a 400-amp charging current or more. If you're not careful, this could place an unsustainable load on your alternator and burn it out. To avoid this, you need to somehow: (1) limit the output current; and/or (2) reduce alternator heating. There are several strategies to do this, both mechanical (e.g., regulating pulley size) and electrical (e.g., using smart regulators like the Balmar MC-612 to de-rate the alternator, or using external rectifiers).

With an AGM bank the size you contemplate and a 70-amp alternator, I think I'd want to find a way to de-rate it by about 20% in order to avoid overheating and burnout. Temperature sensors on the alternator and batteries would be highly desirable as well.

If you were designing the system from new, you'd want a much larger alternator...perhaps 4 to 5 times the capacity.

Bottom line: the 70A alternator will do, but charging times will be longer than would be ideal, especially since you'll want to derate the alternator somewhat to avoid burnout.

Bill
Bill is spot on with what he is saying. I have seen small alts and big alts burned out by AGM's a fair number of times. The way I usually handle it is in the regulator settings to de-rate the alt or use the "small engine" setting. You'll likely want to be in the 50a output rage or so but it should be able to run like this for a good long time. Alt temp sensors also can work but I often find them a tad inaccurate and problematic if not installed "just right"..

The only problem will be slightly longer re-charge times..

P.S. Just noticed the Heart regulator which does not have settings for reduced alt output so it will need to be done in other ways such as pulley ratios..
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