Can alternator be too big?

lumphammer

lumphammer

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I am looking at changing my alternator. Looking at ones from Balmar the smallest size they do is 60A.

I have two 110Ah batteries and will be using the Sterling Alternator controller.

Is there a problem using an alternator with such a high output on relatively small battreries?
 
If the batteries are prepared to accept the full 60A charging rate then the alternator could consume 2.4 HP from the engine. 1 hp = 746W. At 60A/15V then the power output of the alt is 1.2hp assuming 50% efficiency then the alt is consuming 2.4hp. Not a problem if your engine is big enough but might be if its a typical small yacht unit.
 
Reality check though...... the period of time over which a set of batteries are going to demand anything like the full output of the alternator is pretty small..... they are more likely to be demanding 50% or less of that amperage, and even then, not for long...

From a safety point of view, a fully charged set of batteries is probably more important that losing a small amount of HP for a very short period of time.....
 
The alternator size can't really hurt the batteries unless it's grossly oversized. It's the regulator that determines how much energy is pumped into the batteries. This is done by regulating the pressure (voltage) with which energy is pumped into the batteries. There is only a relatively short period of time during which the alternator delivers it's full (unregulated) output to the batteries.

In fact, given the same regulator, a larger alternator won't really make a lot of difference in charging time, as full capacity of the alternator is only used for a very short time during the charging cycle, and only when the batteries are heavily discharged. A 'smart' charger, such as the Sterling you are considering, on the other hand makes all the difference in the world, even with a relatively small alternator.

I agree that 60 amps is not particularly large for 220Ah battery capacity. Actually I'd say it was about right. If you're upgrading from say 50 amp capacity, I'd say you're probably better of keeping the cash in your pocket. The key is the regulator i.m.h.o.
 
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No, I'd go for the 90A Balmar if it fits.

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I agree - it will allow a good charge to be put into the batteries while supplying the load (charging current = alternator current - current into the load e.g. nav gear, lights, fridge)
 
\"losing a small amount of HP\"

Plenty of small auxilliaries suffer the effects of a flat battery bank. My Volvo 2002 is 18hp (well, was 7,500 hours ago) and I have upgraded my alternator to a 80 Ah. Even before the new alternator, with low domestic batteries it would not rev up above 1500 rpm in gear. At low revs they produce just a few Hp, even less when cold. As posted below such an alternator at high output (you will seldom see full output) will require up to 2 Hp as it approaches full output, which on my rig is delivered at just under 2,000 rpm. With the prop turning and absorbing power, and the flat domestic batteries on line, as the engine revs increase and hence alternator output, you reach a point where the combined load is equal to the output of the engine.

Answer, charge in neutral for a few minutes, or set off on the starter battery and switch over once he rev's are up.

Merlin advanced chargers for one do not charge at all for the first 10 minutes of running. This is what they call the 'soft start' system. Bit useless if you have one of those and calculate you need to run the engine for 20 minutes to charge the domestics!
 
Re: \"losing a small amount of HP\"

According to the Merlin website they currently sell Balmar regulators. My Balmar Maxcharge regulator has a 45 second ramp up time and also offers a 'small engine' setting that limits the load on the engine if this turns out to be too high.
 
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And always assuming that the belt driving it is up to it too

[/ QUOTE ] Double belts then with large alternators charging large battery banks.
 
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1 hp = 746W. At 60A/15V then the power output of the alt is 1.2hp assuming 50% efficiency then the alt is consuming 2.4hp.

[/ QUOTE ] 50% strikes me as a remakably low efficiency figure for an alternator. Where do you get such a low estimate from?
 
yes they can.. I fitted a larger Balmar which was supposed (just) to be able to run on one belt... In fact the weight of the genny charging made me break belts at an alarming rate. The weight involved in more powerful alternators is considerable and probably you need to invest in a 2nd belt....
As for the charging element the new bigger alternator was excellent - pity about the boring time spent at sea fitting new belts head down in the engine compartment.
 
I use a 60A alternator on 2 80AHs + 1 70AH with a stirling regulator.

Your setup sounds like a very good charging system to me, a 60A alternator is not particularly high output, and a 230AH bank is not relatively small.

In the most demanding scenario when your batteries are discharged (i.e. 50% capacity) they will only accept a high input up to about 75% charged (e.g. 57 amps for an hour). After that the regulator will not be drawing all the current from the alternator anyway.

A bigger alternator would get through the bulk charge stage a bit quicker quicker but, as others have mentioned, alternators are not free in terms of HP. I would not have anything larger than a 60A alternator on my 10HP engine, you can feel it if the batteries are sucking up power.
 
Is the Balmar not a 6 diode alternator? If your man isn't a techy ( why ask the question if he is?) he may have problems with his sterling smart controller connection. ( I've just had to convert my old lucas on the 4108 to 6 diode in order to accomodate the sterling)
 
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