alternator sizing

[Mark Maulden]

I have an older Volvo MD17C with the original alternator (35a). I'd like to upgrade to higher capacity alternator but I'm constrained to a ~3/8" belt. I have a local electrical shop that deals with a lot of marine/industrial/etc electrical parts and they say they can build or recommend an alternator of over 100a and they say it would be OK with that engine and belt size. Any comments or experience???

 

[tommays]

It will only put out what the battery bank will take i am also still getting by nicly with a 35 amp on two group 27s

 

[Bill Roosa]

Ditto tommays
Batteries will not accept more than around 25% of their rated AH capacity. So a 400 AH bank can never accept more than 100 amps current. And the only time it is going to do that is when it is discharged well below 50% which you should avoid. Once you get above 50% State of Charge (SOC) the acceptance rate falls off rapidly to around 15% (60 amps for the example) at 75% charged and continues to drop till it reaches a float rate of 2-3%.
What you are really balancing is the cost and expense of a new alternator with some additional time to charge your batteries. IF you don't deeply discharge your batteries on a regular basis (bet you don't) this additional time to charge is measured in minutes not hours or days.
I have a 410 AH bank and de-rated 150 amp alternator. It ate belts at full rating so I ramped down till it got to 50 amps max output (1/2” belt BTW). I have never had any problems keeping the batteries charged even on 2 week cruises. The worst case was the 2 week cruise 2 summers ago when we had favorable winds a lot and did not motor except into anchorages. If you have some way of making hot water other than running the engine (and charging the banks) then you might have a problem. Or if you don't like hot showers and don't need to run the engine for hot water could also be a problem in a cruising situation.

Without knowing your specific boat usage facts it is hard to make an estimate. I’d assume that you feel you need to upgrade because the banks are not getting charged. I had some similar questions and produced an electrical system sizing spreadsheet (newly updated to accurately reflect the above battery acceptance limitation) I give out to anybody that wants it. William-roosa@us.army.mil for a copy. What I found, aside from the fact that large alternator are not as useful as you would initially believe is that managing the load is a much more effective way of getting bang for the buck. I found that just by changing the time I turned on high amp loads would result in some very significant reduction in amps drained from the batts. Some was obvious like you can run the refer AND freezer all the time the motor is on at no cost to the SOC. This consume as you produce scheme is the only way I would try and justify having a large alternator (except the case of a 1640 AH bank). Which segues into battery bank size. It is a little understood fact that if you get a bigger bank your alternator will automagicaly charge at higher rates (bank acceptance is a function of total bank size). So without more info I’d recommend spending your limited monies on more batteries and not a new larger alternator.

I can attest to the fact that 150 amps on a single ½” belt is foolish.
So is 125 amps and 100 amps. Even 75 amps dusted my ½” belt and it gave up the ghost after about 75 engine hours. A marked improvement over the 5 engine hours at 150 but still not anywhere near way is normal. The de-rating to 50 amps has resulted in no dust and I’ve had the current belt on for over 100 hours with no signs of excessive wear.
I’m thinking you mechanic is suffering from lack of work and will say anything to get your business. The trick will be to get him to fix it after the belts start breaking at high frequency.

 

[Old Skool Neil]

I'm totally on board with everything Bill offered. However, I'll add this in favor of a larger capacity alternator: if your battery management practices result in an occasional charging rate of 35 amps, your current alternator will be straining at full capacity to deliver it accompanied by a considerable heat rise. A hundred amp alternator will be easily cruising along at 35% of its capacity to deliver the same amperage.

And while we're on the subject of belts, dust and wear, I'd like to take this opportunity to shamelessly hawk a new product available from Moyer Marine:

http://www.moyermarine.com/forums/showthread.php?p=40242#post40242

Even though Moyer is an Atomic 4 specific company, this product works on any V belt driven alternator.

 

[Maine Sail]

Balmar suggests that a single 3/8" belt can handle up to 80 amps. They can, but at max output there will be some dust even with good alignment and clean rust free pulleys. A larger alt can always have the field derated by a regulator like the MC-614..

 

[Jake N Eggs]

For what it's worth, I attended a seminar put on by ElectroMaax at the Toronto Hunter Rendezvous this past week end and they have a system that converts the pulleys to a serpentine belt system. The system looked very good for those who use the battery bank a lot. There products are advertised here on SBO and I assume that they can be purchased here also.

 

[Stu Jackson]

And there's always the Small Engine Mode instead of continually derating an alternator with the more difficult to use reed switch: http://c34.org/bbs/index.php/topic,4669.0.html

It's in the manual for all Balmar regulators (except the ARS4 although it will work with one too).

 

[sailaway78]

Alternator vs Belt Size

I currently have a 70A Hitachi on a 3GM Yanmar. 3/8 belt. This is getting clost to max for the belt, and it does require a little more attention than the origional 35A alternator. Also a little more belt dusting. When the batteries are a little low, 450ah bank, you can hear the engine pull down when the alt. kicks in. I could not recommend anything more than 80amp, and maybe a 55 would be better. You can get these at any auto supply for around 100 bucks.

 

[Bill Roosa]

For the record
I have a balmar A5 regulator and you only have to set the derating once. it remembers the setting between runs so it is a set and forget type setup.

 

[Stu Jackson]

Bill, I understand, however, the point (and advantage) of the Small Engine Mode is that once the batteries are charged up somewhat, all you have to do is flip a toggle switch to increase the AO to full, based on battery acceptance of course. Using the reed switch simply means you've spent the $$ on a big alternator and are only using a percentage of its power all the time. The link within the link I provided gets into a detailed discussion of this issue.

A number of years ago we had a discussion, with jvss I think, about having larger alternators. His point was that battery acceptance will limit AO. So, his point was that if you have a larger alternator it'll work less hard to produce the same output. A 70A alternator with a 70A load, not unexpected on a depleted 400 ah bank, will be trying to work at 100% output. With the same load with a 125A alternator, do the math...

My discussion of the Small Engine Mode covers what we've done.

Your boat, your choice.

 

[rpwillia]

OK, I gotta ask ...

For what it's worth, I attended a seminar put on by ElectroMaax at the Toronto Hunter Rendezvous this past week end and they have a system that converts the pulleys to a serpentine belt system. The system looked very good for those who use the battery bank a lot. There products are advertised here on SBO and I assume that they can be purchased here also.

What is the advantage of a serpentine belt over a the standard belt? I see more surface area to grip, is that it?

 

[Maine Sail]

What is the advantage of a serpentine belt over a the standard belt? I see more surface area to grip, is that it?

Yes that's it... A singe 6 rib serpentine belt can do up to about 210 amps. I am installin an Electromaax serpentine kit and 140A alt in the next few weeks..

 

[Bob S]

Are the pulleys machined from cast iron or aluminum? I thought of converting to either a 1/2 V or a 6 rib belt a few years ago when I installed the 90 Amp Leese Neville. I couldn't find pulleys that would fit the motor. I did do some research, lots of great information on the Gates website. Unlike an automotive application the Electromaax serpentine kit doesn't employ an pulley tensioner. This looks like a very expensive upgrade.

 

[Maine Sail]

Are the pulleys machined from cast iron or aluminum? I thought of converting to either a 1/2 V or a 6 rib belt a few years ago when I installed the 90 Amp Leese Neville. I couldn't find pulleys that would fit the motor. I did do some research, lots of great information on the Gates website. Unlike an automotive application the Electromaax serpentine kit doesn't employ an pulley tensioner. This looks like a very expensive upgrade.

Electromaax makes some of the kits from anodized aluminum and some from steel. I am going aluminum. The nice thing is they are designed to fit right over your existing pulleys as in some boats you literally can't get a gear puller in there to get the old crank pulley off.

The alt kit with pulleys is less than the AGM bank of batts I installed on a customers boat two weeks ago and those will probably need replacement in three to five years....

 

[Old Skool Neil]

I wanted to add that RPM is a factor for output amperage too.

My 100 amp Delco turns at roughly crank speed on my Atomic 4, approximately 2100 RPM flat out under way. During a recent project I took the alternator to a highly respected local shop and had it dyno tested under load. At this RPM the output was about 50 amps.

It's not a big deal for me. I don't hammer my batteries that hard and as I said earlier in this thread, having the alternator cruise along at far less than its capacity is a good thing.

 

[sailaway78]

Size of the battery bank

The size of your battery bank should pretty well be the deciding factor. The bigger the bank, the more alternator you can use. But if your charging a grp. 27 start battery, and two grp 29 house batts, a super big alternator is just throwing money into a bottomless pit.

 

[Mark Maulden]

I see I generated quite the discussion. My particulars are that we have two banks of 245ah each and another 130ah. This is four 6V GC2H and one grp 27 start. Throw em all together and it's about 600ah. 25% of that would be 150a. As far as usage, they can get run down (exectp the grp 27) as in the PNW, we have to keep the beer cold in the summer and we (us) have to stay warm in the winter. I've seen 35a charge rate with the current alternator. Hot water for showers is usually a solar shower tube through the head port from up on deck. I don't see more batteries as the solution. What I see is a bigger alternator that doesn't have to run full tilt but still play nice with the smaller belt. I'm still thinking a max 80a. And btw, my mechanic is me. I just need to find the part. Tnx for the discussion..

 

[mitiempo]

Mark

Your 2 battery banks would be a lot more efficient as 1 larger bank. They would not be taken to as low a state of charge and will last longer as a result. Charging with a large bank as opposed to 2 smaller banks is more efficient as well. As 2 banks of 245 AH they will accept a bit less than 50 amps each..

The larger alternator makes sense and as long as the banks are paralleled as one it will be a good system. A good external regulator makes a lot of sense as well.