Alternators - What's the Difference ?

[stoupidmonkey]

I am going to replace my old alternator, a delco 42amp with 2" foot, on my perkins 4-107, though am not wanting to pay a fortune (more than $200) for a new alternator with an output of ~60-90 amp. Is it possible to use an alternator for a gas engine for this application, or would the rpm's be too low on the diesel engine? Is a marine alternator much better, or is it just for flash supprecent? Any good, cheap brands I should know about?

[Cheechako]

I bought a new 100 amp for my perkins real reasonable from this guy. Unfortunately I havent had the time to install it yet, but you might communicate with him. larry@semarine.com I'm not a big believer that "Marine" means much, but some have obvious improvements, others just seem to have a great paint job. The old Balmars worked fine and inside didnt look much different than the standard prestolite auto unit. Now they are different.

[bunyipdreaming]

i was wondering the same thing. i have a 41 coronado with a 4-107. a high amp balmar surely ain't cheap. i believe my alternator the p/o owner installed is 60 amp and is automotive seems to work ok now but planning on upgrading the whole electrical system in the near future.

[GordMay]

Engine RPM is typically about 1/2 (50%) of Alternator RPM or less because of the pulley ratios. For example; if the Alternator pulley is 2.5", and the Engine crank pulley is 5", the ratio would be 2:1, so an Engine speed of 1250 RPM would produce an Alternator speed of 2500 RPM. On an alternator rated for full output at 5000 RPM, this would only achieveabout 40 - 60% of rated output.

The Rule of Thumb is that the alternator output should equal about 25% of the battery capacity*, so a 60 Amp Alternator should be good for 240 Amp-Hour Battery Bank. Remember, the Alternator will use about 1 H.P. of engine power, for each 25 Amps of Output.
* AGM Batteries may have a higher acceptance rate, allowing a larger Alternator.

The Delco SI format alternator has proven to be a very reliable alternator (internally regulated), at an excellent price.
.
There were 10SI alternators with 37 Amp, 42A, 55A, 61A, and 63A ratings. The 12SI series were built with 56 Amp, 66A, 78A, and 94A maximum output ratings.

[stoupidmonkey]

Thanks for the input, exactly what I needed.

[DeepFrz]

Try to buy from a shop that can convert your internally regulated alternator to an externally regulated one that is compatible with a Balmar regulator.

Many automotive alternators just can not stand up to the rigors of charging a house battery bank. Check the "hot" rating of the alternator you are buying.

Many marine alternators have larger front end bearings to help endure the stress of putting out higher loads.

If your alternator ever runs to hot it will reduce it's output. Check the specs.

There are many threads on this forum addressing the design of a cruising 12 volt system including battery and alternator sizing and selection.

[Cheechako]

Good point, if you have a really large Batt bank, then the external regulator will continue to put out high amperage for a longer time creating a lot of heat. Even though the old Balmar's I've had on many boats looked pretty normal (std. bearings, std case/cooling, but possibly bigger diodes) They lasted pretty well. As far as new ones from whomever, I'd be looking for cooling improvements for sure to make the "marine" worth the money. Had a Balmar (ARS III?) regulator as a spare while cruising. It put out a lot less amps then the Ample power unit on the same alternator....

[DeepFrz]

Just checked a source for max. temp for an alternator and it seems to be 200 - 225 deg. F. Higher than that and the alternator life will be shortened.

Most (many, some,?) automotive alternators state that they are not to be used to charge a "discharged battery". They are only built to handle light loads, so checking the alternators specs. is imperative. Even then they may be stating max. burst amps. out rather than max. sustained amps. out.

[Cheechako]

Most of mine I'm sure you could have fried eggs on after 15 minutes of charging! Wish I'd have put a thermometer on one.... Most car systems nowdays are only built to charge lightly to replenish the starter motor loss etc. Not sure what the older case styles that are commonly used for boats were rated at temp wise....

[hellosailor]

Some of it is philosophy, some is simple math. The most important safety difference will be that a marine-certified alternator will have some type of spark/ignition prevention, typically metal screening to prevent fuel vapors from igniting from internal sparks. If you're diesel, that's something you may not consider important.

And in theory the body will be anodized or enameled, because aluminum castings develop white pox in salt air. Usually this is just a cosmetic issue.

And a proper marine alternator will often have dual fans (pulling and pushing) to compensate for the lower airflow in a marine engine bay. Fans of course can be added or changed.

I'm also a Delco and AC/Delco fan (the companies split from one) and their 12-SI series has one of the widest RPM ranges on the market, so you can choose a pulley size that gives you high output at idle speed, without over-revving at "battle" speed.

On the math side...first is the question of whether it fits, with the same feet and same dimension from the mounting points to the pulley. you may be able to make something fit--but sticking to the same profile is simplest.

It helps if you can compare the output curves of the new alternator to those of the old alternator. The Delcos usually have this information on their web sites, and they have toll-free phone numbers to call for help. You also want to check the ratio of alternator speed to engine speed (which is the ratio of their pulley diameters) to make sure the alternator will be turning fast enough to charge at idle--as fast as possible at idle--without revving past the safe limit at maximum engine speed.

If you look at the charts for output, you can see that sometimes "just" changing the pulley size (which may mean spending $100 to have a machine shop make a custom pulley) can sometimes literally DOUBLE the output you will get at idle and cruising speeds, and radically reduce the time you need to charge batteries. Even if the alternator isn't twice the power.

[Cheechako]

I've always thought a 1/2" dia copper tube wound around the case like a coil and a diapragm pump would be a great scheme. Have it come on for 20 minutes each time the motor is started to suck the heat off the alternator... never did figure out how to attach it to the case securely... most cases being aluminum...

[DeepFrz]

Steve Dashew uses Electrodyne alternators that have remote diodes to reduce the heat buildup in the alternator. The diodes are mounted on heatsinks with a fan that runs when the ignition system is on. A little more than is required of the typical (if there is a typical) cruiser's needs.

Electrodyne Brushless Alternators

[Cheechako]

Rethinking it, if you are going cruising.... $500+ for new alternator might be a good thing. Those new Balmars have cooling slots everywhere......

[Nauticatarcher]

We use Bosch 120amp alternators, 1/3 the price of Balmars, because of gearing can only pull about 80 amps so dont get too hot, run for ever servicable any where in the world

[hellosailor]

Cheechako, find a friend with a milling machine. Then have them micro-flute the entire alternator case running in the same direction as the shaft. Fluting can double the surface area, which can double the heat transfer out of the alternator. Ain't gonna be cheap if you have to job it out. And, of course, you might want to strip it and clean it to make sure all those metal shavings don't go pyrotechnic on you.

But it would be effective.