Alternators - Testing and Philosophy of use

[ThndrSki]

The older I get with boating, the more I have come to expect a reasonable life of use and not take any chances with something that can strand or cause great inconvenience. Partly the reason I now load test my batteries etc etc.

This weekend, with 2 batteries less than a year old, I went to crank and my batteries were dead after a long run a few hours before. I looked behind the alternator and the crusty nut had rocked loose leaving the output terminal very loose on the post. I jumped the boat and put it on the trailer, etc. I haven't had a chance to test the output after tightening the post, going to do that tomorrow but in the meantime, just made me think about general alternator thoughts.

I have a couple questions for the low tolerance members of the group:

1. Do you run through all of the 4 tests in the FSM from time to time or just check the output to the battery on your gauge or a volt meter?

2. Is there a common school of thought of service life before replacement or just monitor, etc?

3. Always recommend the OEM alternator as a replacement, I have a 55 amp OEM Mando that has had a long long life. I see some "bargain ones".

Thanks, just wanted to create a good maintenance and double check study.

 

[gm280]

ThndrSki, if you thoroughly understand how an alternator works, you can easily rebuild one yourself. They even sell rebuilt kits that include the bearings, slip rings, brushes and diode pack/voltage regulator very cheaply. So unless you have the winding's burnt up, you can make any alternator work like new pretty easy. I have done that for my vehicles without any issues. And then you can take your rebuilt alternator to an auto parts store and let them test it for compliance, if you need that extra assurance. JMHO

 

[ThndrSki]

Thanks GM280, is there really any difference in components on the Mercruiser part # alternator and road going versions of GM product?

 

[achris]

Thanks GM280, is there really any difference in components on the Mercruiser part # alternator and road going versions of GM product?

Oh yes! All electrical and fuel system components used in a boat must comply to SAE-J1171. That basically dictates that they are compliant in that they will not initiate a fire or explosion in the presence of petrol vapour.

For an alternator, starter motor and distributor that means flash proof screens internally. And anything else that stops sparks getting out and vapour getting in.

Chris. ...

 

[edsboats]

Do you have a volt meter on your boat? It should read 14 volts with the engine running. It should be part of your scan of the gauges when you are driving the boat. If it only reads 12 volts with the engine running then you need to head back to the marina. An alternator produces 12 volts to run the electronics and an extra 2 volts to put a charge into the battery. 12 + 2 = 14 volts output.

 

[gm280]

achris, you're absolutely correct about electrical parts for an I/O boat setup. But I am not so sure the individual components are much, if anything, different then vehicle internal components in an alternator. I believe the housings are approved to not produce any sparks or shielded from vapors, but the diodes, and regulators I would think are pretty much the same. But again that is merely a guess on my part and I could be totally off. :noidea:

edsboats, you are correct up to a point. When the battery is low, the alternator will produce a higher voltage to charge the battery. But once that battery is charged, the voltage drops so not to over charge the battery and the voltage can read near typical battery voltage (~12.6). So unless the battery needs charged, the voltage will not read ~14 volts all the time. That is why there is a charging regulator in the alternators. JMHO

 

[edsboats]

The 2 volts is like a trickle charge. It is a 12 volt battery but a 14 volt electrical system. The voltage regulator controls amperage depending on how many things are turned on.

 

[achris]

achris, you're absolutely correct about electrical parts for an I/O boat setup. But I am not so sure the individual components are much, if anything, different then vehicle internal components in an alternator. I believe the housings are approved to not produce any sparks or shielded from vapors, but the diodes, and regulators I would think are pretty much the same. But again that is merely a guess on my part and I could be totally off. :noidea:

You are quite right about the diodes and other electronics, also the windings and rotor. Generally, with electrical components, the screening is added to a standard component. A distributor has the 90 degree angled vent, you don't see on a car. Fuel system components are actually designed differently. Carburettors are internally vented and have no fast idle cam. Fuel pumps have a sealed lower section with a port for which to a tube is connected and plumbed up to the carb special fitting, so that should the diaphram part company with itself, the spilt fuel will be directed up to the carb, and flood out the engine, rather than filling the sump with petrol and leading to a rather large ka-boom!

edsboats, you are correct up to a point. When the battery is low, the alternator will produce a higher voltage to charge the battery. But once that battery is charged, the voltage drops so not to over charge the battery and the voltage can read near typical battery voltage (~12.6). So unless the battery needs charged, the voltage will not read ~14 volts all the time. That is why there is a charging regulator in the alternators. JMHO

The 2 volts is like a trickle charge. It is a 12 volt battery but a 14 volt electrical system. The voltage regulator controls amperage depending on how many things are turned on.

Sorry guys. A lead acid battery is actually 13.2 volts when fully charged (6 cells of 2.2 volts). The alternator (when working correctly) outputs between 13.8 and 14.5 volts, regardless of the charge in the battery. The reason is that if you had 13.2 volts going into the battery, it would never reach full charge. Any higher and it would be over charged.

The current in the circuit is controlled by demand. If you switch on a high power load, such as a doof-doof, then the battery voltage drops and the current output of the alternator rises to compenstate. I am trying to think of an analolgy, but it's difficult. Imagine 2 trucks pushing against each other. The 'alternator' truck is quite large and constant, and the 'battery' truck can vary its size (yeah, that's where it all goes pear-shaped). These trucks are the 'voltage' and their movement is the 'current'. When the battery truck is small, then the alternator truck can push it quickly. As the battery truck is pushed, it grows in size, unitl it is slightly smaller than the alternator truck, so the alternator truck can only push it slowly, but it can still push it. If a load is added, the battery truck decreases in size, and the alterntor truck again pushes it quicker, until the battery truck is back up to size... Yeah, I didn't say it would be pretty. If someone else can come up with a better way to explain, please do so. I would love to be able to edit that lot ^^^ out.

Chris.......

 

[ThndrSki]

Just as a follow up, my FSM says the Battery output test should be between 13.9 and 14.7 volts @ 1500-2000 RPM. Mine just barely touched 13.9 volts @ 2000. It's time for a replacement. I agree with Gm280, it would be nice to learn to rebuild the alternator for the most efficient price, but the OEM one gave me 20 years of use so i've also hit the point in my life where if I'm replacing myself, I like purchasing the right tools and new if it makes sense as far as feasibility goes.

To me, boat days are precious and dragging my family to the lake and the boat not performing or breaking down is a huge disappointment to them. I have a mental "pre launch" checklist, really just trying to formalize everything and see if I'm missing anything major.

 

[gm280]

Just as a follow up, my FSM says the Battery output test should be between 13.9 and 14.7 volts @ 1500-2000 RPM. Mine just barely touched 13.9 volts @ 2000. It's time for a replacement. I agree with Gm280, it would be nice to learn to rebuild the alternator for the most efficient price, but the OEM one gave me 20 years of use so i've also hit the point in my life where if I'm replacing myself, I like purchasing the right tools and new if it makes sense as far as feasibility goes.

To me, boat days are precious and dragging my family to the lake and the boat not performing or breaking down is a huge disappointment to them. I have a mental "pre launch" checklist, really just trying to formalize everything and see if I'm missing anything major.

Ha, rebuilding things is not for everyone. I like doing such things because I know how to do it and I like knowing what is going into my equipment. And since it is basically all they do as a re-manufactured unit, I can pocket the labor cost in my own pocket. I have all the tools and can rebuilt things with available kits for near nothing.

They sell alternator rebuilt kits all over the place for a song. And those kits can even include new slip rings. And since that is a part that does wear a lot, it is a good thing to replace if you are rebuilding one. JMHO

 

[ThndrSki]

gM280, I hear ya, I'm into a classic car as well so sometimes you are forced to rebuild things like a waterpump as no remanufactured or new parts exist. Just out of curiosity where would you get a rebuild kit for a Mando 55 amp alternator?

 

[muskyfins]

You are quite right about the diodes and other electronics, also the windings and rotor. Generally, with electrical components, the screening is added to a standard component. A distributor has the 90 degree angled vent, you don't see on a car. Fuel system components are actually designed differently. Carburettors are internally vented and have no fast idle cam. Fuel pumps have a sealed lower section with a port for which to a tube is connected and plumbed up to the carb special fitting, so that should the diaphram part company with itself, the spilt fuel will be directed up to the carb, and flood out the engine, rather than filling the sump with petrol and leading to a rather large ka-boom!





Sorry guys. A lead acid battery is actually 13.2 volts when fully charged (6 cells of 2.2 volts). The alternator (when working correctly) outputs between 13.8 and 14.5 volts, regardless of the charge in the battery. The reason is that if you had 13.2 volts going into the battery, it would never reach full charge. Any higher and it would be over charged.

The current in the circuit is controlled by demand. If you switch on a high power load, such as a doof-doof, then the battery voltage drops and the current output of the alternator rises to compenstate. I am trying to think of an analolgy, but it's difficult. Imagine 2 trucks pushing against each other. The 'alternator' truck is quite large and constant, and the 'battery' truck can vary its size (yeah, that's where it all goes pear-shaped). These trucks are the 'voltage' and their movement is the 'current'. When the battery truck is small, then the alternator truck can push it quickly. As the battery truck is pushed, it grows in size, unitl it is slightly smaller than the alternator truck, so the alternator truck can only push it slowly, but it can still push it. If a load is added, the battery truck decreases in size, and the alterntor truck again pushes it quicker, until the battery truck is back up to size... Yeah, I didn't say it would be pretty. If someone else can come up with a better way to explain, please do so. I would love to be able to edit that lot ^^^ out.

Chris.......

Actually, very nice analogy.:thumb:

What happens when the alternator truck can no longer push the battery truck? Or should I say when the alternator truck can't push the battery truck fast enough?

Or, in regular talk, when the alternator puts out the proper voltage but not enough current. In such a fashion that the drain on the battery is higher than what the alternator can replace it? Is this even a thing? Or is simply when it won't put out at least the 13.8 volts that it is considered "bad"?

 

[achris]

Actually, very nice analogy.:thumb:

What happens when the alternator truck can no longer push the battery truck? Or should I say when the alternator truck can't push the battery truck fast enough?

Or, in regular talk, when the alternator puts out the proper voltage but not enough current. In such a fashion that the drain on the battery is higher than what the alternator can replace it? Is this even a thing? Or is simply when it won't put out at least the 13.8 volts that it is considered "bad"?

There are 2 conditions in which the battery truck could push back the alternator truck. First condition is if the load on the system (like a really noisy Doof-Doof) is so great that battery truck gets so small that it's like the alternator truck is pushing against no load at all. Ever been pushing a car stuck in the mud and it suddenly goes? You fall flat on your face, right?... Just like that...

The other condition is when the alternator truck has lost it's power (literally and metaphorically) and can't push anything. You see that in the voltmeter. If it's not between 13.8 and 14.5 volts (or whatever the book says, they all say slightly different, but in about the same range)...

Alternator output should always be measured on a fully charged battery. That's the No Load Voltage output test. Then you load test them. Turn on as much as is needed to get the current demand up near the rated output. Check the Full Load Voltage output and the actually current being produced.

Hope this helps...

Chris.........