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Alternator on Generator

Old 03-15-2019, 06:20 PM
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Default Alternator on Generator

Assume one has two engines with functioning alternators charging two battery banks. When underway, the batteries are being charged by those alternators. At anchor, one starts the generator that provides 120v to the charger.... and charges the batteries. What function does an alternator on the gennie have in those circumstances assuming that the gennie does not have its own battery NOT serviced by the 120v charger?
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Old 03-15-2019, 06:22 PM
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Does the generator have dc output or its own alternator?
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Old 03-15-2019, 06:24 PM
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Why would a generator have a alternator? I’m lost?
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Old 03-15-2019, 06:28 PM
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Originally Posted by Motorboater1983 View Post
Why would a generator have a alternator? I’m lost?
a regular alternator charging the engine side is common in generators. Avoids the need to have an AC to DC charger/inverter.
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Old 03-15-2019, 10:33 PM
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Most have a decated battery for the generator. I have one battery for each engine And one for the generator. My generator doesn't have an alternator but does charge the battery at about 2 amps. So if the ac charger goes out it won't kill the generator battery. My onan won't run with a dead battery.
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Old 03-16-2019, 12:38 AM
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There is a few options, most are a complete engine, including an alternator to charge it's own start battery, and the engine is simply coupled to the generator, that same engine might be used to power a pump, or some hydraulic device, so, it requires it's own alternator and start battery.
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Old 03-16-2019, 03:39 AM
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Originally Posted by Rarline View Post
Most have a decated battery for the generator. I have one battery for each engine And one for the generator. My generator doesn't have an alternator but does charge the battery at about 2 amps. So if the ac charger goes out it won't kill the generator battery. My onan won't run with a dead battery.
how can a generator charge a battery if it doesn't have an alternator ?
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Old 03-16-2019, 03:43 AM
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Fortu ,

On my last 2 boats the generator was hooked to the house battery . So when it was running it charged the house battery . The starting batteries don't need to be charged when you are using the gen on anchor, so there was no need to turn the charger on .
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Old 03-16-2019, 04:37 AM
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Originally Posted by whatta life View Post
how can a generator charge a battery if it doesn't have an alternator ?
Many of them have a low wattage output inverter that comes off of the generator's 115v production - I think on mine it is 200w @ 12vdc (which is about 1.8 amps).
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Old 03-16-2019, 04:42 AM
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LI32 , Didn't know that , Thanks for the info !
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Old 03-16-2019, 06:02 AM
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It is often recommended that the generator have a stand alone battery under which circumstances, the gennie engine alternator serves a vital purpose.
My question was posed because I am in the process of purchasing a boat and the absolutely useless mechanical surveyor noted in his report that the gennie alternator was " non-functioning". I am prioritizing work to be done and since the vessel is NOT equipped with a gennie stand-alone battery, I believed the alternator to be deferable.
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Old 03-16-2019, 06:41 AM
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Originally Posted by Fortu View Post
It is often recommended that the generator have a stand alone battery under which circumstances, the gennie engine alternator serves a vital purpose.
My question was posed because I am in the process of purchasing a boat and the absolutely useless mechanical surveyor noted in his report that the gennie alternator was " non-functioning". I am prioritizing work to be done and since the vessel is NOT equipped with a gennie stand-alone battery, I believed the alternator to be deferable.
If you are going to run your engines and have a VSR to charge your house battery I agree. The Alternator allows you to shut down the mains at sea without worry but you can compensate for that with the battery charger.
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Old 03-16-2019, 06:42 AM
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Originally Posted by Fortu View Post
It is often recommended that the generator have a stand alone battery under which circumstances, the gennie engine alternator serves a vital purpose.
My question was posed because I am in the process of purchasing a boat and the absolutely useless mechanical surveyor noted in his report that the gennie alternator was " non-functioning". I am prioritizing work to be done and since the vessel is NOT equipped with a gennie stand-alone battery, I believed the alternator to be deferable.
What type of motors are in (or on) the boat? Sounds like you may be talking about an inboard powered or inboard/outboard powered boat. Which almost always use an automotive style engine driven alternator to charge the batteries. As opposed to most outboard motors that use a permanent magnet generator system. However, some also use an automotive style alternator.

Your terminology may be confusing folks. When talking about a standalone 120 volt AC generator (comprised of a piston engine driving an alternator) it is not typical to refer to the alternator portion of the device. It is simply referred to as a generator.

A 120 volt generator could be designed so that in addition to providing a 120 volt AC output to a separate standalone battery charger it could also provide a 12 volt DC output so as to recharge the battery that is used to start the generator. It could also be designed so that one of the start batteries in the boat are used to start the generator. The generator can then recharge that battery when the generator is running.

Is this making sense?
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Old 03-16-2019, 07:34 AM
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I THOUGHT I was acknowledging that setup in my last post. Guess that was unclear. The generator is an Onan. The boat is twin inboard diesel-powered.
I suspect there is general agreement that in the absence of a dedicated battery, the Onan alternator is not a priority item.

Thx
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Old 03-16-2019, 07:42 AM
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Do you have the exact model and year of the Onan? I may be able to pull up alternator specs if it is a regular belt driven unit.
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Old 03-16-2019, 08:23 AM
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Originally Posted by Fortu View Post
I THOUGHT I was acknowledging that setup in my last post. Guess that was unclear. The generator is an Onan. The boat is twin inboard diesel-powered.
I suspect there is general agreement that in the absence of a dedicated battery, the Onan alternator is not a priority item.

Thx
Is it an Onan alternator or is it an Onan generator? Cummins Onan (the company) refers to it as a generator. As in, it generates electrical power. To make it more confusing, they also refer to it as a Marine Genset. Short for Marine Generator Set.

Going back to your original question "What function does an alternator on the gennie have in those circumstances assuming that the gennie does not have its own battery NOT serviced by the 120v charger?"

The function of the alternator portion of the Onan generator is to supply 120 volt AC power to anything on the boat that requires 120 volt AC power. Battery charger, refrigerator, television, hair dryer, vibrating massager, lighting, you name it.

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Old 03-16-2019, 11:14 AM
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Originally Posted by Fortu View Post
It is often recommended that the generator have a stand alone battery under which circumstances, the gennie engine alternator serves a vital purpose..
In my boats I much prefer the generator to run off of the engine starting batteries - I have a 1-2-all-off switch on it to allow me to select which engine to start the generator with; my starting batteries are 8D's. The house bank (2 group 24's) only does the house and an electronics bank with 2 group 24's just does electronics.
Generators take little power to start, and my practice with my twin diesel is either to have at least 1 engine and/or the generator running at all times when away from the dock. I also have two smart battery chargers (Xantrex TrueCharge2 40A & 60A) that run in parallel.
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Old 03-16-2019, 11:59 AM
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Originally Posted by Fortu View Post
I THOUGHT I was acknowledging that setup in my last post. Guess that was unclear. The generator is an Onan. The boat is twin inboard diesel-powered.
I suspect there is general agreement that in the absence of a dedicated battery, the Onan alternator is not a priority item.

Thx
What model Onan?

My Onan has an alternator that charges the generator's starting battery to keep it topped up.

When the genset is running, the AC power can be switched onto to my on-board chargers (have three) and that charges the all batteries.

An alternator on a genset is important to keep the genset battery topped up but if the genset is also powering a charger that is charging the genset's starting battery, I dont know that you absolutely need the genset alternator functioning?
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Old 03-16-2019, 04:44 PM
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Originally Posted by whatta life View Post
how can a generator charge a battery if it doesn't have an alternator ?
I believe the generator end has slip rings that are D.C. Power also to produce 12 volts to run the coil and spark plugs. Not exactly sure but would have to check the manual. I do know that if the battery is low it won't stay running. So I think it charges the battery through the windings to provide 12 volts for the engine.
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Old 03-16-2019, 06:24 PM
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Originally Posted by Rarline View Post

I believe the generator end has slip rings that are D.C. Power also to produce 12 volts to run the coil and spark plugs. Not exactly sure but would have to check the manual. I do know that if the battery is low it won't stay running. So I think it charges the battery through the windings to provide 12 volts for the engine.
diesel boat, not going to have a gas genny
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