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Old 10-19-2017, 11:24 AM   #1
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Two Alternators

Hi All,

2 owners ago, the guy used this 36' trawler for fishing. He noticed the stock alternator didn't really charge batteries at idle, so he installed a second alternator, strictly for the purpose of trying to get the batteries some charge while trolling at idle.

Bring us up to date. I have an 8D for starting the beast and 2 sets of 6V GCB's for HOUSE 1 and HOUSE 2, shore power (of course), a generator and a monster inverter/charger. That 2nd alternator is a PAIN in the rear for trying to move around down below. If it wasn't for the fact I'm very limber and can squeeze into tight spaces, there's no way to easily get to anything in front of or on the port side of the engine.

The original alternator has a 12g Orange wire that disappears into the engine wiring harness. The second alternator has an 8g wire right to the batteries via an amp gauge.

Does anyone see an issue with me removing the second alternator and not being able to sustain electrical systems while under way?

Dave
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Old 10-19-2017, 11:48 AM   #2
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Not crazy about 12ga disapperaring as your charge wire.

I prefer a heavy wire straight to your desired battery bank with appropriate fusing.

Depending on the alternater rating yes...as long as you dont draw more than 50 percent of your rating for other than charging.
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Old 10-19-2017, 12:35 PM   #3
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If your using your boat for low rpm travel such as trolling, you may not have enough pulley rpms to charge your batteries and/or keep your house 12 VDC systems alive. Hereís an alternator output curve from Balmar.

Alternator Output Curves | Balmar
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Old 10-19-2017, 12:39 PM   #4
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If the "second" alternator is in your way but of higher output than the other, why not move the higher output alternator to where the first one is?

BTW - unless the alternator output is very low, 12GA is too small. Even the 8ga used on the "higher output" alternator sounds too small - and it should go straight to a battery bank and be properly fused.

Ken
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Old 10-19-2017, 12:47 PM   #5
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My Balmar alternator (100A) output can be outstripped by load when running slow at night with everything on. Volts drop from 13.8 to to around 13. If it goes into the low 12's I just crank up the gennie and use the charger to augment. Only had to do it a few times.

When trolling (daytime) I run super slow (650rpm, about 5.5-6kts but DC load is less so volts stay up.

I would consider getting rid of the second alt. If you have volt drop issues, you may be able to find a higher output alt that fits where the factory one goes.

Take the belt off the second one and see how your volts do. If ok, take the thing off.
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Old 10-19-2017, 12:50 PM   #6
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Are you sure the 12 and 8 gauge wires are the output wires from the alternator to the battery and not for gauges? 12 gauge wire rating is somewhere around 20 amps and 8 gauge wire is ~50 amps. The wire run length and temperature will affect the capacity of the wire also. What size alternators are they?
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Old 10-19-2017, 01:11 PM   #7
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Taking a SWAG. Do your dash instruments have an ammeter? Usually where the alternator output goes if it leaves the engine harness for parts unknown. It will come back, sometimes orange with a color stripe.
The other place the orange may show up is the starter solenoid terminal that the battery cable connects.
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Old 10-19-2017, 01:15 PM   #8
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Quote:
Originally Posted by Larry M View Post
Are you sure the 12 and 8 gauge wires are the output wires from the alternator to the battery and not for gauges? 12 gauge wire rating is somewhere around 20 amps and 8 gauge wire is ~50 amps. The wire run length and temperature will affect the capacity of the wire also. What size alternators are they?
Is it possible he was charging batteries from the second and running gauges etc. from the other? Is that possible?
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Old 10-19-2017, 01:15 PM   #9
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Quote:
Originally Posted by Ski in NC View Post
My Balmar alternator (100A) output can be outstripped by load when running slow at night with everything on. Volts drop from 13.8 to to around 13. If it goes into the low 12's I just crank up the gennie and use the charger to augment. Only had to do it a few times.

When trolling (daytime) I run super slow (650rpm, about 5.5-6kts but DC load is less so volts stay up.

I would consider getting rid of the second alt. If you have volt drop issues, you may be able to find a higher output alt that fits where the factory one goes.

Take the belt off the second one and see how your volts do. If ok, take the thing off.
I'll try that. Thanks. I will also get some specs on the primary alternator.
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Old 10-19-2017, 03:24 PM   #10
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I have a second alternator. I keep it in the spare parts locker in case the installed alternator fails.
Yea, I know, not the same. LOL
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Old 10-19-2017, 03:26 PM   #11
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Originally Posted by Dave_E View Post
Hi All,


Does anyone see an issue with me removing the second alternator and not being able to sustain electrical systems while under way?

Dave
Tough to say Dave. What is your electrical load is while underway? And what is the output of your other alternator at cruising speed?

If your load is higher, it's obviously not sustainable.
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Old 10-19-2017, 03:29 PM   #12
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There is/was something called a cruise generator. It was mounted to the propeller shaft.
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Old 10-19-2017, 03:36 PM   #13
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Is it possible he was charging batteries from the second and running gauges etc. from the other? Is that possible?
One alternator could be for the starting bank and the engine gauges. The other alternator could be for House 1 and House 2.
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Old 10-19-2017, 03:48 PM   #14
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None of us are as rich as Rockefeller and none of us are certified marine electricians.
Before you start removing and modifying the electrical stuff, it would be worth the money to get a marine electrician out to look at the installation and trace the wires.
WE can cause more and expensive damage than necessary.
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Old 10-20-2017, 08:44 PM   #15
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Have 2 alternators on my engine. 2nd charges house batteries and has an external 3 stage regulator. If you plan 2 cruise and anchor out, it will save you countless hours of generator time. I would get an external regulator and use it for a year before making a bad mistake.

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Old 10-21-2017, 01:36 AM   #16
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You really should have an electrical plan for the whole boat. In order to come up with that plan you need to know how you plan to use the boat.

If you have no generator and you need to charge your batteries from a single engine, you will want a 100+ amp alternator that uses an external three stage voltage regulator. This allows charging at 14.1 volts.

If you just want to maintain while cruising then an eighty amp single wire alternator is all you will probably need. Single wire alternators have built in voltage regulators and produce 13.8 volts.

There is a lot more to this subject, this is only scratching the surface.
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Old 10-21-2017, 06:23 AM   #17
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It want uncommon for 80s vintage boats to have the alternator charge wire go to an ammeter as someone already posted then return to the batteries.

I had it on my last boat and fixed it on a few others. Usually an orange wire, way undersized.

Not hard to identify the batt output of an alternator and follow where the wire goes.

Not every job requires a certified anything. Thst is why box stores and libraries are filled with DIY books. All you have to be is smart enough when you are in over your head. Lacking that or not that energetic or in a rush...sure, then call a pro if the can fit you in when you need them.

Oh, and lots of "pros" arent really all that good in the marine trades so you have to ask around and then for credentials.
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Old 10-21-2017, 07:45 AM   #18
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If you want an ammeter these are cheap, work great, and the shunt allows you to run small wires to the dash instead of a big one.
https://www.ebay.com/itm/DROK-Dual-D...72.m2749.l2649
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