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Old 09-16-2021, 11:15 AM
Mak51 Mak51 is offline
Join Date: Apr 2010
Location: Minnesota
Posts: 46

The voltage would be around 12.3v when motor was at idle and then dropped to 11.8-11.9v when running/higher RPMs.

Almost wondering if the issue was caused by both having too many electronics plus the Minn Kota On-Board Alternator Charger also drawing on the battery. I've moved two of the big graphs to a dedicated lithium battery but would still like to run the on-board alternator charger. I'll continue to monitor going forward, hoping the new alternator holds up.
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Old 09-16-2021, 07:56 PM
REW REW is offline
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Join Date: Nov 1999
Location: .
Posts: 38,955

An alternator can only put out so much current.

In theory, whether an alternator is pumping out 2 amps or 40 amps, there should be little difference in the life of the alternator.

Also, it shouldn't matter how little or how much a battery is used with thinking about the alternator.

Any of today's alternators have built in regulators.

If there is a lot of load on the batteries, the alternator will read that voltage and provide up to its maximum wattage to replenish the drained batteries.

But, that is all that would happen. If you continued to exceed the load of the alternator, the alternator would be unaffected; you would simply find that one or more batteries would not be fully charged and or possibly charged very little because of the load on the batteries.

But, in any event, there should not normally be a difference in alternator life, just because it is producing its maximum power.

Best wishes
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Old 09-17-2021, 11:31 AM
staylor staylor is offline
Join Date: Jun 2004
Location: North Tonawanda, NY, USA.
Posts: 2,513

I also own a 2013 Merc 150 4S and when I was fishing today I pulled some numbers from the voltage gage for your reference. The only battery is a 4 year old cranking battery and I only run a fish finder for accessories. Alternator is Merc OEM and is the original equipment on the motor.

Engine off, fish finder off, ignition in Run position = 12.4 volts

Engine on, in neutral, fish finder on= 13.8 volts

Engine at cruise speed at 3500 rpm, fish finder on= 13.6 volts

Engine at 800 rpm Musky trolling speed= 13.9 volts

I think you must be working the alternator nearly all of the time at close to max output- and this may explain the failures you’ve experienced.
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Old 09-17-2021, 12:06 PM
Hot Runr Guy Hot Runr Guy is offline
Join Date: Nov 2006
Location: West Chicago, IL, USA.
Posts: 15,598

I'd disconnect that Minn-Kota 3-bank alternator charger from the mix, and see if your voltage numbers increase.

If your TM batteries are worn down, that charger is a massive load on your alternator.

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"I've got a car with a trailer hitch, and a pocket full of money. Do you want to sell that boat today, or not?"
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Old 09-19-2021, 08:18 PM
twofishy4u twofishy4u is offline
Slot Fish
Join Date: Nov 2020
Posts: 133

Disconnect the charger and try a normal lead acid battery. Not a fan of AGM's.
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Old 09-23-2021, 05:41 PM
ABQGeo ABQGeo is offline
Join Date: Oct 2020
Location: Windsor, CO
Posts: 8
Default It's working its heart out!

Originally Posted by Mak51 View Post
The voltage would be around 12.3v when motor was at idle and then dropped to 11.8-11.9v when running/higher RPMs.
Whoa! There's a flag right there Minnow. If the alternator voltage is dropping to 11.8-11.9 while running at higher RPMs you've got something that's overloading it for sure and it's getting hot and burning out.

My 150 4S is at 13.6 - 14.1 when running down the lake.
2019 LUND 1875 Impact
Mercury 150 hp 4 stroke
Yamaha 9.9 kicker, iTroll
Minnkota Ulterra 80#, 60"
Lowrance HDS 12 Live
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Old 10-05-2021, 08:08 AM
Snowking Snowking is offline
Join Date: Jun 2003
Location: Germantown, WI, USA.
Posts: 2,130

On my two bank minnkota alternator charger. I installed a switch. It is on the wire that goes to the ignition. The one that is setup so the alternator charger only turns on when main motor is running. If I am not going to be using the trolling motor much that day. I will leave the switch turned off. So less hours where the main motor is charging up the trolling battery’s. Mercury’s are known for using pretty cheap alternators. Unlike a truck. Not to easy to add a bigger alternator to a outboard. If you are running all those graphs at once. Or even three of them. I think you are going to have to bite the bullet. Get a extra electronics battery in the back.
2014 Lund 1875 Crossover
2014 Mercury 150 four stroke
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