New 120amp alternator into system

R

Rory Hinds

New Member
May 6, 2020
27
Boat Info
250 Sundancer 5.7l Merc Bravo II
Engines
5.7l Merc Bravo II
I have a 1995 Sundancer 370 with 2 x Merc 7.4L's that had 55amp alternators and I'm upgrading the starboard side alternator to a 120amp.

My system seems to have the alternators connected to a battery isolator then into a solenoid. The isolator has dual alternator inputs but is only rated to 75amp so I can't use that with my new 120amp alternator.

The question is can I bypass the battery isolator for the 120amp alternator and connect that directly to the solenoid and be good?
 
Thanks I have the 2 AWG wire but didn't think about about adding a second belt. Would you just run the two belts on top of each other?
 
i upgraded to 90 amp alternators several years ago. The shop that made the alternator told me any more amps would require changing the alternator and crankshaft pulleys to ones with 2 V Belts. Do you have V belts or the flat serpentine belts.
 
To make this work it will require at least one boat dollar. Can you return the alternator? Maybe buy two slightly stronger than your current ones but still within the design parameters of your charging system?

what is the need for so much more power?
 
Pretty sure I have one V belt on the alternator and its not the flat serpentine belt but will have to check when I'm down at the boat again today.

I'm wanting more power to keep the house batteries charge while running the inverter while the motors are running. The 120amp alternator I purchase is rated as a replacement for the one I had before so I'm hoping it will be plug and play.
 
Pictures will help everyone understand what we are dealing with.
 
Man that's a lotta power from an alternator. What are running from your inventor? I'm curious.
 
I have a 240 amp alternator and dual group 27 batteries in parallel in my F-250 and the max inverter ford installs is 400 watts. Demanding too much from the charging system will end up costing you with melted wires, blown voltage regulator, cooked batteries, possible fire.

maybe add larger or more batteries to your bank. If you push the system trying to run like a generator nothing good will come of it.
 
I have a 280amp alternator in my Sprinter van that runs as a second alternator for my house bank so this 120amp alternator for my boats 7.4L motor doesn't seem that high to me.

I need to charge my eFoil battery on the inverter which needs a lot of juice. I also do a bit of cooking while cruising at 7knots so the convection oven might be running. Those are the big things I can think of that will come into play.

Also find my windlass drains the battery real fast so having a more powerful alternator will help with dropping and raising the anchor while the motor is running.
 
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Rory Hinds said:
I have a 280amp alternator in my Sprinter van that runs as a second alternator for my house bank so this 120amp alternator for my boats 7.4L motor doesn't seem that high to me.

I need to charge my eFoil battery on the inverter which needs a lot of juice. I also do a bit of cooking while cruising at 7knots so the convection oven might be running. Those are the big things I can think of that will come into play.

Also find my windlass drains the battery real fast so having a more powerful alternator will help with dropping and raising the anchor while the motor is running.
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Sounds like a lot of power demand.

Are your batteries all healthy? No sense putting a hotter charge into a battery that can’t accept it.
 
It’s also okay to have a portable generator for temporary use in a safe place for a short period of time right?

Having a portable generator on a smaller boat is a sore subject as they are so often operated negligently.

If a portable generator is not used as a replacement for a proper generator it has its place.

Totally dead, no power. ran everything down while I slept. Well, then running a portable a couple hours to charge the batteries so you can start up probably won’t kill you.

the problem is often portable generators are used for HVAC, Cooking stove, TV/video games. These are not emergency uses!

Wanting power and demanding it from the engine is how the Honda problem begins. It then progresses into I don’t worry about it since I have never had an issue. Then you’re in a dangerous place.

someone needs to come up with a magical solution other than spending an extra 10-20 boat bucks for a proper generator.

all I can say is don’t demand too much of the engine charging system, it still has to run its self and get you home.
 
all my batteries are brand new AGM's. My boat has a 9k genset but I'd rather use an inverter to save on gas and added noise. On a recent trip I took my portable 2k genny which was great but it was noise so I'd like to have the inverter do the same job but silently.

I have a 3000 watt inverter I'll be installing once I get this 120amp alternator hooked up.
 
Rory Hinds said:
all my batteries are brand new AGM's. My boat has a 9k genset but I'd rather use an inverter to save on gas and added noise. On a recent trip I took my portable 2k genny which was great but it was noise so I'd like to have the inverter do the same job but silently.

I have a 3000 watt inverter I'll be installing once I get this 120amp alternator hooked up.
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Now its sounding weird. You want a larger alternator so you can run your main engines to charge batteries, rather than your genset that is designed for that purpose? And that is to save fuel and reduce noise?
 
Creekwood said:
Now its sounding weird. You want a larger alternator so you can run your main engines to charge batteries, rather than your genset that is designed for that purpose? And that is to save fuel and reduce noise?
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I agree totally, this makes no sense. "Noise", I get it, but a genset designed for the boat is barely audible. Sounds like a lot of money, effort and questionable results when he already has everything he needs.
 
I'd like to use the energy created by the motor while its running to power the inverter. No point to me running the genset if the motors are running and I'm cruising at 6 knots to leisurely get somewhere.

That and not having to power on the genset at night if all I want to do is run the oven for 15 minutes.

The 120amp alternator isn't that large and is designed as a replacement for my 55amp. I see it at using more modern technology to upgrade the 1995 system into current workings and get more out of your motor while its running anyways.
 
Better upgrade the rest of the charging systems technology as well. This way you get the benefit of the extra power rather than the mess cleaning up new problems that are bound to pop up.
 
Yes sure. I upgraded all my batteries to brand new AGM's and had to upgrade the charger too. Now will upgrade the alternator and install the inverter and possible add an extra battery to the house bank.
 
What about the voltage regulator and related components? AGM batteries don’t like to accept a high rate of charge. Where will all the extra amps go when their not used?
 
The new 120Amp alternator has an internal regulator but I'm looking at getting a Balmar 614 anyways. Just trying to figure that out now. Not sure what other related components except for higher gauge wire.
 
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