Charging batteries and 12 V system underway

[Action]

I would think that the charging system (alternators) of a boat would be sufficient to recharge all battery banks and supply enough extra current to operate any 12 volt need when the boat is underway.

I have seen alternators rated at under 100 amps on a number of twin diesel engines. Isn't that a little light?
I was thinking 150 to 200 amp alternators would be desired.

As long as a main engine is running, wouldn't it be desireable to have the alternators producing enough power for any 12 volt need?

What am I missing?

 

[rslifkin]

It all depends on how quickly you need to get things recharged, what other charging sources you have, and what your power consumption is while underway. There are plenty of advantages to having bigger alternators on the main(s), but not everyone needs it enough to be worth the cost and potential complexity of upgrading.

 

[psneeld]

Historically, most twin power boats were not long term cruisers so house banks were small and easily recharged even with small alternators.

My 37' Convertible with twin Cats had OEM 65A alternators charging 2 8D batteries. One was for starting and house.

That guy cruised it from the Chesapeake to Florida and back for almost 10 years. It had no genset but the guy was well off and went from marina to marina. Worked fine for him.

 

[RCook]

My single engine NT 37 has nine g31 12v agm batteries. Four for house, two for starting, two for bow thruster and windlass, and one for stern thruster and downrigger. With external regulation and ACR's, one 105A alternator charges them all just fine. A 145A alternator does it a little faster.

 

[Action]

Thanks for the replies.

And I forgot a piece. The other piece was running a generator while under way. Why run a generator if both engines are spinning alternators?

So noted that charging speed is swifter with larger amp alternators.
I get that larger alternators are more money. I replaced the stock alternator on my express cruiser with a Balmar system that was larger than 100 amps. (I did this some years ago so a lil light on detail) My cruiser biggest draw is a small refrigerator. The rest is lights and a sound system. All of this into two batteries.

The reason for the upgrade was specifically for greater amp output at lower RPM. (There is a whole situation I won't get into for that)

However, I would think there would be less cost at running bigger alternators than running a generator and twin alternators too.

 

[Lepke]

Depends on the 12v need.

Keep in mind, stock alternators rarely put out their rated amps without external control. And then they need a temperature sensor so they don't get too hot.

 

[Action]

Depends on the 12v need.

Keep in mind, stock alternators rarely put out their rated amps without external control. And then they need a temperature sensor so they don't get too hot.

Part of the reason I upgraded the charging system on my current express cruiser boat. The stock alternator didn't get it at engine RPMs under 1500.

 

[Jeff F]

However, I would think there would be less cost at running bigger alternators than running a generator and twin alternators too.

This has been discussed here, and I think the concensus is that there's no appreciable difference in fuel consumption between the two.

Unless you're using an inverter under high loads or trying to recharge your house bank quickly the OEM alternators should keep up just fine. I'm not sure I understand your question, unless it's related to one of those concerns.

 

[Action]

The question is primarily, why run a generator when underway?
Isn't the output of the alternator(s) sufficient to provide enough power?

To elaborate ....
I understand if AC or multiple AC units are running.
Beyond that I am not imagining an electrical load necessary to run a generator when the main(s) are running.

However, I have zero experience with larger boats, with multiple electrical systems.
I have been looking at some and those had rather lower amp alternators. (100 amps or less) It is my inexperience about larger boats*. I would have thought much higher amp alternators would handle the load. And be more or less standard on diesels for this kind of boat. I am just surprised more don't have alternators in the 150+ amp range. Yeah they are more expensive and 250 amps or more are possible.

My experience stops at 30 feet with a single gas engine with a refer, lights and a sound system that has a good draw. I am looking at roughly 40 feet boats with twin diesels. And all of the power systems that kind of vessel has. I saw one with 55 amp alternators. It had a generator. Just seemed strange.

 

[Jeff F]

I am looking at roughly 40 feet boats with twin diesels. And all of the power systems that kind of vessel has. I saw one with 55 amp alternators. It had a generator. Just seemed strange.

It's very common. The expectation was that the alternators charge the start batteries and carry DC operating loads, which will generally be under 40 amps. Easy work. The generator handles the house loads.

As long as you're willing to run the generator as needed it's a tried and true system.

 

[twistedtree]

The question is primarily, why run a generator when underway?
Isn't the output of the alternator(s) sufficient to provide enough power?

To elaborate ....
I understand if AC or multiple AC units are running.
Beyond that I am not imagining an electrical load necessary to run a generator when the main(s) are running.

However, I have zero experience with larger boats, with multiple electrical systems.
I have been looking at some and those had rather lower amp alternators. (100 amps or less) It is my inexperience about larger boats*. I would have thought much higher amp alternators would handle the load. And be more or less standard on diesels for this kind of boat. I am just surprised more don't have alternators in the 150+ amp range. Yeah they are more expensive and 250 amps or more are possible.

My experience stops at 30 feet with a single gas engine with a refer, lights and a sound system that has a good draw. I am looking at roughly 40 feet boats with twin diesels. And all of the power systems that kind of vessel has. I saw one with 55 amp alternators. It had a generator. Just seemed strange.

We have large alternators, yet still run the generator occasionally while underway. It's a bigger boat, with full household amenities.

Why big alternators? Mostly to minimize when we need to run a generator. As someone mentioned earlier, the fuel consumption for both is about the same, though I did an experiment a number of years ago that suggested for more moderate loads the alternators are more fuel efficient, yet for heavier loads the generator was more efficient. The big reason to avoid running the generator is to reduce maintenance.

Bigger loads that the alternators enable with are:
- recharging a large house battery bank
- operational loads while underway. Nav equipment is the obvious one, but the bigger power consumers are the ER vent fans
- Ability to run some large AC loads via inverters. These include a water maker, electric dryer, electric oven, and at least partial HVAC.

Why still run a generator? If the AC loads are consistently maxing out the alternators, I'll run a generator to keep ahead. There are also cases where the loads will consistently be above what the alternators can cover, and then I'll run a generator for the duration. Such loads would be:
- Continuous, full power HVAC
- 2 watermakers at the same time to make water really fast.
- Two or more heavy loads at the same time in any combination, like the dryer while also making water or running the oven. We usually try to run those things sequentially, but that doesn't always work out.

 

[twistedtree]

I should also mention that if we didn't have large alternators, we would have to run a generator all the time. Underway, without any special loads, our power consumption is around 100A @24V, so 2.4kw. Our boat size is right around the transition point for boats that require a generator all the time.

 

[mvweebles]

I would think that the charging system (alternators) of a boat would be sufficient to recharge all battery banks and supply enough extra current to operate any 12 volt need when the boat is underway.

Many larger boats are designed with power hungry creature comforts such as multiple fridges. Ice makersz freezers, entertainment systems, networks, and of course air con. The combined house load can easily surpass practical limits of DC charging/inverting, even at 24v. The owners of theee boats accept they are a generator boat - generator hours often surpass engine hours.

Peter

 

[O C Diver]

My boat has one engine with two alternators, one for the engine and a 220 amp alternator for the house battery. When moving day after day, the house bank easily provides enough power from afternoon through departure in the morning. An hour after departure, the alternator has gone from bulk, through absorption, to float, so 90% recharged. In addition to topping off the bank, there's more than enough alternator capacity to do aload of laundry. For me, generator time underway is when I need air conditioning.

Ted

 

[Action]

Thanks so much. The above helps me to understand more.

 

[rwidman]

I would think that the charging system (alternators) of a boat would be sufficient to recharge all battery banks and supply enough extra current to operate any 12 volt need when the boat is underway.

I have seen alternators rated at under 100 amps on a number of twin diesel engines. Isn't that a little light?
I was thinking 150 to 200 amp alternators would be desired.

As long as a main engine is running, wouldn't it be desireable to have the alternators producing enough power for any 12 volt need?

What am I missing?

What you are missing is that charging a battery takes power from the engine that would otherwise be used to propel the boat. A 200 amp alternator would use considerably more power than a typical 60 amp alternator.

Driving the 200 amp alternator would also require more and heavier belts to transfer this increased load. And of course, increased wire size.

 

[rwidman]

Thanks for the replies.

And I forgot a piece. The other piece was running a generator while under way. Why run a generator if both engines are spinning alternators?

Some people have appliances on their boats that require 120 volt (or 240 volt) AC power to work. Air conditioning, large refrigerators, washer and drier, etc. They typically run a genset 24/7 unless they are on shore power.

 

[CharlieO.]

What would be a good replacement drop in option for an alternator on a 120 Lehman? I would like to add a 400 ah house bank charged with a dc2dc charger.

 

[Action]

Many larger boats are designed with power hungry creature comforts such as multiple fridges. Ice makersz freezers, entertainment systems, networks, and of course air con. The combined house load can easily surpass practical limits of DC charging/inverting, even at 24v. The owners of theee boats accept they are a generator boat - generator hours often surpass engine hours.

Peter

Besides the HVAC system(s), would an inverter with big battery banks handle everything else?
You are suggesting not. Which may come into the depends category.
Depends on "creature comforts" that the boat was equipped with or that I added on. As a side note, I have come to realize my boats are like me. They get heavier with age.

Not sure if larger refrigerators in larger boats are 2 way. (110vAC and 12vDC) My small refer in my current boat is made that way. (which was another reason I upgraded the charging system. Balmar)

 

[Iggy]

Action, the other part of your question really depends on your needs.

My last boat was a 30 footer with 4 group 24s for the house. A total of 320 amp hours. Now with a 40 footer, the demands are greater. More lights, bigger fridge, more and bigger electronics and bigger hot water heater. As we all know!

Point being. I measured using a shunt, my total amps for a few days to get an average. Doing the math.......... Do I install a bigger battery bank? I did and went to lithium, 800AHr. How long can my battery bank last? How much does the Alt put out when cruising? How to charge my bow thruster and starting bank? How not to over load the Alt. And more....

From that, when to run the genset? Do I install solar panels? Per hour of cruising, how much of a charge will go into the batteries?

Its all a learning curve!

 

[Action]

Thanks everyone. I believe I have answered my own question.
Just like life, it depends.

 

[mvweebles]

Besides the HVAC system(s), would an inverter with big battery banks handle everything else?

I was aboard a Nordhavn 55 recently with a very large 24v LFP battery bank, a pair of large alternators, and a pair of stacked inverter/chargers. And he has over 2kw of solar panels. Owner claims he could easily spend a couple days on the hook.

So yes, the system does scale, but certainly not economically due to lack of room for adequate solar panels plus complexity of alternators due to cabling and controls. I would not be surprised to learn he has over $40k in the system - plus the cost of generator. And he still struggles to get it running right, even had to divert to San Diego last year to have it worked on.

The off grid solar folks are driving some impressive technology. As cost of LFP batteries decline and advanced in controls and monitoring improve, the feasibility of an all-DC platform will improve. Even with air conditioning. But for the most part, on boats, there are physical limits for solar panels and limits to alternators. There just comes a point where it's easier and more economical to run a generator.

Peter

 

[Action]

Thanks so much.
I am curious about that point. The economics between the two.

But not now. I need to get the yacht first. And technology is likely to change by that point.

 

[O C Diver]

Thanks so much.
I am curious about that point. The economics between the two.

From my perspective, the fuel consumption is essentially the same depending on the efficiency of the generator and the alternator.

The difference is in almost 6,000 hours, there's been zero maintenance on the second alternator. At some point I'll probably replace brushes and bearings.

There would be more maintenance with 6,000 additional hours on the generator. Replacing the alternator would be less than $1K. The generator would probably be $20K+.

Ted

 

[ranger58sb]

Besides the HVAC system(s), would an inverter with big battery banks handle everything else?
You are suggesting not. Which may come into the depends category.
Depends on "creature comforts" that the boat was equipped with or that I added on. As a side note, I have come to realize my boats are like me. They get heavier with age.

Not sure if larger refrigerators in larger boats are 2 way. (110vAC and 12vDC) My small refer in my current boat is made that way. (which was another reason I upgraded the charging system. Balmar)

I think usually ACs, water heater, and resistance cooktops aren't suitable for battery/inverter use.

Our galley fridge and freezer are 110VAC only, Sub-Zero household units. The boat didn't come with an inverter to service those, but we added that. The frost-free feature is nice, compared to the typical AC/DC units we've had in previous boats.

-Chris