Generator Alternator Configuration

I would like to hear some opinions of an unusual setup I am considering for the generator and it's alternator.

Current configuration:
The main engine alternator currently charges the starting and house battery bank through an isolator. The onboard battery charger also feeds through this isolator, the charger is never on while the engine is running. There is a battery selector switch enabling the house bank to start the main engine. The boat has a 24v stern thruster with and independent bank and charger, I believe there is also cabling and battery switch allowing the thruster bank to start the main in an extreme emergency (I need to double check this next time I'm at the boat). Ideally the thruster batteries be reconnected into parallel rather than series prior to using it however in this sort of situation the potential damage to the starter may be worth it (depending on the situation).

Considered configuration:
The 4.5kw generator to be installed will share the starting battery for main engine (single screw). What I am considering is running the output from the generator's alternator (small, about 30-45 amp) to the battery isolator and putting it's field wire on a switch so it will only charge when the generator is running but the main is not and the battery charger isn't running either. The sensing wire would go on the house battery bank. My rational is that it doesn't make sense to me to make ac power with the generator only to run it through a battery charger for dc power to maintain the batteries, better to use the alternator and free up ac capacity for other things. An alternative solution would be run the alternator output through a smart battery isolator which would charge the starting battery first and it would then charge both banks once the starting battery is satisfies, this would be simpler to operate and avoid someone having to remember the field wire switch but I don't know how much I trust these smart battery isolators and if would fail in the closed circuit position.

The boat should not have a significant dc draw when at anchor and the house batteries shouldn't be require much charging after a day's running. There is a small inverter that can run the coffee maker or TV but it is rarely used. The boat is primarily used during summer in a humid environment so the generator will be run frequently to power the ac and occasionally the hot water heater.

Any thoughts, am I over complicating things?

There are a number of issues in your post. Rather than just giving you my opinion on the issues, let me discuss the whys:


1. Isolators are not the best solution to charge distribution. They have a voltage drop and unless you have a separate voltage sense wire for your alternator's output, it won't charge effectively. Combiners, ACR, Echo Charger, Duo Charger are much better.


2. There is no problem with multiple charging sources: shore power, propulsion alternator, genset alternator, etc all connected to the same load. They will co-exist just fine.


3. Genset DC alternators are often crap: poor regulation and poor heat rejection for continuous maximum output. So you don't want the alternator feeding a heavy load like discharged house batteries.


4. As a result and maybe it is more to save money or complexity, some gensets have no DC alternator. They depend on the shore power charger to keep the batteries charged when it is running.


So given the above and not trying to rewire/configure your entire DC system, I would simply feed the alternator's generator output to its starting battery. That should limit the load to simply replacing the starting amp hours and should co-exist fine with the other charging sources.


David

Thank you for the detailed response. I will have to review the existing charging configuration and consider and upgrade. The sensing wire from the main's alternator goes to the house bank now. It has been doing the job fine so far but the boat has primarily been used for day trips without prolonged house loads. The PO left the shore powered charging on all of the time and rarely if ever anchored out. So far we have been using the boat in much the same fashion with the exception of leaving the shore charger on all of the time but this coming season it will be used for more trips with greater demand on the house bank. I don't anticipate the house batteries ever being heavily discharged but I guess I should reconsider the loads it will realistically see.

Thank you again.

djmarchand said:

There are a number of issues in your post. Rather than just giving you my opinion on the issues, let me discuss the whys:


1. Isolators are not the best solution to charge distribution. They have a voltage drop and unless you have a separate voltage sense wire for your alternator's output, it won't charge effectively. Combiners, ACR, Echo Charger, Duo Charger are much better.


2. There is no problem with multiple charging sources: shore power, propulsion alternator, genset alternator, etc all connected to the same load. They will co-exist just fine.


3. Genset DC alternators are often crap: poor regulation and poor heat rejection for continuous maximum output. So you don't want the alternator feeding a heavy load like discharged house batteries.

So given the above and not trying to rewire/configure your entire DC system, I would simply feed the alternator's generator output to its starting battery. That should limit the load to simply replacing the starting amp hours and should co-exist fine with the other charging sources.
David

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I agree completely w/ David's (usual thoughtful) response.
I agree charging start or house batt'ys are better done via modern multi stage shore powered batt'y chargers than from an alternator unless you have a sophisticated regulator / system.

I posted an article comparing various batt'y combiners in the Library Misc section - Battery Isolator Comparison - Sailing Today Oct 20

In 2015 I reconfigure my Mainship charging system along the lines David suggests and am very satisfied. I eliminated 2 isolators Mainship installed at the factory - 1 in main engine alternator output - 1 in 110V batt'y charger output. I did add a Sterling CVSR to handle charging & isolation of house / thruster banks from the min eng alternator.

My gennie batty is now charged by gen alt only and so far no problem - back-up can be a set of jumpers or a batty switch to allow emergency connection to another source. Running a shore power charged from 110V gennie output for house / start batt'ys makes sense to me as the amperage used is fairly small compared to most gen capacities and you get the improved multi stage charging... which is better for the batteries.

Just my thoughts based on a somewhat similar arrangement.

I would not share a starting battery with the main engine. I would (and do) have a separate battery only for generator starting. Hooked up to nothing else.
That way if (and when) you discharge too deep the gen battery is always ready to either start the gen to recharge the main battery, or to jump start the main engine.
I have been there once or twice.

jleonard said:

I would not share a starting battery with the main engine. I would (and do) have a separate battery only for generator starting. Hooked up to nothing else.
That way if (and when) you discharge too deep the gen battery is always ready to either start the gen to recharge the main battery, or to jump start the main engine.
I have been there once or twice.

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What he said. Let the AC charger keep it charged.

Then I'd install a battery on/off switch wired from the gen starting battery to the main starting battery. That way you have instantly jump the two together if one goes flat.

Thanks again to all, I'm sure you've saved me from future headaches.

Nothing wrong with main and gennie sharing a starting battery. I have set up many that way with no issues. The starting battery is dedicated to starting only and has no house loads to flatten it. Keep a manual cross tie switch so you can use house bank if needed to start either.

Also common practice to disable the gen DC charge circuit and let the house AC charger do the job.

Ski, your way will work. That is exactly how I've had it on a few vessels. But for a belt and suspenders approach many builders and owners prefer the genset with its own alternator and start battery. When in remote spots a stand alone genset is pretty comforting.

"I don't anticipate the house batteries ever being heavily discharged but I guess I should reconsider the loads it will realistically see."

Now is the time to install a SOC , state of charge meter.

Just guessing at batt loads does not work because many items can not be guessed realistically , like refrigeration. Or a light in the head on for a day or two.

"2. There is no problem with multiple charging sources: shore power, propulsion alternator, genset alternator, etc all connected to the same load. They will co-exist just fine."

This is true BUT the outputs are only additive with almost dead batts.

Usually what ever has the highest V output will feed , and the rest of the charge sources will be idle as they see the higher voltage and do not charge.

For any system , contemplate FAILURE.

If one batt goes dead ?
If one electronic combiner dies?
If a switch is inadvertently left on , or switched on at the "wrong " time?

Contemplate multiple failures for a true bulletproof setup.

The boat does have a Clipper BM-2 State of Charge Meter that can measure both starting and house batteries. It will be helpful in getting actual data for the house loading.

The refrigerator is dual voltage and runs off of ac when it is available, which would be the case with the generator running. That said it would only take that ac circuit tripping or the generator shutting down to start pulling dc amperage, that certainly could happen.

The bulk of my boating experience on this size boat comes from 3 different styles of boat with dramatically different power management schemes.

1) Mid 30'-40' sailboats with stock charging from the auxiliaries. These are racer/cruisers rarely motored and house batteries were used sparingly. Cooking is propane fueled and refrigeration (if any) was cold plates supported by a belt driven compressor on the auxiliary. Most sailing trips are 3-5 days and between motoring into a marina or anchorage or running the aux a little above idle for an hour, the batteries kept up with the navigation electronics and lighting loads.
2) Chesapeake dead rise styled charter boats with sparse accommodations, the most significant house battery loads these boats saw were fishing at night with spreader lights, sounder and gps left on (sometimes the radar at anchor) and a lectra-san running every time a customer drained his used beer. This would be for a max period of 6 hours, and the boat did not have any provision for shore powered charging. Stock charging from the internally regulated stock alt on the cta cummins.
3) Custom convertible with twin mains and generator. This boat was run charter fishing in the bay and offshore for 12hour day trips and occasional overnights. The generator ran anytime the boat was away from the dock and we used ac power like gluttons. The reverse cycle a/c units and residential refrigerator ran at all times as well as the dc charger. Power conservation was never considered.

Question-I started my generator and saw that I was getting a reading of 11.5 volts on the alternator meter. I think I had the house battery charger on at the time. Will that make the generator alternator not charge?

ddalme said:

Question-I started my generator and saw that I was getting a reading of 11.5 volts on the alternator meter. I think I had the house battery charger on at the time. Will that make the generator alternator not charge?

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Depends on a couple of things, first does your house charger feed your generator battery as well? If not it would have no impact at all. If it does than I would expect the voltmeter to read more than 11.5 regardless of the source of the power. If the generator battery is completely stand-alone I would check your wiring connections at the alternator, particularly the field wire and what controls it.

The general consensus of this discussion seems to be to either leave that starting battery separate and let the small generator alternator service it alone OR use a quality-multi stage AC supplied charger to maintain all of the battery banks.

ddalme said:

Question-I started my generator and saw that I was getting a reading of 11.5 volts on the alternator meter. I think I had the house battery charger on at the time. Will that make the generator alternator not charge?

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I agree with what GDavid said above. More definitively if you measured 11.5 volts which is pretty near a dead battery, the house battery charger was not supplying voltage to that battery and the generator alternator was not putting out anything.


If either were connected and supplying charging, the voltage would be 13 or more.

So the house battery charger is probably not connected to the genset battery and the genset alternator isn't working if it is connected to the genset battery.

The remaining big question- how did you start your genset with a battery that reads 11.5 volts? It probably should not have started.

Methinks you have some wiring or switch setting problem.

David

the gen set battery started the gen fine. The readings that I'm getting is on the gen panel volt meter. Next time I'm down to the boat, I'll test the battery with my volt meter and see if I can figure it out being gauge or battery or generator alternator. I just love getting a nice ER yoga workout.

I support having a separate start battery for the genset. It got me out of trouble early in my ownership of this boat. Install a dedicated regulated small solar panel for the battery too.