One engine two alternators.

[Stoaty]

I am posting this here guys as I think you lot would give me a decent answer.

The engine in my boat is a Lister HRW6M. The alternator is as supplied originally a Lucas AC5 24v 35Amp. It has an external controller which I have set to high charge (29.4V). I have two battery banks. Engine start consists of 2x 12v 125AH (125ah @ 24v) and a domestics bank of 4x 12v 110AH (220AH @ 24v) Deep cycle leisure batteries.  After a weekends fishing and a night at anchor somewhere  the domestics will be down to about 50-60% charged. When we get underway the alternator charges about 30 amps and it takes 3-4 hours before it starts to drop. At night with the Nav lights on, radar, demisters etc the alternator may be charging as low as 10 Amps. As most of our journeys are 2-3 hours the output isn't good enough. My solution is to add a second alternator, probably a second hand truck or bus one, running in parallel with the original.  I have plenty of room on the front of the crankshaft for another pulley and would mount it securely to the engine room floor. The way I see it both would charge until the voltage is too high for the second one and the original one would carry on with the final charge phase.

Now people are telling me I can't do this. They are telling me the alternators would fight each other and it would cause spikes (I have sensitive electronics). Others are telling me the alternators have to be synchronised or the world will catch fire.

I can't understand this as we are talking DC power here

Tell me I can do this guys.

[apogee_man]

Why not just replace the Lucas with a higher output unit rather than all the hassle of mounting a second one?

I would think something in the 100 - 150 amp range...

Just my $.02,

Steve

[Stoaty]

I know. But I have this urge to keep the original one on the engine. Once it's off It would never go back on. Also when your 60 miles offshore 2 alternators are better than one.

[westcoaster]

Ford F250/F350 diesel superduty pickup trucks are available over here in North America with dual alternators from the factory.

It can be done. Perhaps a search for a charging circuit diagram for that pickup might shed a bit of light on how Ford does it....

[Quisp]

You said you have 2 battery banks. Why have the original alternator sharge the starting bank, and put a high output alternator to work charging the other? That way there's no worries about them banging heads.

[trigzy]

2 DC Alternators wont fight each other.  Whichever one has the voltage set highest will work the hardest.  On our buses, there are two different types of factory configuarions:

A)  2 Identical alternators (Aircooled, 140A each) with internal regulators run in parallel, each hooked up like the other one wasn't even there.  Voltage regulators are set the same, so that load is spread out roughly the same.  Unless the diode bridge self destructs in the worst possible way, even if one fails, you still have some power.

B) 1 (280A Oil cooled Delco Alternator) set at 28.8V to provide the main charging source.  1 (Aircooled 140A) alternator set at 27.6V, only picks up load if the main alternator is overloaded or fails. 

Either one of these systems could work on your boat.  Use proper marine alternators, fuses on each alternator, and solder and crimp the crimp connections for durability.  Theres rarely such a thing as "wire to big".  If you need more specifics, let me know.

Steve

[mobile_bob]

anytime you use two alternators driving the same battery bank it is wise to use fail safe diodes
fail safe diodes are large diodes one of each on the output of each alternator
this allows both to charge through each diode to the battery, but not back into one another,

normally you won't have a back feed, but
if you have a rectifier bridge fail to short in one alternator, the other alternator will seek the short and be damaged
as well.
the failsafe diode does what it is designed to do, that is keep the backfeed from happening and damageing the one
remaining alternator.

bob g

[mobile_bob]

i also do not like fused alternator outputs, if for some reason you blow a fuse under heavy charge
the alternator voltage will spike to well over a hundred volts and may very well take out your regulator
or diodes in your rectifier bridge.

ymmv

bob g

[Stoaty]

You said you have 2 battery banks. Why have the original alternator sharge the starting bank, and put a high output alternator to work charging the other? That way there's no worries about them banging heads.

That's a lot easier to say then do. The wiring is complex. There are several conduits merging in the engine room eventually ending up in the main fuse box (see below). I don't have a wiring diagram for the boat. I haven't been able to tell which cable in the fuse box comes from the alternator. I plan to wire it in with the shore power charger.

[Stoaty]

anytime you use two alternators driving the same battery bank it is wise to use fail safe diodes
fail safe diodes are large diodes one of each on the output of each alternator
this allows both to charge through each diode to the battery, but not back into one another,

normally you won't have a back feed, but
if you have a rectifier bridge fail to short in one alternator, the other alternator will seek the short and be damaged
as well.
the failsafe diode does what it is designed to do, that is keep the backfeed from happening and damageing the one
remaining alternator.

bob g

Would a fail safe diode cause a voltage drop and thereby prevent the batteries from fully charging?

I may already have one. In the picture below, bottom left is a black object with two blue and two yellow wires on it. I have no idea what it is. We are going down the boat today for a long weekend I will get some better pictures.

[jzeeff]

i also do not like fused alternator outputs, if for some reason you blow a fuse under heavy charge
the alternator voltage will spike to well over a hundred volts and may very well take out your regulator
or diodes in your rectifier bridge.

On the other hand, not using fuses/circuit breakers has some serious issues also.  So what is the solution?  Perhaps zener diodes or MOVs to absorb the spike? 

The same issue occurs at home when you have inductive loads and then the circuit breaker trips.  Other devices on the circuit may not survive.

The rust at the top of the box reminds me what it was like to maintain wiring on a boat.  I soldered where I could and used LPS3 on other connections.

[mobile_bob]

voltage drop across the fail safe diodes affecting charge rate?

yes, you will need to either remote sense or turn up the controller to offset the FSD voltage drop


after seeing the breaker box, not sure i would want to get involved with this project at all.

the alternator power cable should go directly to either the battery or the switchgear that directs to either of two banks
but not any great distance in my opinion, and not through some cumbersome box with other circuits, again in my opinion.

going some distance is going to have as much or more voltage drop than a FSD at higher amperage.

looks like you might wanna think about getting an expert to take a look at what you have, draw up a schematic for you
and access what the hell you have there,, looks like frankenboat to me.

point being, i sure would not want to go to sea with a system such as that without a good schematic and a better understanding of the system.

as for the second alternator, that makes perfect sense to me, redundancy seems prudent for something very remote like being out at sea or on the moon,, which being out at sea is about as close to as one can get as far as being remote and not able to run down to the corner parts store.

if you decide to go the 2nd alternator, maybe connect it to go direct to the batteries and work only at specific times? such as in a failure at sea?  or maybe its time to just upgrade the original to something much larger as steve suggests,, there is a time to make upgrades
and this might be one of them, just save all the oem stuff for putting back to original if you like later. i am all for keeping things stock, but
when it comes to safety, you gotta draw the line somewhere?

good luck

bob g

[jzeeff]

If you get an alternator with remote sense, you don't need to worry about voltage drops (from wire or from diodes).  But I wouldn't turn up a voltage to account for a voltage drop - the voltage drop is proportional to current flow, so when you are drawing low current, the voltage will then be too high.

[mobile_bob]

the cabling should not be heating in the first place, so change due to amperage should be quite small

so turning up the voltage will not have a marked affect on voltage at the battery from high current to low current
if however it does, you gotta look at why?  too small cable guage or some faulty connection somewhere.

this is why i am hesitant to give advice on this application, who knows what exactly he has there?
without knowing what he has there is no way of making a recommendation, and without being able to inspect
the final installation??? well, it seems to me a good way of setting this thing up for problems somewhere down the road...
or out to sea.

he needs professional guidance here, on site to access exactly what should be done in my opinion.

i know i don't want to be responsible for a fire, injury or worse.

bob g

[apogee_man]

Stoat,

I have worked on many boats and that is one of the nicer electrical boxes that I have seen.

Certainly, it would have been better if they labeled things, but still very nice imho.

You just need to turn off all the power (both AC and DC) and tone back to the box using a clip on tone tester.  Should make tracing the circuit very easy.

Guys, his wiring is pretty much standard in the boating world.  Odds are, his engine harness goes back to the box shown as a tie point, then goes up to his instruments both in the pilot house and on the bridge (if it has one).

They would have run back to the main junction box so his ammeter works to indicate his charging.

Also, on a boat, they will not run a lead straight to the batteries or starter like on a car or truck, because they need to be able to shutoff ALL the power with the battery disconnect switch(es).  They will usually feed a box like this directly from the battery disconnect switch(es).

I certainly wouldn't be afraid of the wiring that I see in the pics as that box is one of the nicer that I"ve seen.  It's NOWHERE near a frankinboat and I can say that with 100% certainty because I've worked on more than a few...

The original builder of the boat may have copies of the schematics.  They are pretty good at keeping that info available and would certainly be worth a phone call to check.

Regards,

Steve

PS - this will give you an idea of the tester that I'm talking about.  Home Depot should have something similar.  This one is expensive but you should be able to find one for less money.  Very handy to have around!  I use mine for all sorts of stuff...  Need to make sure you have ALL of the power turned off though!!!

http://www.professionalequipment.com/tempo-classic-tone-probe-kit-701k/telecom-tools/