Twin alternators

[boatmike]

It is quite common to have twin alternators when there are twin engines. However, most setups I have seen have twin engine batteries with one alternator to one battery. However, there is no real need for twin batteries if you have a good single engine start battery with enough cranking capacity and a seperate service battery that can be switched in if it goes flat. In this instance, assuming you have a smartcharge and smartbank fitted with the relay fitted between the two banks, it is claimed that you can effectively parallel the two alternators. In any case, even with a two engine start battery plus service batteries the alternators will get paralleled when both are charging the service batteries which is usually the biggest load on the alternators regardless of which system you have. However, reading literature on paralleling twin alternators it ain't that simple and I see systems where each alternator feeds a diode splitter and the diodes are "commoned" on the battery side to stop the charge from one alt feeding back to the other. My inclination is to fit a splitter on each and feed one side of both to the service battery and the other to the engine battery with the relay of the smartbank/smartcharge between them. Does this make sense? I think not having the splitters might avoid voltage drop, but as the alternators are rated at 14 volts that should not be a show stopping issue but do I need them? Note, this in a MoBo with engines running all the time and a seperate onboard generator so maximum charge rate to batteries (open lead acid) may not be as important as a sailboat..... Opinions please!!

p.s. Smartbank/Smartcharge already fitted and diode splitters already exist from a previous setup. There is no cost issue. The question is do I need the diodes or not with twin alternators in this setup or should I connect them direct.

 

[lw395]

I think the diode splitters are more to stop the 'domestic' loads flattening the start batteries than to protect the alternators from each other.
Voltage drop will matter, compensating for the diode voltage will mean your battery gets from half to 90% charged a lot quicker.
You could consider a voltage switching relay system to avoid the drops.

In my simple view, two start batteries with nice short wiring to each engine sounds nice, simple and (almost) fool proof.

 

[pvb]

p.s. Smartbank/Smartcharge already fitted and diode splitters already exist from a previous setup. There is no cost issue. The question is do I need the diodes or not with twin alternators in this setup or should I connect them direct.

You don't need diodes, indeed they will reduce the charging voltage.

I've heard of Smartbank, but what is Smartcharge?

 

[boatmike]

You don't need diodes, indeed they will reduce the charging voltage.

I've heard of Smartbank, but what is Smartcharge?

Sorry, that was me. Its a smartbank advanced linked to smartgauge. Seems to work very well. No such thing as smartcharge!

You seem to agree with Merlin that the alternators can just feed a common terminal. Where I am coming from is industrial alternator sets that need quite sophisticated crossover networks to parallel. Do you agree that with simple DC alternator output the built in diodes in the output side stop one alternator screwing up the controls on the other then?

 

[pvb]

Do you agree that with simple DC alternator output the built in diodes in the output side stop one alternator screwing up the controls on the other then?

Yes.

 

[halcyon]

Sorry, that was me. Its a smartbank advanced linked to smartgauge. Seems to work very well. No such thing as smartcharge!

You seem to agree with Merlin that the alternators can just feed a common terminal. Where I am coming from is industrial alternator sets that need quite sophisticated crossover networks to parallel. Do you agree that with simple DC alternator output the built in diodes in the output side stop one alternator screwing up the controls on the other then?

With 2 alternators you will get hunting, once you reach regulation voltage. The alternator with the lower regulation voltage will stop charging, increasing load on the one still running. This will cause a drop in output voltage, and the first one starts charging, then goes through the cycle again. But not a problem, we had twin alternator units running this way since 1983 with no long term problem.


The posh answer is to run both alternators from a single regulator.


Brian

 

[boatmike]

With 2 alternators you will get hunting, once you reach regulation voltage. The alternator with the lower regulation voltage will stop charging, increasing load on the one still running. This will cause a drop in output voltage, and the first one starts charging, then goes through the cycle again. But not a problem, we had twin alternator units running this way since 1983 with no long term problem.


The posh answer is to run both alternators from a single regulator.


Brian

That is in line with what I had read in fact Brian. I think there is a way of organising the posh answer by linking regulators but I am not sure how to do it. The Prestolites I have have external regulators and I have e-mailed Prestolite to see if this can be done relatively simply..... While I think as you say you can run both the way you and pvb suggest it would be good to get a total 180Amps........

 

[pvb]

...it would be good to get a total 180Amps........

What are you going to do with 180A? You'll need a very big domestic battery bank to absorb that.

 

[boatmike]

What are you going to do with 180A? You'll need a very big domestic battery bank to absorb that.

800amp hours

 

[William_H]

What are you going to do with 180A? You'll need a very big domestic battery bank to absorb that.

This comment is misleading. The alternator capacity is a max or limiting value. The batteries do not need to "absorb" the power rather they take current as they need depending on battery charge state and capacity.
The 2 alternators will probably mostly jog along supplying about 5 amps each. With better alternator life at lower current.
Alternators in their internals actually have diodes so an alternator connected permanently to a battery will not discharge the battery. 2 alternators paralleled to one battery will not affect one another. What does happen is that the 2 regulators sensing one battery will interfere with one another. ie one regulator first senses low batt voltage so gets it's alternator to increase output. This will mean that the other alternator regulator seeing more voltage will not increase output so you can get an inbalance. If the most active alternator reaches its capabilities then the other will back it up by increasing output. Not necessarily a problem but twin engined small aircraft usually have a balancing regulator. Try an aircraft electrician at a small airport for more info. regards olewill

 

[boatmike]

I think all of that is true and I am hoping my e-mail to Prestolite will provide the answer.
Also in the med my boat will be like a greenhouse and I am hoping this alternator output will allow me to run the aircon through an inverter with the engines running without running my gennie.

 

[halcyon]

The 2 alternators will probably mostly jog along supplying about 5 amps each. With better alternator life at lower current.

The voltage you see is set by the battery, not the alternator, this only limits the maximum voltage.

The reason we originally did the 3 bank VSR was to increase available charge current, back in 83 32 amp alternator was big . But also the domestic load, lights,fridge, navigation equipment used up power, and in some cases dramatically reduced battery charge levels. By linking both alternators, it gave capacity to cover domestic load, and battery charge until available alternator voltage was reached. At which point the alternator with the lower regulator voltage would start cutting out, leaving the higher voltage alternator to do the work, which was now lower amperage. Though as said you get hunting at this point with the lower voltage alternator cycling in and out.

Running two alternators from one regulator I'm not happy with, yes it gets round hunting, but you loose redundancy if one fails. This was one of the original reasons for the 3 bank VSR, if one alternator fails, the other will cover all three battery banks.

If you run a bow and or a stern battery bank, than that is another story.

End of the day it's swings and round-a-bouts.

Brian

 

[boatmike]

Halcyon, can you please clarify. It seems to me that the alternators will both run at 14volts. I understand that the amperage they supply is related to the state of charge of the battery. Are you saying they will both run normally until the batteries are near maximum charge and showing 14 volts and the hunting phenomena only starts to be an issue at that point?
I have aircon that pulls about 4 amps at 240v which is approx 960 watts. This equates to a load of around 80-90amps 12v through an inverter dependant on efficiency (if I have that right) so with 2 90amp alternators running I should get about half load on them with the engines running and the aircon on. In other words the batteries will still get charged with the engines running and aircon on and have load to spare for other things while underway. Do you agree? If so hunting when at maximum charge with healthy batteries is no problem?

 

[halcyon]

Halcyon, can you please clarify. It seems to me that the alternators will both run at 14volts. I understand that the amperage they supply is related to the state of charge of the battery.

If you take away the regulator the voltage will carry on going up, as per a alternator smart controller. Which increases charge voltage by by-passing the internal regulator.

The alternator produces a AC output made up of around 9 pulses per rev, these are fed through a diode array to produce a pulsed DC that charges the battery.

The alternator is rated at watts at a nominal 14 volt into a resistive load, now they divide the watts by 14 to give amps as it is better marketing. It only limits amps to limit voltage, it does not generate amps to suit the battery level, the battery excepts amps depending on state of charge and available volts.

The voltage you see is generated by the battery, and it's charge level, as the battery level increases the voltage required for the chemical conversion increases. A bit like a big variable resister that increases with recharge level, so a flat battery is around 13 volt, 80% recharge 14.4 volt. If you recharge at a high level due to chemical conversion problems, the voltage will rise faster, but not the recharge level.

To stop the voltage going above the regulation voltage, the regulator stops the earth to the coil in the rotor that generates the magnetic field, that in turn kills the output. Thus you have no amps, so battery voltage falls, and the regulator starts conducting, restoring the magnetic field and the output current. This starts charging the battery again, and the voltage increases until it exceeds the regulation voltage and coil earth is again cut. So the regulator controls voltage by cutting amperage, this could be for one pulse, a with a full battery a few seconds.

With the two alternators they are not going to have identical regulation levels, so the low one will cut out first. This increases load on the remaining one, so voltage will fall, thus the low cuts back in. This will carry until the higher one can maintain the voltage at the load, at which point only one alternator will work.

A battery is basically a capacitor, and you will see a higher voltage than if you had a resistive load. So with the inverter running you could see a lower voltage that could not reach the regulation point, it all depends on type of load and alternator size.

The problem with it all is that a battery is not a bath, that you just fill with amps from the alternator.

Brian

 

[boatmike]

Thanks for that Brian. I have read your explanation carefully several times and I believe the light is dawning.

Interestingly I have now had an answer to my e-mail from Prestolite which confirms that the two alternators will load share without problems so I think we are OK as we are.

Again this forum has been a great help and thanks to everyone who contributed. Your time and interest is much appreciated.