Alternator output not constant

[Budawang]

I've noticed when I'm motoring that the output from my alternator briefly drops out every couple of minutes. It only does this for a few seconds before going back to its normal output of 40A. It takes about 10 seconds to increase back to 40 A. I recently connected my alternator to a DC to DC charger as part of a lithium battery installation so it may have something to do with that. The alternator belt is nice and tight. Any ideas what's going on?

[Sailor 99]

Originally Posted by Budawang

I've noticed when I'm motoring that the output from my alternator briefly drops out every couple of minutes. It only does this for a few seconds before going back to its normal output of 40A. It takes about 10 seconds to increase back to 40 A. I recently connected my alternator to a DC to DC charger as part of a lithium battery installation so it may have something to do with that. The alternator belt is nice and tight. Any ideas what's going on?

Do you have external regulation on the alternator?
Overheating?
Overloading.


What is the rating on the alternator. If it is 40 amps then it simply can't run for any length of time putting out 100% 40a is not a lot for an alternator that your using to charge big batteries.



I have seen my 120 amp alternator ramp to 90 -100 amps back when we used to rely on it for recharge when cruising

[Fortytwo]

My MPPT solar charge controller drops charging when my batteries are charged and doesn’t start charging solar until my bank is down 1.5 amp/hrs.
Maybe a similar thing?

[Budawang]

Originally Posted by Dark Horse

Do you have external regulation on the alternator?
Overheating?
Overloading.


What is the rating on the alternator. If it is 40 amps then it simply can't run for any length of time putting out 100% 40a is not a lot for an alternator that your using to charge big batteries.



I have seen my 120 amp alternator ramp to 90 -100 amps back when we used to rely on it for recharge when cruising

The alternator is regulated by the DC to DC charger. It's probably not overheating as it happens soon after the engine is started. I think it is a 100A regulator but now that you mention it I should double check.

I'd like to be able to generate more than 40A but the DC to DC charger is limited to that amount.

Another weird thing I noticed just now is that the tachometer wasn't showing any RPM when I started the engine while on shore power. When I switched off shore power it started to work again....

[smac999]

How is this wired? Alt to engine battery. Then dc to dc from engine to house What dc to dc? I have seen the steering dc to dc overheat. So it would cut out. Therefore the alt would also drop from say 40-10 since its only charging the engine battery at that point. And it would cut back in and out.

Replaced with mastervolt. So much better.

Is this external reg?

Tach cutting out on a balmar reg is normal with full / external charged batteries. Because it stops the alt from charging.

[skipgundlach]

I believe this to be a characteristic of alternators with a tachometer lead out.

when the battery shows as full the regulator says I'm done. That results in no RPM and also the output dropping off.

When your load has taken enough charge that it needs more it re-engages.

The solution to that characteristic is to add a small resistor in line with the tach. Typically 100 ohms should do it.

[stormalong]

Originally Posted by skipgundlach

I believe this to be a characteristic of alternators with a tachometer lead out.

when the battery shows as full the regulator says I'm done. That results in no RPM and also the output dropping off.

When your load has taken enough charge that it needs more it re-engages.

The solution to that characteristic is to add a small resistor in line with the tach. Typically 100 ohms should do it.

A small resistor in line with the tach will do nothing. A small resistor from your ignition switch to the field coil will keep the alternator functioning when the regulator turns off. 100 ohms will provide .12 amps of field current at 12 volts. That should do it but I would not go lower. A larger resistance may be called for depending on how much voltage the alternator produces with that field current. Some experimentation is needed with your particular alternator. And that resistor has to be at least a two watt resistor. Anything smaller will burn up. .12 X 12 =1.44 watts.

[goodoldwoody]

Originally Posted by stormalong

A small resistor in line with the tach will do nothing. A small resistor from your ignition switch to the field coil will keep the alternator functioning when the regulator turns off. 100 ohms will provide .12 amps of field current at 12 volts. That should do it but I would not go lower. A larger resistance may be called for depending on how much voltage the alternator produces with that field current. Some experimentation is needed with your particular alternator. And that resistor has to be at least a two watt resistor. Anything smaller will burn up. .12 X 12 =1.44 watts.

Hi Stormalong

I’m having the same issue of alternator dropping out with fully charged batteries. So I've added various loads to keep it ( the tacho) going. Running fans and so on.
I’ve been wondering about applying your suggested resistor for a long time and have been reluctant to try it because I don’t want to overcharge my batteries. My alternator drops out at 15v so by adding the said resistor may cause it to rise above that then risk damaging electronics plus whacking up to another 10 amps into batteries.
Thanks for your thoughts.

Woody

[Sailor 99]

Originally Posted by Budawang

The alternator is regulated by the DC to DC charger. It's probably not overheating as it happens soon after the engine is started. I think it is a 100A regulator but now that you mention it I should double check.

I'd like to be able to generate more than 40A but the DC to DC charger is limited to that amount.

Another weird thing I noticed just now is that the tachometer wasn't showing any RPM when I started the engine while on shore power. When I switched off shore power it started to work again....

This would be due to the DC to DC system saying it has all the power it needs so it shuts off the field power on the alternator.

In a flooded cell application you never really take off the field. When the regulator sees that it is full it goes into float mod but I don't think you float LI batteries.

You tricking it into float mod as suggested might fix your problem but it might cause problems and I bet that kit did come cheap.

I think a phone call or email to the manufacturer or people you bought it from would be good. This will not be the first time they hear of this problem.

[stormalong]

Originally Posted by goodoldwoody

Hi Stormalong

I’m having the same issue of alternator dropping out with fully charged batteries. So I've added various loads to keep it ( the tacho) going. Running fans and so on.
I’ve been wondering about applying your suggested resistor for a long time and have been reluctant to try it because I don’t want to overcharge my batteries. My alternator drops out at 15v so by adding the said resistor may cause it to rise above that then risk damaging electronics plus whacking up to another 10 amps into batteries.
Thanks for your thoughts.

Woody

Hi Woody,

Your response is exactly why I suggested some experimentation might be needed. For the tach to work the alternator does not need to be producing much power. So, the thing to do try the 100 ohm resistor and see what voltage is produced. If it is too high try 200 ohms. The way to test is to turn off your external regulator by disconnecting it from your ignition switch and try with the resistor. The power produced with a 120 milliamp field coil current is totally dependent on the alternator. Most alternators need 2 to 3 amps to produce significant power.

Brian.

PS. Is this the Woody that used to own Lapis about three decades ago.

[billgewater]

Originally Posted by Budawang

The alternator is regulated by the DC to DC charger. It's probably not overheating as it happens soon after the engine is started. I think it is a 100A regulator but now that you mention it I should double check.

I'd like to be able to generate more than 40A but the DC to DC charger is limited to that amount.

Another weird thing I noticed just now is that the tachometer wasn't showing any RPM when I started the engine while on shore power. When I switched off shore power it started to work again....

Yes. Your last observation is common. Your mains battery charger should be turned off as it can upset the pulse signal from the alternator to the tachometer.
This problem can be intermittent in that if you speed up the engine, the tacho may suddenly come to life.

[sailorboy1]

What regulator is it?

[goodoldwoody]

Originally Posted by stormalong

Hi Woody,

Your response is exactly why I suggested some experimentation might be needed. For the tach to work the alternator does not need to be producing much power. So, the thing to do try the 100 ohm resistor and see what voltage is produced. If it is too high try 200 ohms. The way to test is to turn off your external regulator by disconnecting it from your ignition switch and try with the resistor. The power produced with a 120 milliamp field coil current is totally dependent on the alternator. Most alternators need 2 to 3 amps to produce significant power.

Brian.

PS. Is this the Woody that used to own Lapis about three decades ago.

Hi Brian.....Not the same Woody
Thanks for your info I’ll try that.
Many thanks.
Woody.

[skipgundlach]

Originally Posted by stormalong

A small resistor in line with the tach will do nothing. A small resistor from your ignition switch to the field coil will keep the alternator functioning when the regulator turns off. 100 ohms will provide .12 amps of field current at 12 volts. That should do it but I would not go lower. A larger resistance may be called for depending on how much voltage the alternator produces with that field current. Some experimentation is needed with your particular alternator. And that resistor has to be at least a two watt resistor. Anything smaller will burn up. .12 X 12 =1.44 watts.

Thanks for the correction and clarification.

I'm not an EE so cast about in the dark, repeating a scholarly discussion elsewhere.

Your solution sounds right.

For us, I just put up with it, as our load, while motoring, is high enough that it doesn't last long, and, as well, I've got a musician's ear and so am more attuned, perhaps, than others, to change in pitch (ergo revolutions per minute).

Thanks again.

L8R

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