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Hi all,

Been boating for around 6 months all been going well, until a week ago, when I notices the Domestic batteries where not getting charged. Running a Beta 38 with dual alternators and twobattery sets, single starter and four domestic batteries, these are not linked in any way. The started runner circuit seems to run OK.  So I did the following:

  • Checked alternator output, 10v - so not good, so I ordered a new alternator 100Amp 14.1V.  This also was confusing as I would of expected to at least see the battery voltage across the alternator.
  • After replacement, still only getting 10V (slight panic as the new alternator was £150)
  • Got on blog and someone mentioned isolator switches, but both mine where replaced when I changed to position of the isolator switches about 6 months ago, and  they where not cheap !!!!
  • As a test, I bypassed the isolator on the domestic side and now get 12.8V from alternator (was expection 14.xx) seems like battery voltage......Oh dear
    • so the alternator seems to be working but not at the correct voltage  - did it get fried due to a faulty isolator ?
    • the batteries are getting some change, but not whats needed to get them to a fully charged situation.
  • I check both isolators, ie domestic and started, and both seem to be intermittent, sometimes they work, other times not, and pushing the key in seems to affect their operation.  Seems like both need to be replaced.

Just got back from boat, slighty frustrated, but slighty happy about the fact that the isolator switches seem to be the issue, but worried about alternator output.

Questions:

  • Did I fry the alternator, due to a faulty isolator switch, I have the original which I have go repaired and was going to use as spare
  • Why only 12.8V. is this linked to above and/ is this the battery voltage showing
  • Any suggestions on a good make of isolator

hope that makes sense.

Kev

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Intermittent isolator switches might just need to be stripped down and the contacts cleaned.

If the alternator has been run without a battery connected then that could certainly cause damage; without anywhere for the current to go, the voltage can rise out of control and blow things.

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8 hours ago, Giant said:

Intermittent isolator switches might just need to be stripped down and the contacts cleaned.

If the alternator has been run without a battery connected then that could certainly cause damage; without anywhere for the current to go, the voltage can rise out of control and blow things.

Not quiet true. It is if the battery is disconnected while the alternator is running & charging but if the switch is intermittent that could well have happened.

The most important measurement we need is the charging current when you measured 12.8 volts.

The closer you get to maximum alternator output the lower the charging voltage. As the batteries charge so the current falls the voltage rises until it reaches the regulated voltage setting.

If the domestic batteries are really flat (as 10V would suggest) then the charging voltage will be low for a while but then gradually rise so set it charging for several hours. Run the engine  at whatever speed gives the highest ammeter reading and gradually reduce the speed as the curent falls or run at between 1000 and 1200 rpm.

Having gone through all that 12.8 does seem a bit low to me but try a good long charge first.

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2 hours ago, Tony Brooks said:

Not quiet true. It is if the battery is disconnected while the alternator is running & charging but if the switch is intermittent that could well have happened.

True, the battery needs to have been connected initially to have some current flowing - it's when it becomes disconnected whilst running that's the risk. But as you note, that's exactly what can happen with an intermittent switch, especially under engine vibration.

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Yes unfortunately an intermittent isolator could well have fried the diodes in the alternator. There are some pretty awful isolators out there. Tell us your specific make and model of isolator and we will be able to tell you if it's any good - or not. Good makes are Blue Sea Systems, BEP Marine amongst others.

  • Greenie 1
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We are on our 3rd domestic isolator. The first was unmarked, the second was rated at 300 amp continuous so should have been ok for a Victron 3000 watt inverter but didn't last as long as the unmarked. We now have a Blue Sea one rated at 500 amp.

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Just wondering - most / many  /some alternators (especially higher output types) need a sense wire connected, usually via a signal lamp, else it won't give any output.

? worth checking.

If you have the make and model number of the alternator, it would help.  

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I don't think you are talking about a sensing wire. A sensing wire goes to the battery or very close to it so the alternator can detect and compensate for any voltdrop between its output terminal and battery.

The wire with the signal (warning) lamp in it is used to provide the initial excitement by producing a small magnetic field in the rotor. If this was not done in most cases the residual magnetism in the rotor would not produce enough voltage to "open" the diodes do the alternator would not charge. Once its charging the excitation current is provided internally in most alternators.

 

At this stage, rather than worrying about the possibility of blown diodes, I would think the OP is best advised to run the engine at charging speed for an hour or so to see if the voltage starts rising. If it does keep charging and see how high it gets. If it does not then its time to think about diodes.

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Hi All and thanks for the speedy feedback.

The alternator is a iskra 100A 14.1V will check the model number tomorrow when back on boat

The more I read your comments I'm almost certain that the intermittent isolator switch has fried the alternator diodes.  

But I'm going to try Tony's suggestion of running the system for an hour or so and then see if we get any improvement in voltage.   In the mean time i'm searching for a local alternator repair shop !!!!!

Looking at the blue sea isolators I see there is an option for Alternator Field Disconnect (3001) which protects the diodes, is this a good route to take to add another layer of protection for the alternator ?

Again Thanks for the guidence, feeling at lot more confident.

Kev

Edited by Kev Challen
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Sadly, if it turns out that the Alt has been damaged by a faulty isolator, it once again reinforces my (and others) opinion that the BSS is wrong, and alternators should bypass the isolators. It doesn't help the OP of course. 

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I would say that AFD is not really worth it. That type of isolator is not easily knocked accidentally to the off position. So you are protecting against an intentional switching off of the isolator whilst the engine is running. Seems unlikely unless you are in the habit of lending your boat to numpties! Having AFD doesn't mean that a repeat of the same situation can't occur - if the isolator did go intermittent (which is highly unlikely for that quality of product) then the AFD isn't going to help. Also with the AFD you have more wiring, more chances of a bad connection etc isolating the alternator field current inadvertently. And presumably more cost!

1 minute ago, WotEver said:

Sadly, if it turns out that the Alt has been damaged by a faulty isolator, it once again reinforces my (and others) opinion that the BSS is wrong, and alternators should bypass the isolators. It doesn't help the OP of course. 

Ours does.

 

OOps - did I say that out loud?

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In motorsport cars are required to have a big battery isolator switch that can be reached by marshalls to quickly disconnect the electrics after a crash. If the engine is still running then the alternator can be damaged in the same way as a boats alternator can when a spinning alternator suddenly has no load. Some motorsport isolators have an extra set of contacts that connect the alternator to a resistor to dump the output to earth when the isolator is operated. Never heard of one of these being used on a boat. Possibly due to the current rating of the main switch not being high enough for house batteries, but they should definitely work for an engine battery and alternator in a split system. An example in the link below. Any thoughts? Why don't boats use this? Boat safety scheme? Cost? No ones thought of it?

Jen

https://www.pegasusautoracing.com/document.asp?DocID=TECH00109

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13 minutes ago, Jen-in-Wellies said:

In motorsport cars are required to have a big battery isolator switch that can be reached by marshalls to quickly disconnect the electrics after a crash. If the engine is still running then the alternator can be damaged in the same way as a boats alternator can when a spinning alternator suddenly has no load. Some motorsport isolators have an extra set of contacts that connect the alternator to a resistor to dump the output to earth when the isolator is operated. Never heard of one of these being used on a boat. Possibly due to the current rating of the main switch not being high enough for house batteries, but they should definitely work for an engine battery and alternator in a split system. An example in the link below. Any thoughts? Why don't boats use this? Boat safety scheme? Cost? No ones thought of it?

Jen

https://www.pegasusautoracing.com/document.asp?DocID=TECH00109

No reason not to use it, but then again not much reason to use it. The better solution is to not turn off the isolator when the engine is running! Or use an alternator field disconnect contact as mentioned earlier.

And actually, thinking about it, if the resistor has a low enough resistance to take say 100A without the voltage rising to more than say 20v, that means that at 14v it will still be taking a lot of current and dissipating a lot of heat. So it would have to be very large and/or get very hot to deal with the case where the isolator was switched off and the engine continued to run (maybe for hours, until someone noticed).

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24 minutes ago, nicknorman said:

No reason not to use it, but then again not much reason to use it. The better solution is to not turn off the isolator when the engine is running! Or use an alternator field disconnect contact as mentioned earlier.

And actually, thinking about it, if the resistor has a low enough resistance to take say 100A without the voltage rising to more than say 20v, that means that at 14v it will still be taking a lot of current and dissipating a lot of heat. So it would have to be very large and/or get very hot to deal with the case where the isolator was switched off and the engine continued to run (maybe for hours, until someone noticed).

Good point. On a petrol engined race car the engine will stop within seconds of the isolator switch disconnecting the ignition. On most diesel boats the engine would continue running, so the resistor might be dumping alternator load for hours, rather than seconds. With an electric stop solonoid you could rig up something clever with relays to automagically stop the engine, but it is all getting rather complex for something that will hardly ever occur.

Jen

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3 hours ago, Kev Challen said:

Hi All and thanks for the speedy feedback.

The alternator is a iskra 100A 14.1V will check the model number tomorrow when back on boat

The more I read your comments I'm almost certain that the intermittent isolator switch has fried the alternator diodes.  

But I'm going to try Tony's suggestion of running the system for an hour or so and then see if we get any improvement in voltage.   In the mean time i'm searching for a local alternator repair shop !!!!!

Looking at the blue sea isolators I see there is an option for Alternator Field Disconnect (3001) which protects the diodes, is this a good route to take to add another layer of protection for the alternator ?

Again Thanks for the guidence, feeling at lot more confident.

Kev

To get field disconnect to work you will probably have to have the alternator modified so an external switch can break the feed to the rotor. It was easy in the days we had separate regulators but now they are part of the alternator, often with the brushes attached makes it more difficult.

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It would help if you could use a clamp a meter on the alternator power out lead as well as measuring the voltage.  As already mentioned if at 12.8 volts you are getting 100 amps then the issue is you have flat or dead batteries and no amount of fiddling with new alternator or isolators will fix it.

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10 hours ago, Tony Brooks said:

The most important measurement we need is the charging current when you measured 12.8 volts.

9 minutes ago, Detling said:

It would help if you could use a clamp a meter on the alternator power out lead as well as measuring the voltage.  

Or... 

6 hours ago, Tony Brooks said:

I would think the OP is best advised to run the engine at charging speed for an hour or so to see if the voltage starts rising.

Which it would appear that OP is going to try: 

5 hours ago, Kev Challen said:

I'm going to try Tony's suggestion of running the system for an hour or so and then see if we get any improvement in voltage.

:)

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7 hours ago, WotEver said:

Sadly, if it turns out that the Alt has been damaged by a faulty isolator, it once again reinforces my (and others) opinion that the BSS is wrong, and alternators should bypass the isolators. It doesn't help the OP of course. 

Mine do as well, always have done on every boat I have owned, its the only sensible way to wire it. My cables are of a length that they can go via the switch for a bss examination.

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