How much does it take to damage alternator?

[Oscarpop]

Our boat has a slight flaw in the design.

From the helm it is easy to knock the ignition key of our yanmar engine into the off position while keeping the engine running.

I have heard that this can damage the alternator.

However, can you please advise how quickly this will happen, and what the symptoms of a failed alternator would be.

thanks.

 

[Talulah]

Our boat has a slight flaw in the design.

From the helm it is easy to knock the ignition key of our yanmar engine into the off position while keeping the engine running.

I have heard that this can damage the alternator.

However, can you please advise how quickly this will happen, and what the symptoms of a failed alternator would be.

thanks.

Turning the ignition key off won't damage the alternator.
It's when the engine is running and someone disconnects the charging wire that damage is done as the generated charge has no where to go. This is sometimes achieved by turning the main battery isolator off depending upon how the system is wired.

Sympton of failed alternator is battery light remains illuminated on engine panel when engine is running.

I should add that turning off the switch happens all the time on school boats.

 

[Norman_E]

I did that once to my Yanmar, and no damage resulted, possibly because the engine was just ticking over in neutral, so that the alternator was not putting out much power. I also agree that it is probable that only disconnecting the charging cable is likely to result in a burned out alternator, but I would not like to test the theory by turning the key to off with the engine at full revs.

 

[Oscarpop]

Turning the ignition key off won't damage the alternator.
It's when the engine is running and someone disconnects the charging wire that damage is done as the generated charge has no where to go. This is sometimes achieved by turning the main battery isolator off depending upon how the system is wired.

Sympton of failed alternator is battery light remains illuminated on engine panel when engine is running.

I should add that turning off the switch happens all the time on school boats.

Thank you for that. I was having kittens about having to buy a new alternator.
I checked the battery light and it went off again, so I hope its ok.

 

[Bobobolinsky]

Surely it would just stop charging as the exciter power is on the key circuit (Otherwise you would always have a flat battery)

 

[vyv_cox]

I have read on these pages, written by people more knowledgeable than I on electrical matters, that the damage occurs when the alternator is reconnected. It seems it's the surge that kills the diodes.

 

[prv]

I usually turn my "key" (actually a switch) off while the engine is running - I then pull the stop control. Doing it this way round avoids the annoying beep as the oil pressure falls once it's stopped.

It's the connection from alternator to battery that shouldn't be broken while running, not the switch in the panel. That's just people getting battery switches and ignition keys mixed up with each other.

Pete

 

[SvenglishTommy]

If it's a diesel engine then it makes no difference if the key/switch is on or off, you can use it all day with no effect to the alternator.

 

[lw395]

If it's a diesel engine then it makes no difference if the key/switch is on or off, you can use it all day with no effect to the alternator.

The alternator generally needs the current through the warning light to start up, although it may have enough residual magnetism to self start, given more revs.
Switching off the 'ignition' will not affect a normal, standard, car-type alternator, it will keep charging and regulating. If you have more complex system, it is possible damage could occur, depending on the detail of it.

 

[rob2]

Unfortunately on my installation the engine stop button only works with the key "on", so I had to switch back on to stop the engine. As Viv say, that could be the critical moment! When damage does occur, it is normally confined to the diode pack so not as expensive as a new alternator. Taking it to an automotive repairer should see it fixed in no time and at reasonable cost. It is possible that you could lose one diode in the bridge rectifier which will halve the throughput of the bridge and if it happens to be on the pair which are not tapped for the indicator then the light won't show the failure even though you'll be getting precious little charge - so don't panic, but keep an eye on the charge.

Rob.

 

[VicS]

I have read on these pages, written by people more knowledgeable than I on electrical matters, that the damage occurs when the alternator is reconnected. It seems it's the surge that kills the diodes.

No Vyv the surge occurs if the battery is disconnected, ie isolator opened, while the engine is running.

Imagine all the BMWs belting along the motorway. Someone suddenly stops the flow by jack knifing a lorry. No where for the BWMs to go ... big pile up.

Same thing happens to the electrons belting through the wires . Someone opens the isolator... no where for the electrons to go ... big pile up of electrons = voltage surge which blows the diodes in the alternator.

Sorry about the analogy

 

[Blueboatman]

If it helps, I have inadvertently turned off the ignition on my Yanmar at least twice-but not tried turning it on again until stopping the engine with the 'right' lever, ahem. And once was from 2000rpm, oversize alternator fitted, seems fine..so far

I now have a bit of luridly coloured string between the key and the stop lever to say 'oy wake up'. Doubtless you could epoxy a wee block of wood or plastic to prevent the key getting knocked, long term?

 

[vyv_cox]

No Vyv the surge occurs if the battery is disconnected, ie isolator opened, while the engine is running.

Imagine all the BMWs belting along the motorway. Someone suddenly stops the flow by jack knifing a lorry. No where for the BWMs to go ... big pile up.

Same thing happens to the electrons belting through the wires . Someone opens the isolator... no where for the electrons to go ... big pile up of electrons = voltage surge which blows the diodes in the alternator.

Sorry about the analogy

Seems to me that opening an isolator is more like your motorway collapsing due to a bridge disappearing, or one of those holes appearing due to subsidence. Plenty of space for the BMWs to dive into

 

[maby]

The damage is done by driving the alternator round while the battery circuit is disconnected. It is not possible to come up with a simple rule for what can cause this in terms of switch and key movements since it is all a question of how the manufacturer has configured the electrics.

A basic diesel engine - without any complex engine management systems - does not need any electrical circuits to operate - unlike a petrol engine. To stop it, you need to cut the compression, or interrupt the supply of fuel to the injectors - both mechanical operations. Older boats have a purely mechanical system with some kind of lever that you pull or push to stop the engine - which has no connection to the electrical systems. Modern boats tend to try to provide a more "car-like" experience by implementing the engine stop via press buttons, or even the ignition key - all they are doing is operating the mechanical stop mechanism via some form of solonoid or servo. You need to consult the manufacturer's instructions to know what is safe - on our Jeanneau there are separate "ON/OFF" buttons that switch the electrical circuits and a "STOP" button that stops the engine. The manual warns that pressing the "OFF" button with the engine running may do damage - though I have done it accidentally a couple of times without any obvious problem. The engine continues running normally after the "OFF" is pressed, but the rev counter, indicator lights and engine room blower all stop. The correct sequence is to press "STOP" for several seconds, wait for the engine to come to rest, then press "OFF".

 

[VicS]

The damage is done by driving the alternator round while the battery circuit is disconnected.

No, the damage is done if the load, eg the battery, is disconnected from the alternator while it is running. Interupting the current causes a voltage surge which may be large enough to blow the diodes in the alternator.

This is why with a 1,2 both,off switch it is important to always switch via the both position, never via the off position when cahnging batteries over while the engine is running

You can completely disconnect the alternator then start the engine and run it without any damage being done.

 

[Rivers & creeks]

No Vyv the surge occurs if the battery is disconnected, ie isolator opened, while the engine is running.

Imagine all the BMWs belting along the motorway. Someone suddenly stops the flow by jack knifing a lorry. No where for the BWMs to go ... big pile up.

Most new BMW's have forward looking sensors to warn of obstructions so I think in this scenario they'd all slow down before hitting the lorry. Plus all those X3 and X5's will be able to off road their way round the obstruction. A lot of Renault and Vauxhall drivers would die though.

 

[Graham_Wright]

No, the damage is done if the load, eg the battery, is disconnected from the alternator while it is running. Interupting the current causes a voltage surge which may be large enough to blow the diodes in the alternator.

Would a zener, say 16 volts across the output solve this? Perhaps a series resistor would limit the resulting current?

 

[mjf107]

Would a zener, say 16 volts across the output solve this? Perhaps a series resistor would limit the resulting current?

This has been discussed before, the problem IIRC is that the damage happens very quickly the magnitude of the current is such that the protection circuit may well be damaged its self interrupting the spike, so you don't no if it will offer and further protection. I believe that modern regulators have zener protection but how it actually works and whether its reliable or not I don't know
Michael

 

[Talulah]

This is why with a 1,2 both,off switch it is important to always switch via the both position, never via the off position when cahnging batteries over while the engine is running

Because the likelyhood of this happening is so high I've never understood why people wire their batteries this way.
i.e By all means have the lead to the starter motor going through the 1,2,both, off switch but have the charging wire from the alternator going direct to the battery through a separate isolator if you feel it necessary.
I suspect most people just don't consider anything different though. As standard the charging lead from the alternator is connected to the starter motor and ipso facto to the battery. Personally, I have disconnected this wire between alternator and starter and have the alt wire going direct to the battery. Hence, no damage done if people play about with the 1,2,both, off switch.

 

[VicS]

This is why with a 1,2 both,off switch it is important to always switch via the both position, never via the off position when cahnging batteries over while the engine is running

Because the likelyhood of this happening is so high I've never understood why people wire their batteries this way.
i.e By all means have the lead to the starter motor going through the 1,2,both, off switch but have the charging wire from the alternator going direct to the battery through a separate isolator if you feel it necessary.
I suspect most people just don't consider anything different though. As standard the charging lead from the alternator is connected to the starter motor and ipso facto to the battery. Personally, I have disconnected this wire between alternator and starter and have the alt wire going direct to the battery. Hence, no damage done if people play about with the 1,2,both, off switch.

What you describe is the favoured way by our N American friends.
I believe it is what is described in Calder's book.

In the UK there seems to be a preference for the battery isloator switch to completely isolate the battery when its off, except for any essential "on ways on" circuits, rather than leave bits of engine still connected

I hope you have suitably rated fuse in the alternator connection close to the battery and that you remember to physically disconnect the battery before waving spanners etc around in the engine room.