The consequences of disconnecting batteries with diesel running

Yara

Yara

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So, the rule is never isolate the battery while the diesel is running, or it will destroy the alternator.

I have a Volvo 2001 with a non-functioning alternator. I am guessing maybe a PO did isolate the batteries. Some suggest that the consequence would be blown diodes. If so, are they buried inside the alternator or would they be somewhere external? Are there other consequences of the disconnect like burning out the wiring? The alternator is a Paris Rhone
 
You will probably get away with disconnecting the battery but it is not worth taking the risk as it has the potential (sic) to damage the diodes.
The diodes are internal. The wiring will not be damaged as it just a high induced voltage caused by a collapsing magnetic field that may damage the diodes. Easiest option is to take it to a local auto alternator repair business who can test and repair it.
If you want to open it up you may find some information here:
Alternator handbook
 
I took my old Paris Rhone to the alternator shop, they dragged an old boy from out the back to look at it as nobody had seen one in 35 years.
 
Yara said:
So, the rule is never isolate the battery while the diesel is running, or it will destroy the alternator.

I have a Volvo 2001 with a non-functioning alternator. I am guessing maybe a PO did isolate the batteries. Some suggest that the consequence would be blown diodes. If so, are they buried inside the alternator or would they be somewhere external? Are there other consequences of the disconnect like burning out the wiring? The alternator is a Paris Rhone
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Over 50 years ago a mechanic told me not to disconnect a battery when the engine was running as it would ruin the regulator.
But a few years ago on a sea trip a battery which had not been secured properly fell over and the battery lugs rested on the metal diesel tank and shorted out both batteries. The battery was welded to the tank and was levered off. The lugs were disconnected from the battery. Fortunately the engine was running and ran for 5 hours. The alternator suffered no damage as when we got to port and bought new batteries it was still charging.
 
My impression is that it's a theoretical risk, but a fairly rare one. There will usually be enough of a load from all the bits and pieces on a boat to protect the alternator from the battery being disconnected. OTOH, disconnecting the output from the alternator will make the risk less theoretical, but I wouldn't like to say it's a certainty, even then.

Take the alternator to someone who knows what they're doing and they'll sort it or sell you a replacement. If it's the original on a VP2001, it doesn't owe you anything, after all.
 
Jim@sea said:
Over 50 years ago a mechanic told me not to disconnect a battery when the engine was running as it would ruin the regulator.
But a few years ago on a sea trip a battery which had not been secured properly fell over and the battery lugs rested on the metal diesel tank and shorted out both batteries. The battery was welded to the tank and was levered off. The lugs were disconnected from the battery. Fortunately the engine was running and ran for 5 hours. The alternator suffered no damage as when we got to port and bought new batteries it was still charging.
Click to expand...

Shorting the battery and alternator is not the same as disconnection. The trouble is caused when the circuit is broken . All the electrons rushing along the wire suddenly have no where to go and all pile up to create a big voltage peak. It is this high voltage peak that can damage the alternator.
 
scottie said:
It’s not an urban myth but can and does happen but not every time
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Indeed.
There are dozens of different regulators and many different diodes.
Some are undoubtedly tougher than others.

The failure may also not be immediate. Some parts may fail months later.
I was told by a bloke in an auto electrical place in Portsmouth that yacht alternators quite often mysteriously fail in ways which vehicle alternators don't.
 
My understanding is that the sudden collapse of the magnetic field will create a back EMF (voltage) that can if large enough be greater than the reverse voltage spec of the diode thus blowing the diode.
 
It can damage the alternator, but that's not a given. For one thing, most modern boats have more than one battery bank, so even if you disconnect the engine battery, the charge still goes to the other batteries via the split charging system. There also might be loads on the circuits that will carry enough current to save the diodes.
 
Well, I once blew all the instruments on a boat by inadvertently isolating the batteries. Had to do a transatlantic delivery without boat or windspeed data which was a bit of a pain.
I'd always understood it allows the alternator voltage to go wild which doesn't hurt the alternator but it seems likely to wreck anything electronic that's connected at the time.
Is this not correct?
 
Slowboat35 said:
Well, I once blew all the instruments on a boat by inadvertently isolating the batteries. Had to do a transatlantic delivery without boat or windspeed data which was a bit of a pain.
I'd always understood it allows the alternator voltage to go wild which doesn't hurt the alternator but it seems likely to wreck anything electronic that's connected at the time.
Is this not correct?
Click to expand...
With a "machine sensed" alternator the regulator should control the volts but perhaps in your case the alternator was" battery sensed" and the regulator was not seeing the alternator output voltage , only the voltage of the disconnected battery.
 
Slowboat35 said:
Well, I once blew all the instruments on a boat by inadvertently isolating the batteries. Had to do a transatlantic delivery without boat or windspeed data which was a bit of a pain.
I'd always understood it allows the alternator voltage to go wild which doesn't hurt the alternator but it seems likely to wreck anything electronic that's connected at the time.
Is this not correct?
Click to expand...
This could happen with some regulator circuits.
The regulator is designed to have a battery connected to it, which means the volts can only change at a limited rate.
Take away the battery and the regulator may not be able to respond fast enough, or it may even oscillate.
Running with no battery connected is not in the design requirements for an automotive alternator, it may not even be in the 'wish list'.
Perhaps more so with older units, these days more cars have battery fuses, so disconnection of the battery might be considered.

It's possible that the instruments were not as robust as they should have been.
I forget the exact numbers, but some military vehicle kit I have worked on has to cope with something like 40V spikes on the '12V' line. Positive and negative!
 
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