Damage to Alternator ?

[Moonfire]

I made the mistake of briefly (about 2 seconds) turning off the battery switch to the engine while the enging was running. Would this have likely blown the alternator in such a short time? What sort of damage is generally done through such action? Many thanks if anyone can advise, John

[btrayfors]

John,

That depends mostly on how the alternator is wired. If it's wired directly to the batteries and not thru the switch you turned off, then it will be OK.

However, it's wired thru the switch (which is NOT a good idea, but many boats are wired this way) then it's likely you've blown one or more diodes in the alternator.

Is the alternator still putting out?

Bill

[S/V Illusion]

If you want to know the answer, simply put a voltmeter on the alternator output to see if it still works

[smurphny]

Originally Posted by btrayfors

John,

That depends mostly on how the alternator is wired. If it's wired directly to the batteries and not thru the switch you turned off, then it will be OK.

However, it's wired thru the switch (which is NOT a good idea, but many boats are wired this way) then it's likely you've blown one or more diodes in the alternator.

Is the alternator still putting out?

Bill

Bill, can you explain this? Most systems/factory wiring harnesses are wired through the engine on/off switch and then routed to battery banks or not via the battery selector switch. Why would diodes be affected by switching the banks? I recently burned out an alternator and assumed it was from momentarily switching polarity while screwing around with new fuses. Maybe something else going on?

[btrayfors]

Yes, many boats are wired with the output of the alternator routed to the starter solenoid. From there, it depends. Many boats are wired, as I said before, with the starter wire going to the 1-2-ALL-OFF switch common lug (along with other house loads). This is because many boats use the selector switch to choose either house batteries or start battery, or to combine them.

IMHO, this is a very bad idea...for several reasons, not just the danger of blowing diodes if you switch to OFF. For example, with the start circuit effectively tied to the house loads thru the common lug, when you activate the starter you draw down the voltage considerably, possibly affecting onboard electronics. Worse, when you let up on the starter button and the starter stops, its magnetic field collapses and puts a very healthy voltage spike on the circuit...again, possibly damaging onboard electronics over time.

Even with a make-before-break selector switch...as many modern ones are...if you switch to the OFF position you have cut the path from the alternator output to both battery banks. Therein lies the problem...this can blow your alternator diodes very fast, unless the field current is somehow disabled simultaneously.

A MUCH BETTER solution is to wire the alternator's output directly to the house batteries, though an appropriate fuse located near the batteries. Then, use a voltage-follower device or a battery combiner to take care of maintaining the start battery.

Bill

[Moonfire]

Hi Bill - I've measured the output on the starter battery (which is new) and it's 12.7v with no load and 13.9v with the motor running, so I'm hoping I'm ok - except I would have expected the reading to be nearer 14.7v. Do you know if it is possible to partially damage an alternator output? You comments much appreciated. John

[btrayfors]

John,

Most alternators these days are set for a voltage output of about 14.4VDC. Older ones are often set lower. If you measure the voltage at the battery and the battery is discharged, you may see something lower. Also, the size of wire and the quality of the connections between the alternator and the battery can explain voltage drop.

The fact that you're seeing higher voltage when the motor is running indicates that the alternator is at least partially working. Yes, it's possible for an alternator to partially fail, e.g., when one or more of the diode-trio burns out. This causes a reduction in the amperage output of the alternator, though it may put out the same voltage.

Here's where a clamp-on DC ammeter is useful. You can then measure how much amperage the alternator is actually putting out. Even with a perfectly functioning alternator, you might not see a lot of amperage if the battery is fully charged or nearly so.

In an addendum to my earlier post re: 1-2-ALL-OFF selector switches, some models these days have AFD....automatic field disconnect...which if wired correctly will protect the alternator's diodes even if switched to the OFF position. The Blue Sea Systems 9002e200 and the Guest 2100 are two examples.

Bill

[fairbank56]

Originally Posted by btrayfors

Worse, when you let up on the starter button and the starter stops, its magnetic field collapses and puts a very healthy voltage spike on the circuit...again, possibly damaging onboard electronics over time.

Starters are activated by a solenoid with a heavy duty contactor. When you release the start button/key, the contactor opens. Only then can the magnetic field collapse and produce a spike, but it has nowhere to go as the contacts are open. Your battery and any loads are on the battery terminal side of the solenoid, not the motor side.

Moonfire,

The damage to the alternator would be instantaneous. Doesn't matter if you leave the switch off.

Eric

[smurphny]

Originally Posted by btrayfors

Yes, many boats are wired with the output of the alternator routed to the starter solenoid. From there, it depends. Many boats are wired, as I said before, with the starter wire going to the 1-2-ALL-OFF switch common lug (along with other house loads). This is because many boats use the selector switch to choose either house batteries or start battery, or to combine them.

IMHO, this is a very bad idea...for several reasons, not just the danger of blowing diodes if you switch to OFF. For example, with the start circuit effectively tied to the house loads thru the common lug, when you activate the starter you draw down the voltage considerably, possibly affecting onboard electronics. Worse, when you let up on the starter button and the starter stops, its magnetic field collapses and puts a very healthy voltage spike on the circuit...again, possibly damaging onboard electronics over time.

Even with a make-before-break selector switch...as many modern ones are...if you switch to the OFF position you have cut the path from the alternator output to both battery banks. Therein lies the problem...this can blow your alternator diodes very fast, unless the field current is somehow disabled simultaneously.

A MUCH BETTER solution is to wire the alternator's output directly to the house batteries, though an appropriate fuse located near the batteries. Then, use a voltage-follower device or a battery combiner to take care of maintaining the start battery.

Bill

Thanks Bill. That is great info. I think I will reroute my alternator direct to house batteries with a battery combiner as you have described. Nigel Calder recommends doing it that way as well. The old 1-2-all-off switch is too easy to forget and do some damage. Now I'm wondering if my burnt alternator was from switching momentarily into the "off" position while changing batteries although I always leave them alone in one position or the other once under way. I had no idea that the field circuit needs to be opened simultaneously.

[btrayfors]

Originally Posted by fairbank56

Starters are activated by a solenoid with a heavy duty contactor. When you release the start button/key, the contactor opens. Only then can the magnetic field collapse and produce a spike, but it has nowhere to go as the contacts are open. Your battery and any loads are on the battery terminal side of the solenoid, not the motor side.
Eric

Eric,

Yes, you're correct. The opening contacts of the contactor (solenoid) should keep the spikes generated by the starter motor at bay. However, the electromagnetic coil in the contactor itself creates unwanted transients (spikes) when power is removed.

See, e.g., Transients and Voltage Drops - Good and Bad Wiring Techniques for Electronics

I've never measured the magnitude of these transients, but on general principles I think it's a good idea to separate onboard electronics from starting circuits whenever possible.

Bill

[fairbank56]

The coils in the solenoid (there are two) are also disconnected from any loads when you release the button/key. A spike may be produced, but again, it has nowhere to go. I had never heard of this being an issue, either on boats or automobiles.

Eric

[fairbank56]

Just proved myself wrong about this with some test's in the shop. While the spikes produced from both the motor coils and solenoid coils are much higher on the motor side and solenoid side, there is also a significant spike produced on the other side of the switches (solenoid contactor and start button/switch) i.e., the side that may have other electrical/electronic devices on them. The reason being is that it takes a finite amount of time for the switch contacts to actually fully open. During this time, when the magnetic fields are collapsing, there is resistance between the switch contacts as they are opening that the spike makes it across. This is what causes arcing and pitting of the switch contacts. This all happens in an instant but I can see it causing problems to sensitive loads. Typically, power feed to the engine wiring harness and the engine gauge panel comes directly off of the starter solenoid battery terminal.

Eric

[GordMay]

Originally Posted by btrayfors

Eric,

Yes, you're correct. The opening contacts of the contactor (solenoid) should keep the spikes generated by the starter motor at bay. However, the electromagnetic coil in the contactor itself creates unwanted transients (spikes) when power is removed.

See, e.g., Transients and Voltage Drops - Good and Bad Wiring Techniques for Electronics

I've never measured the magnitude of these transients, but on general principles I think it's a good idea to separate onboard electronics from starting circuits whenever possible.

Bill

Voltages, both positive and negative, are generated whenever a magnetic field collapses around in inductor.
When you press the 'start' button to start the engine, the start button activates a solenoid.
The solenoid is an electrical relay, that can safely handle the large amount of current needed to start the engine. It sends current rushing into the starter motor. The starter motor begins to spin the engine and causes it to start.
Because the starter motor must work hard to spin the engine, it requires a large amount of power from the battery. This builds up a very large magnetic field. When you let go of the starter switch, this magnetic field has to go somewhere. It collapses, and for a very brief instant it generates a terrifically high voltage transient or spike. A 12 volt starter /solenoid combination can generate a spike of 200 volts or more! It doesn't last too long, but it sure can have a devastating affect on your equipment. In reality, because the connection that the solenoid makes is not perfect, but rather quite 'bouncy' a whole series of spikes are generated each time you start the engine. These spikes cause arcing on the solenoid contacts, as they open.
Over time, the transient spikes cause damage to the solenoid, leading to eventual failure.

[fairbank56]

I'm seeing around 100-200 volts on the starter motor itself and double that on the solenoid. On the battery terminal side of the solenoid where your other loads may be connected, I'm seeing around 20-50 volts. It's difficult to get an accurate reading as the scope probe is also picking up spikes just being in the vicinity of the starter without being connected to anything.

Eric

[smurphny]

As a follow-up, I have ordered a battery combiner and new switch :BlueSea "Add A Battery Dual Circuit System Dual Circuit Sys W/ACr & 5511E." Will try to route alternator directly to house battery side when installing. I am worried about the voltage spikes you guys have described. Electronic devices are usually internally fused with very low amp fuses and I have never had a problem with them blowing. If a voltage spike of 200v were to get into radar, etc., I would think it would build up enough amperage to blow them?? I assume the spike gets into both + and - sides of the circuit. It is easy enough to shut electronics off to break the + side of the circuit but the neutral sides of the circuits are always connected. Is there a "backfeed" through the - side that can cause damage? Coming in through the back door there would be no fusing. I'm wondering if some sort of double pole switching for electronics would be in order.