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Did I kill my M25 Alernator?

newgringo

Member III
Just when I thought all was hunky dory after a week without shore power I noticed my Universal M25 not cranking over very fast. Serious. No AAA service for this. Some quick checking showed the Alternator only producing 13.2 volts and sagging lower if loaded over 5 amps. Not good. Well a new Alternator solved that problem. $260 for a Prestolite drop in replacement.
But, here is my question - A couple of times the Engine Key Switch has been turned off before pulling the fuel cutoff knob. Looking at the wiring, this cuts DC power to the Regulator with the Alternator still spinning. Did I kill my own Alternator? If I did is there a wiring fix I can install to never do this again?
 

Loren Beach

O34 - Portland, OR
Senior Moderator
Blogs Author
Having just "killed" an alternator by interrupting the + output, and ruining the diodes... I know one way to do the evil deed... :(
Maybe one of the electricians on the site can chime in.

I have never heard that turning off the key while the engine was running would harm it, though.

Loren
('88 M25XP)
 

rwthomas1

Sustaining Partner
Cutting DC power to the regulator should not hurt the alternator. AFAIK, this would simply cut power to the field circuit and that should simply shut down the alternator output. I do know that disconnecting the load, the battery bank, that is from a charging alternator will kill it. Like switching the battery switch off while the engine is running. RT
 

Dave Hussey

Member III
I agree with what Rob said...the key will only turn off the external excitation power to the field coils. Now here is a surprising but I think cool fact though...the residual magnetic fields which remain in the field cores (the electro magnets) will continue to 'self excite' and the alternator will happily continue to put out charge current to your batteries! Cool huh?

BUT Definetly do not ever switch the main battery disconnect while the engine is running, or you will likely let the smoke out of a diode or a regulator.
 

Rocinante33

Contributing Partner
What about switching from one battery bank to another, or to "both" while running? I have done this in the past & didn't observe any problem. Should I NOT switch at all for safety's sake?:confused:
 

Chris Miller

Sustaining Member
while not changing batteries while running is a common "best practice", doesn't that make a second battery bank all but useless? If I run down my house bank at anchor, then switch to the start battery to get the motor started- how am I ever supposed to recharge the house bank if I can't change batteries?
I think somebody heard a rumor somewhere along the way... and it turned into this monster. I think the real issue is turning the batteries OFF, right?
 

Dave Hussey

Member III
Chris, et.al.,
my understanding is switching battery banks when the alternator is not turning, i.e., the engine is off, is OK. It's when the alternator is turning, that switching banks is tenuous. Here's why: The typical battery switch opens the circuit momentarily between the alternator and the load (battery, or batteries, depending on the position of the switch) so the alternator diodes are rectifying and suddenly there is a current spike as the contacts open, and when the contacts close, there is a large surge of current, as the resistance across the contacts goes from infinity (open) to zero (closed) and the (typically low) internal resistance of the batteries absorbs the charge. These surges can exceed the diode ratings, and >phfttttt< $$$$$$$$ !!

:esad:
 

Chris Miller

Sustaining Member
Chris, et.al.,
my understanding is switching battery banks when the alternator is not turning, i.e., the engine is off, is OK. It's when the alternator is turning, that switching banks is tenuous. Here's why: The typical battery switch opens the circuit momentarily between the alternator and the load (battery, or batteries, depending on the position of the switch) so the alternator diodes are rectifying and suddenly there is a current spike as the contacts open, and when the contacts close, there is a large surge of current, as the resistance across the contacts goes from infinity (open) to zero (closed) and the (typically low) internal resistance of the batteries absorbs the charge. These surges can exceed the diode ratings, and >phfttttt< $$$$$$$$ !!

:esad:


Thanks, Dave-
But doesn't that render the alternator useless for your house bank after day 1 of a cruise?
  1. Boat is at dock w/charger
  2. start boat w/ house bank
  3. motor out, keeping topped up as we go
  4. turn off motor and draw from the house bank while sailing
  5. switch to "start" battery to restart motor to anchor, etc...

Now what? how do I charge my house bank if there isn't enough charge in it to start the motor?
I understand the electrical issues, but why bother with a 2 bank system in the first place? Seems like I'd be better off to put another 100amp/hrs for that extra "start" battery space and just call it good? Hope it starts?
The switch just seems extraneous if I can't use it...

Is the suggestion then to always use the house bank and only use the start battery in emergencies?

Still confused:unsure:
 

Geoff Johnson

Fellow Ericson Owner
I believe a good battery switch is supposed to be "make before break" meaning that switching battery banks will connect the second battery before disconnecting the first. However, I don't do that since there is always the chance that someone (including me) will get carried away and switch everything to "off" when the engine is running. I always start my engine from the house bank (and leave the switch on "both") and have a spare starting battery that can be linked in parallel with another manual switch. I also installed the "ZapStop" thingy across the diode. For the truly paranoid, you can get a battery switch that disconnects the excitor? circuit to the alternator when switched to "off".
 

Mike.Gritten

Member III
We have our charging system set up as follows to eliminate any problems as noted above.
First, we replaced the "1-2-OFF-BOTH" switch with 3 battery switches, House, Start and Combine. Pretty obvious as to what each one does. Next we set the alternator to charge the House bank. We did this because apart from starting the engine, the Start battery has little to do. We DO charge the Start battery however with a voltage sensitive charge relay that closes to send some of the juice to the Start battery when the voltage is in excess of a predetermined setting (14.2 volts if memory serves). Once voltage drops down below another pre-set limit (12.7 volts? when the regulator is in "float" mode or the engine is off and not charging) the relay opens to separate the Start battery from the charging circuit. This way we never have to worry about draining the Start battery accidentally and we have none of the diode-related voltage drops associated with battery isolators.
We have had this system now for 18 months and it has worked flawlessly! We have never had to "Combine" the two banks to start the engine.
As mentioned in another post, we have also installed a product called "Zap Stop" that is supposed to save your alternator diodes if you accidentally turn off the key with the engine running. I have not done this (yet) so I can't report whether or not the device works. It cost around $30 at WM IIRC.
 
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Chris Miller

Sustaining Member
Thanks, Mike. That was going to be my next question- do I have to install multiple switches... seems like the way to go. Although that "Zap-stop" thingy might do in the short term.
Thanks!
Chris
 

rwthomas1

Sustaining Partner
The simplest thing to do is to wire the alternator directly to the house bank, not through the 1-2-both switch. This way it is impossible to disconnect the alternator from its charging load accidentally. The 1-2-Both switch can continue to be use to select which bank is being used and also will charge both banks when set to Both. Setup like this the switch can be turned at any time and there is no way it will harm the alternator.

The regulator I have has an optional solenoid that combines the banks when charging and is based on some logic circuit, etc. It appears to work flawlessly. RT
 

Geoff Johnson

Fellow Ericson Owner
FWIW, while my starting battery can be paralleled and charged manually, I just forget about it for the season (haven't had to use it yet). It's an Optima Bluetop AGM and essentially does not self-discharge (maybe 1 percent per month).
 

newgringo

Member III
A Consensus, sort of

As starter of this thread this is what I make of it so far. We are talking about two different things:
1: Will turning off engine key, thus cutting power to the alternator regulator kill the alternator? I gather from those responding that it should not. Thus probably no action or modifications are needed.
2: Opening the load from the alternator output terminal WILL KILL an alternator and is to be avoided at all costs. Several good suggestions to avoid this presented. I kind of like the idea of wiring the alternator output to the house battery with no chance of ever interrupting the circuit. A couple of downsides would be to charge the other / starting battery, the batteries would need to be charged in parallel (not recommended practice but I have done it for years) and the alternator output terminal would always be hot (just like a car).
That's my take so far. Many good thoughts.
 

Geoff Johnson

Fellow Ericson Owner
I could be wrong, but I think that turning off the "ignition" key only turns off the fuel pump. I don't think it affects the power to the exciter wire to the alternator (and even if it did, it would turn it off and de-energize the alternator's fields and stop the charging).

As for wiring the alternator directly to the battery bank, you would probably have to fuse the hell out of it - you lose you means of disconnecting the batteries from the charging circuit.
 
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Tom Metzger

Sustaining Partner
We go through it every year

As discussed every year:

1) Turning off the key switch does no harm to the alternator. It does shut down the alternator and the fuel pump.

2) Switching batteries with the 1-2 both-off switch does no harm to the alternator. Only going to the Off position is harmful. There would be no point in having the switch if you couldn't operate it. As someone pointed out, the contacts are "make before break".
 

Emerald

Moderator
Thank you Tom for the simple distillation. I would add that if you are really worried about getting off some place remote and accidentally switching to "OFF", then install an alternator with an external regulator that is compatible with a battery switch that has an alternator field disconnect. Here's a link to the type of switch I mean:

http://bluesea.com/products/9002e

I would also add that if nothing but from a fire safety standpoint, it is imperative to have a fast way to "turn off" the electrical system and stop feeding current from batteries into a shorting/fire situation.
 
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newgringo

Member III
Final Rewire for Charging and Alternator Protection

Well after all this discussion I have rewired the Alternator charging the way I think it will work better. This what I did:
1. Wire Alternator output directly to battery #1 with #6 wire thru a 50 amp fuse.
2. Disconnect and abandon factory wire from alternator (orange #10) to ammeter at both ends.
3. Rewire factory ammeter for + from battery connection at starter solenoid
4. Rewire factory ammeter for - to key switch
And this is why I did it, with other comments:
1. Wiring direct allows a full 14+ volt charge into battery #1 or BOTH if so selected
2. Alternator can never be "switched OFF" and damaged
3. 50 amp fuse is more to protect an engine worker (me) if alternator battery terminal accidentally shorted.
4. Alternator charging health is now determined by the voltage reading from my Garmin GPS which has a built in digital meter. 14.0 to 14.5 volts means normal charging.
5. The ammeter now only shows loads thru the key switch (fuel pump, blower, glow plugs etc)
6. A downside is that the ammeter now never shows actual charging current. Only key switch loads.
Now installing a high priced solenoid type battery isolator (not a cheap diode type) would have been better but this is cheaper and very effective. So I am done with this one and can think about other projects, like winter sailing.
 
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tdtrimmer

Member II
Direct Alternator to Battery Must Have a Breaker

For those interested in wirng the alternator directly to the house battery, it is important that a 60 amp + circuit breaker be installed between the alternator and the battery. This will protect the alternator and wire should a short occur. It also enables you to disconnect the battery from the alternator in an emergency (short circuit). Check out the Balmar web site. They emphasize the importance of the circuit breaker. I use a similar setup and it works well.

I use a dual outpput Balmar alternator direct connected to each battery bank (with circuit breakers). The 1,2,both, off switch is between the batteries and the loads (house and starter). Thus, I use the switch to select which battery bank to use, not which battery bank to use and charge. This setup has worked flawlessly for 6 years.

Tom
E38 1981
 
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