Alternator Field Disconnect?

[Roy M]

Extemp, If I was reading correctly you are asking why one would use the small engine mode switch to reduce alternator output to zero? Imagine you are anchored in a quiet cove for several days. Suddenly a nasty storm crashes on your head, winds are blowing directly into the anchorage, square waves are blocking your escape route, and you need every ounce of engine power to make your escape. The engine is already going, so you don't need to focus on your batteries at the moment, as you have bigger issues at the moment. It's time to bail, as fast as you can. Flip the switch, taking the alternator load off the engine, pull the anchor and get the hell out of Dodge City ASAP. Then, when you are clear of the nasties, flip the switch back to regular mode and charge up those batteries as you head out of danger.

[Extemporaneous]

Originally Posted by Roy M

Extemp, If I was reading correctly you are asking why one would use the small engine mode switch to reduce alternator output to zero? Imagine you are anchored in a quiet cove for several days. Suddenly a nasty storm crashes on your head, winds are blowing directly into the anchorage, square waves are blocking your escape route, and you need every ounce of engine power to make your escape. The engine is already going, so you don't need to focus on your batteries at the moment, as you have bigger issues at the moment. It's time to bail, as fast as you can. Flip the switch, taking the alternator load off the engine, pull the anchor and get the hell out of Dodge City ASAP. Then, when you are clear of the nasties, flip the switch back to regular mode and charge up those batteries as you head out of danger.

Thanks Roy, I'm with you 100% on that one.
I was referring to what you pointed out to me earlier (Find the lug (terminal 13 - and +)that goes to an installed toggle switch in the cockpit, which you can label "Small Engine Mode".).
It says that it throttles the alternator back to 50% output, not 0%. So I guess my question is would I get that much more power to my prop taking the alternator output from 50% to 0%? I have a 120amp balmar alternator in a 12ton + boat and a 46hp Diesel engine.
It's probably a difficult to know, but I would suppose there are times that EVERY little bit counts.
The "Small Engine Mode" is a pretty cool feature though.

Thanks again,
Extemp.

[Extemporaneous]

Originally Posted by GordMay

For more on AFD, Goto: Alternator Field Disconnect - Resources - Blue Sea Systems

Thanks Gord, more good stuff. It seams to be physical and therefore should ALWAYS work. It also says it has to be use in conjunction with an external regulator. Perhaps that's why it didn't work for jdoe71.

Originally Posted by jdoe71

AFD doesn't work with some alternators, like the Prestolite that used to be on our engine. Even though it is described as being field excite, the thing actually is self excite once it's making power. Turning off the field through the AFD does nothing, it just keeps on making power and would blow the diodes if disconnected from loads.

jdoe71, was your regulator internal or external?
It would be nice to figure out why it did not work in your case. I'd hate to think I'm safe only to blowup my alternator.


I guess one could disconnect the field connection without disconnecting the batteries and then check for current running through the output cable to the battery. Anything risky about trying this? Besides clutsing out while working in close proximity to power and moving parts.

[Extemporaneous]

Okay, I'm convince to go with an AFD. I also have a XANTREX Zap-Stop Alternator Protector to install. I'm wondering if I get an AFD if there may be other occasions/situations besides protection from momentary turning off of the battery switch where the Zap-Stop would provide protection to my system or it would be redundant.
If I only had one I'm convinced the AFD would be the one to get as the Zap-Stop says "momentary".
I'm I on track?

[GordMay]

The Zap-Stop Diode connects between the alternator output and ground.
It is just a high voltage Zenner diode, with a reverse breakdown voltage (± 20 - 25VDC) a bit higher than the operating voltage of the alternator.
When the (transient) voltage exceeds this level, the diode starts to conduct, instantly shorting it to ground.

The alternator (rectifier) diodes are designed for high amperage, at relatively low voltages.
The Zap-Stop diode is designed for high voltages but will only handle high amperage for the few milliseconds it takes for the regulator to regain control.
I don't think it could stand up to something like a loose sense wire, on a high amperage alternator, where the voltage and current output stays high for any length of time.

A Battery Selector Switch with Alternator Field Disconnect (AFD), has an “extra” pair of (small) terminals, which open as the selector is rotated toward the OFF position. If connected in series with the Alternator Field wires, this auxiliary switch will interrupt the connection to the alternator’s field, preventing damaging transients.
Accordingly, the AFD auxiliary switch will only work when you have access to the Alternator Field Wires. This is the typical situation with Externally Regulated Alternators; but an Internally Regulated Alt’ can have the Field Wires brought out (<$50 at an Alt Shop).

Every multiple battery switch I have worked with has been a “Make-Before-Break” design, with the switching sequence “OFF”– “1”– “ALL”– “2”. This allows you to select between 1, 2,. &/or All (Both) without ever disconnecting the batteries.

I always use both an AFD-type Battery Selector Switch & a Zap-Stop Diode.

[Sandero]

Am I understanding this correctly that one may need to go full throttle for some conditions? And an alternator will use up some needed horser power?

I have found in normal conditions that my stern goes down and all I am doing with more throttle is burying it and trying to drive over a bow wave... impossible and a waste of fuel and I don't go any faster. No?

[Hud3]

I got hit by a 50+ knot squall east of Nevis last June. I had my 56 hp Yanmar revved up to 3000 rpm, and was barely able to maintain a couple knots through the water in order to keep the bow into the wind and waves. I once calculated that my boat needs only 18 hp to motor near hull speed in calm water with no wind. Wind and waves make a lot of difference.

[Extemporaneous]

To continue with my battery install, I have another question.
I have Balmar ALT-120A alternator with a Balmar Max Charge Digital Regulator MC-612-H and a Balmar Digital Duo Charge Controller. Balmar’s Digital Duo Charge connects between the house and start (secondary) batteries -- keeping the two banks separate until the unit senses 13 volts at the house battery. Once voltage is reached, the Duo Charge supplies up to 30A to the secondary bank.

If/when I combine the house batteries with the starting battery and the engine is running, will I have to isolate or take the Duo Charge Controller off line? I don't know what would happen if the two battery banks were combined and at the same time the house batteries were supplying the Duo Charge Controller with power to charge the starting battery which is already being charged by the alternator because all the batteries are combined.
Anyone?
If the Duo Charge Controller has to be isolated when the house and starting batteries are combined, could that somehow be done with an AFD battery switch or would that portion of the AFD switch be too small to handle the potential 30amps that the Duo Charge Controller can put out?
My brain hurts.
Any help would be appreciated.

Extemp.

[Pblais]

Can anyone tell me when and/or why I would use an Alternator Field Disconnect?

I have a 62 HP and have the disconnect. It was installed by the PO before going through the Panama Canal. They thought they might come up a tad short making the required speed through the canal.. I've never felt the need for it. The switch was not the best wiring. I'll probably get rid of it. With 62 HP I don't need more. I've rarely used full throttle but when I have I was glad to have it. Beating into heavy chop is all I have ever been in when I wanted all power. This is near gale conditions. I can't imagine with a 100 amp alternator I could ever need more and not have it. Under sail when not on the nose I can make more sail power than with the engine even in double reefing situations.

[Badsanta]

Originally Posted by Extemporaneous

To continue with my battery install, I have another question.
I have Balmar ALT-120A alternator with a Balmar Max Charge Digital Regulator MC-612-H and a Balmar Digital Duo Charge Controller. Balmar’s Digital Duo Charge connects between the house and start (secondary) batteries -- keeping the two banks separate until the unit senses 13 volts at the house battery. Once voltage is reached, the Duo Charge supplies up to 30A to the secondary bank.

Extemp.

Take a break, you are thinking to hard.

If you are combining your batteries it is because your starting battery does not have enough power to start your engine. So if it is not running you are not charging and you wont have 30 amps going into your starter battery. You wont have to isolate it. With your set up you should never have to combine your batteries unless your Duo Charge Controller fails.

[Extemporaneous]

Originally Posted by Badsanta

Take a break, you are thinking to hard.

If you are combining your batteries it is because your starting battery does not have enough power to start your engine. So if it is not running you are not charging and you wont have 30 amps going into your starter battery. You wont have to isolate it. With your set up you should never have to combine your batteries unless your Duo Charge Controller fails.

At first blush, you are correct. If though, you get the engine started you would then have the condition which I'm wondering if I need to guard against. Also I'm new at this and so do not know if there are other reasons to combine besides a dead starting battery.

[Badsanta]

Then I would check with Duo Charge for the correct answer. They would know.

[sabray]

do not know if there are other reasons to combine besides a dead starting battery.[/quote]
My set up allows a manual switch between house and starting batteries. So if either one drops out I can bring the power over either way. A blue Seas relay connects the chargers solar or alternator when one bank reaches a preset level charged level.

[fabricator john]

in drag racing we use a alternator dissconnect all the time, the battery kill switches we use have a second small set of terminals on it for this (run alternator wire to the trunk,the batts are there anyways),on a race car if you hit the batt disconnect on the back and it has a alternator it will keep running,so these break the wire too,,,,,,,,also a few cars do run a switch to break the field during the pass to free up a few ponies,i've seen this done with a relay kicking switched off a w.o.t. switch like used for rev limiters and nitrous oxide.
john...2670 explorer..."IPANEMA"

[Badsanta]

Originally Posted by sabray

do not know if there are other reasons to combine besides a dead starting battery
My set up allows a manual switch between house and starting batteries. So if either one drops out I can bring the power over either way. A blue Seas relay connects the chargers solar or alternator when one bank reaches a preset level charged level.

I question the wisdom, if my house bank is down that is many times larger than my starting battery and I combine it with a dead house bank, It would almost instantly drain the starter battery. So now you have neither house nor starter. I hope you have a gen set. But its your boat, combine away. But I separated my start circuit for a reason. IMO