Battery switch and alternator field circuit

[andy65]

My battery switch has an extra connection for 'alternator field circuit' - should I connect a wire directly from the alternator to allow current in to the batteries even if I switch off the batteries with the engine running, or is it used for something else?

 

[Barnacle_Bill]

Re: Battery switch and alternator field circuit

Be nice to know what you are working on motor wise.

 

[drewpster]

Re: Battery switch and alternator field circuit

The field disconnect on your switch is used as a safety measure. Battery switches are meant to be placed at the battery. Meaning the switch is used to disconnect the battery from all loads. This includes your charging wire from your alternator or stator. If you accidentally switch to OFF while the engine is running the alternator will be damaged.
The idea is to route the field control circuit through the battery switch where it will be disconnected the same time the battery is turned off. This saves the alternator by shutting off the field circuit along with the switch.

 

[Boatist]

Re: Battery switch and alternator field circuit

If it an older boat with an external voltage regulator then the field disconnect circuit will turn the alternator charging off before the switch opens from the battery. You connect that contact to the field winding on your alternator. Problem is most newer boat have internal regulars inside the alternator so no way to disconnect the field winding.

If your switch is a "MAKE BEFORE BREAK" type and everything is in good order then you should be able to turn the switch between batteries without blowing your alternator as long as you do not turn thru the off posistion.
If you do not know if it is a "MAKE BEFORE BREAK" then only safe way is to turn the switch only when engines are shut off.

Another option would be to install a battery isolator if you have two batteries or more as this will also protect your Alternator.

 

[andy65]

Re: Battery switch and alternator field circuit

Sorry, but I haven't understood what the risk is, if I disconnect the battieries while the engine is running - will the alternator produce too much current that has nowhere to go and will destroy the alternator or the gps etc?
So if I want to test the alternator, removing the battery contacts while the engine is running and checking the current over electric system is not the way to do it?

 

[bruceb58]

Re: Battery switch and alternator field circuit

No...you don't want to do that. You can fry your alternator in a heartbeat.

 

[Boatist]

Re: Battery switch and alternator field circuit

If you disconnect the battery with the engine running the voltage will go so high in a fraction of a second that it will blow all the output diodes and probably also the voltage regulator. Back in the old old day with generators you could disconnect the battery and the motor would still run and nothing would be damages but alternator work a lot different.

Keep in mind that even a bad ,coroded, battery terminal or cable can have the same effect. This is one of the reasons I like a battery isolator as both batteries would have to disconnect to blow the alternator.