Why do I have an on/off switch coming off my alternator?

[jacktheflyer]

Originally Posted by seabreez

I rebuild alternators and never heard of leakage current. Also that big terminal is hot all the time as some are connected to the big lug on the starter. The diodes prevent the current from coming back to the alternator.

I had problems with a Moto Guzzi where the battery would run down after being idle for two weeks. I measured a 40mA discharge through the regulator. Thinking it faulty I bought a new regulator and it was worse.

[captstu]

The exciter wires on my twin 200 Amp alternators were switched - so I could get maximum power out of the engines when docking and maneuvering.

Lacking the exciter switch, the alternator load was enough to slow the engines down when accelerating - lots of smoke due to overload.

Maybe the P.O. needed the extra power and thought switching off the alternators would give them this power.

Foolish idea - as noted above,

I would not switch the alternator load wires - but I would put the A Both B switch into OFF when servicing the engine alternator.

[rwidman]

Originally Posted by Dave Lochner

It is referred to as a "service disconnect." ................

I would think a "service disconnect" on an alternator would be pretty unusual and unnecessary. Many boats have a switch to disconnect the battery and this would serve the same purpose without the risk of damaging the alternator.

And of course, disconnecting the battery serves as a "service disconnect" as well.

I suspect the PO installed the switch based on erroneous information he was given on the Internet.

I would remove the switch and just connect the ends of the cable together or replace it with a continuous piece of cable.

[Safari38LH]

We had a new alternator installed in Canada. They wondered why we didn't have a service disconnect and so put one in. I think it might be required up there?

[basssears]

I do not have a setup like this, but what I have read in the past is that people will install a switch so they can disconnect their alternator when they're going to be motoring all day, so they don't boil their batteries off.

As pointed out, you can't do this with the alternator spinning or you'll kill it (same as if you turn you 1/2/both/off battery switch to off while your engine's running), but you can shut your engine down, disconnect the alternator using this switch, fire the engine up again and then you aren't charging your batteries for 10 hours straight.

Here's a gentleman who did it:
Stories of Aeolus- Our Gulf 32 Pilothouse: Replacing alternator

-- Bass

[Frugal]

Probably the previous owner used the switch to disconnect the alternator charging when not needed.
I have found with large storage and solar array it is of little use to have a fully charged system at dawn. The alternator has survived this 90%(ish) disconnected state over a couple of years with no ill effects. You just need to ignore the charging light staying on.

[mitiempo]

Originally Posted by rwidman

I would think a "service disconnect" on an alternator would be pretty unusual and unnecessary. Many boats have a switch to disconnect the battery and this would serve the same purpose without the risk of damaging the alternator.

Modern (and best) practice is to wire the alternator output directly to the house bank positive post, fused close to the battery. The start battery is charged with an ACR or B2B charger. The switch is a service disconnect and is a good idea. It shouldn't be front center with the other main battery switch(es) but in an out of the way place such as the engine compartment.

[Rconwell]

On an older diesel sometimes the alternator will kick in too soon and kill the engine before it can get up to speed especially if thee alternator is more powerful than the original. On ours we have to leave the alternator off until the engine warms up a bit then turn it on. Our rebuilder told me how to wire it and says there will be no problem. we been doing it for about 3 years now without a problem.

This was my first post ever.
1973 Yankee 30 MKIII with a Bukh 10 diesel
Washington State

[Dave Lochner]

Originally Posted by Frugal

Probably the previous owner used the switch to disconnect the alternator charging when not needed.
I have found with large storage and solar array it is of little use to have a fully charged system at dawn. The alternator has survived this 90%(ish) disconnected state over a couple of years with no ill effects. You just need to ignore the charging light staying on.

It is not necessary to turn the alternator current off with a switch when charging batteries, the regulator does that. As the battery charge acceptance rate decreases the regulator decreases the alternator output.

[rwidman]

This is entertaining of course but it's the blind leading the blind. Lots of unbelievable stuff posted here.

[sartorst]

[QUOTE=rwidman;2316329]This is entertaining of course but it's the blind leading the blind. Lots of unbelievable stuff posted here.[/
I agree the PO must have based his wiring off of the crap he read on the internet