Alternator isolator

[dansken]

Setup: One house battery bank (LIFEPO4, 24 volt system), 2 alternators directly to house bank.
Question: With the engine off and the house bank engaged (mainswitch on) both alternators will be "hot". Being an aluminium boat I don't like the idear of having a standing positive electrical potential there which could find its way through the engine and create corrosion - the alternators are supposed to be isolated , but even so.
The "electrical one-way valves" (isolators) I find on the marked are aimed at isolating multiple battery banks: any advice on how I could get my alternators isolated from the batteries when the engine is off?

[Lancerbye]

Some people use a solenoid switch and an oil pressure switch to disconnect both field voltage to and supply voltage from the alternator. I just use the ignition switch via an oil pressure switch to disconnect and apply field voltage. The internal diodes in the alternator take care of the rest.

[dansken]

The supply voltage to the external regulaters is indeed cut-off at engine shut-down and With that the also Field voltage. However, there is still 24 volt on the B+.

[Blue Stocking]

How about a large NO relay, able to handle the alt. output, switched from IGN ON.

2 Alt.? built in redundancy ?

[dansken]

Not sure what a NO relay is...
instead of one large case I have two small case 24/75 which fit in the engine room - and provides redundancy.

[Dsanduril]

NO = Normally Open

A big relay/contactor that is opened when the engine is off. If you go that route it should be at least two-pole and cut off both the field and the battery connection. If the relay opens accidentally while the alternator is running and the field does not cut off at the same time the diode bridge in the alternator will get fried.

That same diode bridge is what isolates the alternator when it is shut down, if it fails then your alternator will be toast anyway and you'll probably repair it pretty quickly.

You could also use one of the market isolators, they're just another diode in the line. Cost you power and voltage drop.

[dansken]

OK. thanks for Your input! get the feeling that you think that the diodes in the alternators will provide sufficient protection..?

[Dsanduril]

Originally Posted by dansken

OK. thanks for Your input! get the feeling that you think that the diodes in the alternators will provide sufficient protection..?

Personally, yes. If the diodes don't block all flow you end up with a drain on your battery that no one wants to have. When diodes do fail they're more likely to fail open.

If you really want the isolation I would go with a relay rather than a diode, as the diode will cause voltage loss, you'll need a heat sink,...

[smac999]

every boat has hot alt posts even if connected to engine battery. because most engine switches are never shut off either. I wouldn't be worried.

if your alt is isolated case ground then you are already better off then 99.9% of other boats.

the newer battery isolators (like the promariner) don't use dides and act like a combiner instead. you'll still end up with hot posts, as they purposely allow this as some alts need 12v on the post to start up.

the old school diode blocks are a waste of power.

if you're really worried I would put in manual battery switches and turn them off when not using the boat. if a solenoid or relay failed you just blew up your alternator. plus I wouldn't want them to shut off that alt line until after the engine has fully shut down and stopped spinning. which would be a delayed time after the key is stopped.

[hellosailor]

I've had alternators (plural) fail in the past, where a diode went bad and allowed current to flow, draining the battery. So they definitely can and do fail that way.

I wouldn't use an "isolator" as those are simply more diodes and no matter how expensive they are, they also cause voltage drop and of course can fail in the same ways as alternator diodes.

A manual switch would be the most failsafe thing to install. A relay, a good robust relay, probably as good since relays are also pretty reliable. Assuming you can find one rated for enough power, in constant use, at an affordable price. Otherwise? Another manual battery switch and you just have to make sure it is engaged properly.

Since diodes tend to fail during heavy loads (making/breaking the circuit especially) it might be more effective to put an ammeter in the alternator output. Then just make a habit of checking it after you shut down. As long as it reads "00.00 amps" after you have shut down, the diodes have not shorted out, and it really should remain good until the next time. As a side benefit, you get an ammeter to read when the engine is running.