Battery isolator coming from Alternator

[DROdio]

One of the members on this BBS is graciously helping me with my RV electrical system, and he's told me to get a battery isolator to run from the alternator that's being used to charge my 20 deep cycle batteries.  I don't want to keep bugging him so i thought I'd throw this out to everyone:  What is the battery isolator for?

I just bought a 200 amp 12v alternator that we're going to use to recharge the battery bank.  And I found a battery isolator here: http://www.emarineinc.com/products/alternators/isolators.html (although i'm not even sure it's the exact thing Nick's looking for).  But I can't figure out exactly why it would be needed.  Any insight would be appreciated!

[FloridaCliff]

Normally its used to seperate the Start batteries from the House batteries.

They are only connected when the engine is running.

The solenoid is setup to only connect them when it sense the voltage from the gen/alt.

This way you dont drain the starts while you are parked.

Cliff

[DROdio]

aaah, got it, thanks!

[Buffalo SpaceShip]

The trouble with isolators is that they use a diode as a "one-way" valve. The diode creates a voltage drop that you have to account for. The Shottkey (sp?) diode has much less drop, but some nonetheless.

A solenoid by its nature doesn't have a voltage drop.

Also, if you're buying a second alt. (as opposed to the coach alt.), you don't need an isolator at all, since both systems would be isolated. If it's a replacement for your coach alt., then that's a different story.

BTW, my Buffalo has the stock alt. @ 24v and a second 12v alt. to charge the house/ inverter batts. I didn't set this system up, or I would have just used the existing 24v system to run a BIG 24v inverter and center-tapped or used a Vanner for my few 12v loads... and used a solenoid to isolate them.

Just an FYI,
Brian Brown