using a 12volt relay for 2nd battery

This the type of "relay" you need for the task you are contemplating......

http://cgi.ebay.com.au/DUAL-BATTERY-ISOLATOR-SOLENOID-EASY-TO-DIY-100AMP-ABR_W0QQitemZ260256521739QQihZ016QQcategoryZ30862QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

solenoid

hey cooper, you're probably talking about say a 30A relay or the like? That will work in the way you describe however the charging current can be up to the full load of your alternator, which can be as high as 90A so you'll need a big sucker. I got one here in the shed, (100A) bought it to do exactly what you are planning however......Then I started reading about voltage spikes that these relays put back into the system, and went for a relay like Roachie is talking about above, it cost me about $120 which is just $30 more than the 100A relay so I went for that.

Works a treat

Cooperdog,
We used to use relays before isolators became fashionable. Like Derek says, the terminals burn out - can't take the continuos current - often 30-40 amps when a discharged battery is getting charged. In addition, if the relay is energised while cranking, then a heap of current (say 100+ amps) will go through the relay. To prevent this current surge, we would add a self-resetting circuit breaker, and sometimes they would stuff up.

To solve those problems, people started using the solenoids that Roachie guided you to - they are $25 well spent. Solenoid is just a high current relay.

Cheers
phil

the relay you describe is a mini relay good for spotlights etc, you need a continiuos rated solenoid capable of 300 amps intermittantly , good silver contact solenoids can run at $ 80 , you can still run the config you want to operate the solenoid. make sure there is the capabilty at the back of the alternator . watch out for taking the trigger feed from the light terminal . back emf from disengaging the solenoid contacys of your battery isolation system you have proposed may in fact destroy your regulator.

Cooperdog

Why not use a VSR or Electronic isolator. Simple 3 wire connection and switches on and off automatically.

These are very common and popular. Manufacturers include Redarc and Piranha.



Regards

Derek.

beware diode protected systems . volt drop applieies and the second battery never fully charges

Have a look at the Red Arc system.
This One -
They are as good as any and simple to install.