Thanks everyone for their input and ideas, especially drivesafe for sparking the idea in the first place.
I had two batterys, both N70ZZ. The first was a 17 Plate 770CCA starter, the other an 85Amp/hr Wet Cell Deep Cycle.
I had an Arrid Smart Relay (just a glorified headlight relay but much bigger and with spike protection and a 1 min dely on the coil after activation) between the two bats. The Arrid was hooked up to the ignition so that when the motor was going they joined and when it switched off they seperated.
The aim was to extend my battery life while at
camp without buying a bigger battery or leaving me stranded with no go in my bats.
What we did:
1. Build a "Low Battery Cut Off" kit from Altronics (cost us about $15).
2. Take the ignition plug off the arrid and replace with lead from cut off switch
3. Plug "input" side of cut off switch onto the Arrid "+" Terminal (from the main battery).
4. Run wire and mount L.E.D. on dash from cut off switch.
All done, mod took about 1/2 an hour to do (and that's only cos I had to pull the dash apart to put the LED in).
Result:
Not only did I get more run time out of my gear but it also seemed to make the batteries run more efficiantly. I'm not sure of the science but I think because the batteries are sharing the load 50/50 they are not working as hard and seem to last longer than when running by themselves. (if that makes any sense at all).
The system is fully automatic (the way I like it). When the car is switched off it now leaves the two batteries connected in parralel until the voltage of the starter battery drops below 11.9v (under load) then cut's the power to the coil on the Arrid Relay which seperates the batteries. The Waeco Cooler keeps running off the deep cycle and the starter has heaps left to crank the surf over in the morning. It worked REALLY
well and was VERY EASY to do. I'd recommend it to anyone.
The LED on the dash is on 24/7 until the starter battery cut's out, it let's me know if both batteries are running or if one is running, just handy to keep an eye on things.
Previously after 12-14 hours of running my Waeco Cooler (drawing about 6-8amps) my deep cycle was getting pretty sad, like totally rooted basically. After running the cooler under the same conditions with the new setup the starter bat had'nt even cut out yet, it was just about too though as the battery was down to 11.9v under load (about 12.2 without load). I wanted to see how much it had left in it so I turn the high beams, spotties, fan on full, air con and all interior lights (2amps) on and turned the key. (first thing in the morn too). The Surf turned over twice and happily sprung to life.
VERY happy with it. So Thanks guys, Explore oz has saved me $$$ again!! :-)))