Deep Cycle Batteries

Sounds OK, but be aware that once the solenoid is energised ie when attemrting to start the car (ignition on), for a split second, full cranking amps are going to try and feed from the second battery, this may fry the solenoid contacts and/or over heat the wiring. Might be better to switch the second battery via a manual switch after the car has started? However, you need to remember that if you forget about it overnight - two flat batteries. Also, charging sensing is still only going to sense the original battery and will prevent full recharge of the second battery as the alternator cannot "read" it clearly.

I basically agree with desert...
What you've done is good, but you will probably never FULLY charge your second battery with that setup. ("Smart chargers go through 3, and occasionally FOUR stages of charging ie: BOOST-ABSORBTION-equalisation- and FLOAT--- all at different voltages)
Actually by fitting the battery somewhere other than the engine bay, you'll probably dramatically extend the life of the battery. Batteries hate being flattened, but they also HATE excess heat. I had a mate who moved the battery from under his bonnet to his boot and tripled the time he was going between battery changes (I wouldn't go that far as you'd need humongous cables- but anything you can do to keep your battery cool helps...)
Do you have solar or some other way of charging- if so accept the fact that the alternator will NOT fully charge the battery, even after driving all day, and try to top it up fully when and if you can. Even if you occasionally have 240v available and top it up with some other charger, it would help. It does not do a wet cell battery any harm at all to have a good "overcharge" occasionally, as long as you watch the eletrolyte level.

Hi David & Desert,
thank you for the advice on my setup.
I do not have a solar panel connected to my system but I have been considering one.
While travelling I carry a 700w gen set and at one stage I had a battery charger fitted in the circuit that used to charge the battery when the genset was running. The genset usually ran after we had stopped for the night and the DS auxillery battery would run the fridge after we ritired for the night and supplied power for the auxillery lights after the genset had been shut down.
I no longer have a battery charger in the circuit but am reconsidering installing one after the comments that you made regarding charging rates from the vehicle alternator.
If either of you know a system that I can install that will allow full charging to take place I would be very much oblidged

I use a LEAB charger on my camper trailer and also for topping up my aux battery in the vehicle. It's a bit dearer than most, but is waterproof and can cope with vibrations, will cope with generator power between 170-240 volts), etc. The australian distributor is http://www.bcaaust.com.au.

For me the charger was much cheaper than premature failure of my 3 deep cycle batteries from undercharging.