Air compressor - which battery

Go the crank in my humble opinion, it charges first usually and the compressor wont suckk too much out of it anyways unless you are trying to inflate a bouncing castle or somethingLaterally Literal
Seriously Cerebral

Fish,
In my opinion, sorry Bonz, I think you should connect all the accessories to the second battery to give it as much work as possible, this "works" the battery a bit, unless you have accessories running of the aux battery all the time, one failing point for the auxillary battery is lack of use. My personal rule is that all my accessories come off the aux battery, to my way of thinking the primary battery was designed for all the electrical equipment installed originally on the vehicle, and one reason for fitting the aux battery is not to overload the primary battery, so in my opinion you may as well use it. I have a solenoid system with a delayed charge circuit, so the aux battery is always getting a charge after a delay of about 1 minute I think it is. Keep the shiny side up

IMHO I'd use the Aux battery for all accessories, including your compressor.

Primary battery is designed to supply high current for a short period of time (cranking) and running the vehicle's electrical system whilst being charged by the Alternator.

Auxilliary (deep cycle) battery is designed to supply a lower current (things like flouro lights, radios, fridges, etc etc...) over a longer period of time whilst not being charged. Think of it as a storage tank for all your electrical juice.

Keep the primary battery in it's best condition for the most important job of all. Starting the vehicle :-)

Use the Auxilliary battery for everything else. Whilst it's not wholly desirable to completely discharge the deep cycle auxialliry battery, it can handle more discharges than the cranking battery can, and even if it becomes completely discharged, if you keep your primary battery for it's intended purpose you will still be able to drive away.

Anyway, just my humble opinion.

Cheers,

Luke.

:-D

Fish,
I run the compressor off the cranking battery with the engine running.Bob

As some of you know, I build the Outback battery chargers, but i wont try to sell one to you now!
We receive 100's of calls per day, so we are quite up to scratch.
Your second battery, being a deep cycle will take longer to recharge, so the less amps you pull out of that battery the better as the fridge is removing enough, unless its an optima or odessy, which will recharge a lot faster.
If you run an accessory like the A/comp for maybe 20 mins it wont effect the cranking battery, as your alternator will pump charge vert fast into the cranking battery, unless it is isolated, and then when full will start to trickle charge the deep cycle.
The deep cycle, with an overnight camp will normally take around 6 hours of driving for the alternator to charge, but if you had a cranking battery for an auxillary, this will be charged in around 1 hour.

Well I only know what I know.

But on this I'm with Luke and Martyn.

All my non standard accessories run off the aux battery. This includes the compressor and the soon to be fitted winch. I run a chloride battery which I'm assured will run the winch, fridge,lights and compressor.

I have never been in trouble because the aux battery was flat. I have been in plenty of places where if the motor won't turn over it is a long wait for the NRMA or an even longer walk to get help. I know I've done it.

Keep the main battery for what the vehicle manufacturer intended it for, the car.

Duncs