Run it off either battery. If you are running engine whilst using compressor then current draw is not an issue. If running off deep cycle second battery with engine off should have enough power to pump up 4 tyres without worrying about flattening . I always run engine regardless of which battery I'm using.

 

You're right in that you'd always want to be running the engine whilst using compressor, as they draw at least 25 amps and in that sense it perhaps doesn't matter so much which battery it's connected to.

However, it partly depends what type of dual battery separator you're using. For example, if it's a voltage sensing unit (for example the popular redarc SBI12), then if it's connected to the aux battery you'll find that the compressor draws more than most alts put out at idle, so it will pull the voltage down and the separator thinks you've turned off the engine so it separates the batteries. Five seconds later it senses happy voltage at the main so reconnects them. You will actually hear the compressor changing tone every five seconds as it just has aux on its own for five seconds, then both batteries and alternator for another five.

To avoid this you could wire it so that when you turn the compressor on, the output also powers the link function on the separator. That will force it to stay joined while the compressor is running.

Or... To cover your behind you'd be better to just run it from the main battery in the first place, if in any doubt.

 

My vote is main and engine running while pumping, makes a differennce, I have an old Piranah 150 system that has been on my last 3 4x4's, this one for 17 years. Early last year I replaced my old BigRed pump with a new 150psi that takes a tyre from 15psi to 38psi in 2 minutes so it works hard and pulls a few amps, it got a good workout over my recent 13,000k trip over to WA and back on as many backroads as I could find.

 

Thanks for the replies guys, much appreciated.

Lee, I am a bit like Gee and have a Piranah unit, but is the 180. I currently run a smaller compressor off the second battery but don't hear it change pitch.

Looks like the main battery is the way to go.

 

I only have the single ARB in car compressor mounted under my drivers seat, but I have it running off my deep cycle battery with no issues, but always have the car running too when inflating, and have never had the issue with the dual battery isolator.

I am running the 150A projecta isolator though and the cut off voltage is 12.8v.

It seems to be if you always the car running while using the compressor it's really just personal preference. The main battery would likely be the better option, but if you already have high gauge wiring etc in the car for the deep cycle, and if it's easier, then no issues running off the second battery.

 

That explains why the compressor dies when i don't have the aux battery plugged in. Thanks Lee

 

It's usually better to run it off the starter battery like you would do with your starter motor or winch. Starter batteries are specifically designed to handle high outputs of power and recover faster. Plus it would be getting most of it's power supplied straight from the alternator when the engine is running so less stress and power drain for the battery. You can use a deep cycle but they are mainly designed to handle lower current items like fridges, camping lights etc and are able to run them for longer periods of time. I've always had mine wired up to the starter battery.

 

My preference is to always run stuff off the main battery that you will always run when the engine is running and this should be the case with a compressor.

Another potential I see depending on your dual system is that if running off the second battery and the main is down a bit then potentially the second could drop quite low and if the main is also low then the controller may not switch over to the dual for a long while.

 

Thanks all, the starter battery it is.

 

just idle up the engine and it wont matter which battery you run it off.
the 120a battery pretty much doesnt charge at all at idle. but 1k rpm and its gets the compressor running like its supercharged.