Battery Isolator Install issue

I am installing a compressor etc on the truck and wanted to use a battery isolator from a dual battery setup so that the compressor cant run untill the alternator is charging .
Got it all wired but ran into an issue I didnt think of. The battery islolator expects to see voltage on both sides of the switch.
Now my idea is to install a small 12v bike battery in the back so it will charge that etc and see the voltage. I will run my strobe lights or something from that.
Anyone see any other way I can get 12v on the second side of the switch and still have it working as an isolator so the compressor wont run until the engine is running

Your solution isn't bad. If that second battery is a gel battery put it in an acid-proof case. I had one of those little gels leak badly when I tried charging it off the car alternator. The larger deep cycle gels are a little sturdier.

I am looking at it this way. The compressor is rated at 17 amps with a duty cycle of 30% . The best part is it will only take 3 minutes to fill the air tank to 120psi. So in theory a smaller 4ah battery should be able to handle the task with no dramas.Now as long as the pneumatics dont leak then I shouldnt have any issues.
Might go visit the boys at the battery factory tomorrow and see what they can suggest.

If the compressor will only take 3 mins to fill the tank is it really worth isolating the circuit in the setup you have chosen?
Running extra batteries and such when you dont have to sounds very complex and unneccesarily expensive.
Why not connect a relay to a switch that has power only when the key is turned to accessories so you can run the compressor when the car is running or just turn the key to run the compressor if it only for a short once off period so as not to flatten the battery..

What brand isolator is it?

Its a Projecta DCB100
The biggest thing I am worried about is the compressor trying to run when the engine is cranking. One of the guys at work doesnt have anything but straight ignition switched relays and he has to start straight off or the battery dies. I want the compressor to be automatic but do not want it to start until the engine is running and the battery is above 12volts

Whats his battery like tho?? Could be run down, a compressor doesnt drain that much power over a short period of time.

What if you supply a voltage to the aux battery positive side on accessories or additional wire constant power to aux battery inlet?
that way the unit will recieve a reading of power on both sides and it will cut out once the main battery sides voltage drops below cranking volts.

Yes I can do that but I would have to install diodes otherwise the compressor would try to run off the voltage loop cable
It does some weird ass things
With no second battery and the earth not connected its full volts at 14v but with nothing on the secondary side and the earth connected the supply volts only reads like 6 volts
Have to do more measurements me things n see how good my earthing is

What if you install a relay thats hooked up to a switch powered by something that is only live when the engine is running like the alternator.
Am I right in saying that wire is only live once running?
Then you save stuffing around with the dual battery system and cant run the compressor unless the engine is running

I actually tried that as un the relay from the alternator but when the relay coil was connected I got the alternator light come up on the dash. The electronics of the truck are a pain in the butt.
I ended up getting a small 4.7ah bike battery that will do everything needed as well as supply power to all the strobes and aux lights I use.Battery is about the size of 3 packs of smokes.Just have to make up a battery mount and we should ( repeat should) be good to go

Bugger well good luck with it hope it works out for ya.

Well as a test tonight I ran the compressor to test the seals and lines and the battery straight out of the box had no issies at all runnign the pressures up to 100psi so should all do the job nicely

Found my air leak it was the pressure relief valve so I replaced that and so far seems fine
I did find another issue though. I was getting funny voltage readings on the Battery Isolator. It wasnt charging when it was meant to and generally not playing the game.
I took a stab at it and went with a bad earth. Sure enough I ran a temp earth with and problem solved. So after all that be careful of wiring in the tub. There doesnt seem to be any decent earth .Never found it with the old installation as I had run an earth for my lightbar.
Anyway problem now solved so just have to run another earth for the tub.