I have been given a smart relay switch for my dual bat set up and I am confused as if it will work. It is ment to allow the main to charge up to 13.7v and then connect the two so that the second can charge. Then when the motor is switched off it should disconect as volatge returns to normal. It is rated at 140 amps. I have run and reved the engine and tested for volts. It sits at 13.5v on the bat and 12.4v on the coil. Is my alt under charging as the book says it works between 12.5 and 14.5 at 3000rpm. How does this work? Is this smart relay suitable?

From your description, the smart relay sounds perfectly suitable for a dual battery setup.

If that's not a typo, 12.4 at the coil when you've got 13.5 at the battery is too low (13.5v is also low at the battery when idling). Is this a kombi? If so, the problem is that the voltage for the coil comes from the battery the long way - via the ignition switch! The low voltage is probably b/c of corrosion on the terminals. Clean the fuses, where the battery earth strap joins to the body and the terminal on the solenoid where the battery positive connects. See if that helps.