I seem to recall from physics or chemistry lessons many years ago that a simple cell had 2.2v. There are 6 cells in a 12v battery so in theroy you should be able to measure 2.2 x 6 = 13.2v. However there will always be a volt drop across the multiple cells so a realistic voltage reading is 12.6v.
My battery is new, the car has been standing for a week while i do other work on it.
The battery is now flat.
I'll do that blue wire on the receiver soon.