Hi Peter, usually something around 8V.
The ballast is switched out of circuit when you turn the key to 'Start', i.e. when starter motor is in circuit to give a fatter spark for starting.
So, yes, you will need to buy a coil that is rated to run at 12V continuously...
I'll see if I can find a name/number on mine (and attempt to measure those plates at the same time...
