Funton,
Don't get lost in the techno stuff like 'back EMF' and forget to remember that the the rigs talked about here have been converted to full 12 volt systems...lamps, ignition, charging systems and batteries, everything, except the starter, (usually) as it is tough enough to handle the increased voltage and keep on going.
So to get net, the entire truck is now 12 volts...except for the starter. It isn't just a case of sticking a 12 volt battery into a old 6 volt bolt.
Stuart
