Glow plug supply volts, 8v or higher?

[MadVlad]

I've had an issue with slow starting and subsequent white smoke in colder weather so I replaced the glow plugs yesterday.  All four were dead.  I noticed afterwards that the bar that supplies power to the glowplugs is only reading 8 volts with the Battery fully charged.  Is 8v the correct supply voltage for the glowplugs?  The weather is mild at the moment so I can't tell if the starting issue is fixed but I'd like to know if there's an issue with the relay too..

[Perfection]

All four plugs dead? How you checked to confirm it was dead?

[MadVlad]

I couldn't get any resistance reading using a multimeter before I removed them and then, once I removed them, tried using jump leads to see if I could get them to glow - no luck there.  It's a 2009 model year car with nearly 200,000 miles on the clock and I've no proof that the glow plugs were ever replaced.  This is my first winter with the car so I've no idea if the previous owner had any starting issues in colder weather.