voltage at glowplugs?

when i tested the voltage at my glowplugs earlier this week with a circuit tester, i was only getting around 7 - 7,5 volts: the voltage climbing to this gradually and then cutting off as the relay clicked off. i put it down to the fact that my van's crappy underpowered car battery was on its last legs. 

today i got a new heavy duty van battery and fitted it. when i measured the voltage across the plugs with the ignition turned on this time, the voltage was better, but still peaked at about 8,5 volts before the relay clicked off.

is this right?  i was expecting that the voltage across the plugs would hit 12v straight off and be a constant 12v while they were on, but instead the voltage seems to build up [from about 5 volts initially] as long as  the plugs are on and - even with a heavy duty new battery fitted - it only gets to somewhere between 8 and 9 volts before the relay cuts off.

ah - that sounds encouraging.  thanks, i didnae know that. i just tend to assume everything on a motor's electrical circuit uses the full 12v. 

are those the manuals that were on the 'rabbitsonthemoon' site [or whatever it was called]? if so, i've got them already, thanks

cheers.  i'll run a few of those checks at the weekend.  unfortunately the manual disnae mention anything about the voltages involved.  the testing seems to be based more on when and for what amount of time voltage is present, rather than the voltage itself.