9 Volts enough for glow ?

Hi everybody ! checked the voltage on the glow plugs today - because even at long glow time ( about 45 seconds - 1 minute the engine seems to start not that easy as i should.even if its san old 1 ) showed up to be about 9 volts ... is that enough ? glowplugs themselves seem to be fine... testred them by ohm-meter...

thanks !

Alex

Most likely battery on way out or bad earth.
Battery distributors have battery tester normally
Also jump lead from battery to engine block will prove if you have a bad earth