By 6v do you mean a 4-cell pack??? In a perfect world a 4-cell Ni-MH pack would read 6 volts (1.5 volt/cell x 4 cells = 6 volts). Now in the real world the max voltage is going to depend a lot on the condition of the pack itself. If  it's truly fully charged it should read anywhere between 5.5 volts - 6 volts when it's removed from the charger.

Ken