Yes you can use it fine. These wall warts have an open circuit voltage quite a bit higher than than 6 volts, probably closer to 10 or more. When you connect them to your battery, the internal impedance of the charger drops the voltage down to what the battery needs to charge. You can not accurately measure the current with a conventional multimeter due to the form factor of the current pulses. It is not a steady current but is a rectified sine wave or half sine wave depending on whether or not the design uses a full wave inverter or a half wave inverter. If you want to see what the maximum output voltage of the charger is, put a capacitor (rated for 15 volts or more) on the unloaded output and measure the voltage. The capacitor will build up to the peak voltage and hold it steady so that a conventional multimeter on the DC voltage range will measure the voltage. When charging a 5 cell battery, just leave it on the charger a bit longer than you would the 4 cell unit. At these low currents, you will not damage your battery by leaving in on for more than the normal 15 hours.