7.2V is the nominal voltage and will be higher hot off the charger.
Do you know what type of battery it uses?
If it's Lithium ion the voltage will be about 8.2V hot off the charger, if its NiMH then it could be as high as 9.6V.
The dimmer won't make any difference because it uses PWM and doesn't actually change the voltage. If you know the current required but the LEDs you could just use a resistor to drop the required voltage, look up Ohm's law on Wikipedia.