You will not want to exceed 3 volts as the receiver will get squirrelly. It can be set up for more voltage, but that requires changing a couple resistor values and re-tuning it. I recommend you buy one of those small DC to DC converters, about 2 amps, and put it between the battery and the receiver. They have an adjustment pot you can set to get the voltage to exactly 3 volts. They are on eBay all the time for just a few bucks. Sometimes they are called bucking converters, just look for the adjustable types. Pololu also carries them.