Watch the 18 Volt restriction on the Airwire receivers. I have used 14.4 volt Li-ion batteries from Batteryspace.com. They usually charge to 18 volts fully charged. ALWAYS fully charge and then test a new battery before using it so as not to go over the 18 volt max OR magic smoke and a burned up receiver. (Been there, done that.) Another option is the QSI sound decoders and their GWire receiver with the Airwire controller. Max voltage is 35 volts. A lot safer, more costly, but you get great sound!