I am working on a wireless sensor application at my internship. The sensor network is working, and I am using two Xbee radios, one at the sensing end and the other at the receiving end which routes the information into a central MCU. The issue is, the sensing radio needs to be standalone and thus will run off of a battery. As of right now, we are seeing about 30-40mA being drawn from the circuit, running off of a 5V power supply that I am acquiring from an Arduino for prototyping convenience. The device needs to be able to operate alone for a long period of time, as we would rather not force our clients to change the battery every couple of weeks. I have very little knowledge of battery options out there. My question is, what is the best way to power a remote sensing circuit and allow for minimal power consumption so that it may be powered for as long as possible?