Thanks for the reply,
I am working on Handheld device for measuring and logging data in which we have logging interval. interval settable form 1 min to 255 min, in such situation won't be able to use solar power solution i need my project to be run on batteries. which should last for at least 6 month in case of Xbee s2 series module. should i use regulator for input voltage from battery and outbut of 2.8Vdc will it work for me to the end of battery capacity?