I guess many have already noticed this about their Android HTC EVO: When I turn on the 4G it seems to consume battery power at about 3 times the rate as compared to when I have it turned off! Most phone modems and other devices are designed to be used intelligently in a rationing sort of way.
A phone modem is on, when download is in progress. It is turned off at other times. Even if you do not download anything, activating the 4G antenna will bleed charged electricity from your battery faster. It appears that detecting 4G signals requires turning on a 4G antenna because it has a signal booster that consumes energy. On old fasioned TV antennas, battery or AC powered signal boosters are used to improve reception and your EVO has effectively the same feature.
My EVO was running down in about 4 or 5 hours which led me to believe batteries needed replacing. I purchased two for the very low price of four dollars apiece at Amazon which they shipped to us for free in two days! The low battery life problem continued even with the new batteries. So the obvious conclusion was the problem was not low batteries but something else.
Quentyn Kennemer, a member of Quora, writes about other battery life extending tips somewhere out there on the web. He also writes about how to handle living in regions like Artesia, Roswell, El Paso,TX, or Carlsbad, New Mexico where folks are getting ripped off by Sprint who advertises 3G unlimited data that does not work.