Please make sure that Viber is the cause for the battery drain by isolating Viber (having no other apps running) on a device with stock ROM (custom ROMs may cause this problem).
We conduct strict testings in our labs with several devices and other than when making a call (when it is natural for the battery to be used), we did not come across any significant battery drain when Viber was simply running in the background. Furthermore, over the course of over 15 hours, on average the battery use was only 2%.
You can find additional information on tests performed by Viber users themselves, which show similar results, at:
http://whrl.pl/Rc0APn