I've kept a spare for my Android phones in the car since 2009 - sometimes for many months at a time and fully charged. I never noticed more than a few percent loss in charge. Keeping them at 50% charge may increase the life of the battery - but makes it 50% worthless as a spare. Given how cheap batteries are, I always keep a few fully charged ones around and spend the $10 or $20 a year for new batteries.