Well actually it's not good for the battery to fully charge them. Batteries don't like very low or very high charges.
Try to keep the battery level between 30%-70%.
I'm not piling on here, but I don't agree with this at all. Apple's recommendations offer nothing to back your assertion up -
http://www.apple.com/batteries/maximizing-performance/ - so I'll ask that you back up your assertion with a citation, if you're willing? I think you can't back it up, and my small company has maybe 150 laptops and iOS devices I can tap for statistics. Lithium-ion batteries have a finite life - read Apple's own offering, under "It makes charging easier" (
http://www.apple.com/batteries/why-lithium-ion), and I'd offer that it contradicts your statement, or, IOW: FUD.
I leave my iOS and rMBPs on a charger pretty much all of the time, and carry a spare charging source with me. My current set up is a late-2013 rMBP that I use as a portable workstation and my tethered-to iPad Air 2 - the cycle count on my rMBP is 1 (yep, 1 discharge cycle) and on my Air 2 it's 35, with the design capacities at 103.9% and 100% (respectively). I showed my links, earnestly awaiting yours…
Cheers!