The battery does not have to be fully drained before recharging. Modern batteries supposedly don't have a memory. I wouldn't recharge it any chance you get. Just before going to bed or something like that.
You don't want to run it competely out of power! Doing so could limit the life of your battery. However, while charging, you don't have to worry about "over-charging" it! Once fully charged, it won't let any power go through, preventing the limited battery life also. Great feature!
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ive heard when you first get any device with a rechargable battery, you should let the battery run completely dry the first time before charging. this is just a rumor i heard so i dont know if there's any truth to it but every time i get a new phone or something, i always drain it before i charge.
really doesnt seem to make much difference from what i hear.