Bought it in September, 2011.
Almost every night I disconnect it and try to drain it till less than 20%.
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I reckon you should read Apple's battery guide...
So you mean, that if I'd work somewhere where I can't have charger, I'd do something very harmful for it?
What are you, 14? The battery only lasts for a certain number of cycles. It's great to use it on battery power for a while every day, but there is no need to drain it the way you're doing. The only thing that will amount to is a battery that will hold less of a charge once you actually need to use it where you have no charger, and in the end having to switch the battery out faster than if you would maintain the battery properly.
As I said, you should read Apple's battery guide. But instead you chose to be an ****** about it, why?
Because I just hate people which pretends that they are some genius + chats *****. I can't stand them. And yes, you're not exception.
My brother has the same Macbook which I have, and now his battery lasts just 1hour when mine lasts 3hours on the same specs brightness etc. Guess why? Because all that time he been holding it on charger. Just 2-3 times drained battery.
Good luck man, I'm not responding to your next dumb pretending being a genius.