Is it bad to leave the Macbook Pro Retina 13inch charging all the time? I connected my Macbook Pro Retina 13 inch to an external monitor, keyboard, and mouse and in order to leave it closed, you must have it connected to power. When my Macbook is closed, does it ruin my battery because it's always closed?
The battery charging automatically stops when battery is full. 

So you can leave it plugged in as long you wish.