I recently adquired a MacBook Pro 13-inch with Retina Display and how should I charge it? Can I just leave it connected to the source or it will damage the battery even more?
MacBook Pro with Retina display, iOS 8.1.2
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The most common way is to put the MacBook to charge when battery is low and let it charge the battery completely (you will see a green light in the charger when battery is full). You can leave it connected the time you want to, as the battery will not overcharge. However, if you are going to leave the Mac plugged in all the time, unplug it at least once a month to use the battery and keep its lifetime.
To supplement Mende1's good advice, these Apple support articles may be useful:
https://www.apple.com/support/macbookpro/batteries_power/
Ciao.
How to charge my MacBook properly?
