iFixit announced new batter repair kit for MacBook Pro retina models released in 2012-2015. Here you can see what they had to say:
This batter replacement kit costs $89.99 to $105.99 allowing the MacBook Pro to last longer and get more juice out of it without having to treasure-hunt for outlet.
Here is the full direction: https://www.ifixit.com/Wiki/Retina_MacBook_Pro_Battery_Kit_Guides