Does Exist any way to change battery of MBP 2012 13" with a bigger one? The question is simple, can i make an upgrade of my battery? I mounted a OWC datadoubler, and now i'm using an SSD and HDD configuration. But my battery life dropped from 6 to 3h 30... i would like to install a bigger battery, does exist a way?
Please please please don't go to eBay or some other website to purchase a battery that claims it has a larger capacity. I have had too much experience with bad batteries, lies, and incompatible products. My customers will come in with batteries from those places and they burnt out within a week or never worked to begin with. 

If you battery is truly defective or has issues then an AASP can run a quick test, usually before they even check the unit into their system, and see how healthy the battery is and then if it is defective order you a new one. I do this each and every day because I like to show people their batteries health before carrying out a repair/replacement.