@matteo80904 try what iFixit recommends: For optimal performance, calibrate your newly installed battery: Charge it to 100% and keep charging it for at least 2 more hours. Then use your device until it shuts off due to low battery. Finally, charge it uninterrupted to 100%.
Now if you continue to have issues with the battery, I would suggest you contact iFixit. Since you bought the battery on iFixit, contact the service department: eustore@ifixit.com or support@ifixit.com and let's see if they can help you further :-)