Why the batery MacBook Pro 13 only lasts 3 hours i bought it one and a half month ago?

Answer 1

Apple recommends that:

The battery needs to be recalibrated from time to time to keep the onscreen battery time and percent display accurate and to keep the battery operating at maximum efficiency. You should perform this procedure when you first use your computer and then every few months after that.
The calibration procedure for Powerbooks and MacBooks (See links below):

1. Plug in the power adapter and fully charge your PowerBook's battery until the light ring or LED on the power adapter plug changes to green and the onscreen meter in the menu bar indicates that the battery is fully charged.


2. Allow the battery to rest in the fully charged state for at least two hours. You may use your computer during this time as long as the adapter is plugged in.


3. Disconnect the power adapter while the computer still on and start running the computer off battery power. You may use your computer during this time. When your battery gets low, the low battery warning dialog appears on the screen.


4. At this point, save your work. Continue to use your computer; when the battery gets very low, the computer will automatically go to sleep.


5. Turn off the computer or allow it to sleep for five hours or more.


6. Connect the power adapter and leave it connected until the battery is fully charged again.