Migration assistant destroyed battery life I got a new computer from work: Macbook Pro 2015. Opened it up and the expected battery life read 14 hours. 

Used Migration Assistant to upload my old computer, a 2013 Macbook Air. Migration worked fine and the laptop was plugged in the entire time.

Now, a day later, with the laptop on idle and literally zero apps open and the screen on medium brightness and all that, the expected battery life is 4 hours. 

How is this possible? 

EDIT Added details.
The 14hrs of battery life the MBP reported initially was almost certainly an incorrect reading.

The first thing I would do is reset the System Management Controller (SMC) and see what readings you get then. 

Reset the SMC


Shut down your MBP
Unplug the power cable and any other hardware (keyboards, mouse, drives, etc)
For 10 seconds, press and hold at the same time the shiftcontroloption keys (on the left side of the built-in keyboard) and the power button
After 10 seconds let go of all keys and the power button 
Plug in the power cable
Turn your MBP back on with the power button.


Once you've done that see if you notice any wild fluctuations in what the system reports in terms of your battery life.

Run Apple Diagnostics

After resetting the SMC you can also follow the steps below to run Apple Diagnostics if you're still concerned:


Fully shut down your MBP
Restart your MBP
Immediately press the D key and keep it down until you see the Diagnostics screen appear
Wait for Diagnostics to finish (this typically only takes a few minutes)
Once complete, one of two things will appear on the screen:


a No issues found message
a brief description of any errors found plus further instructions

If the diagnostics test does find errors, take a note of what they are


Note: If pressing and holding the D key at Step 3 doesn't work, start again at Step 1 and, at Step 3 press and hold both the OptionD keys instead. This will try and run diagnostics from the internet instead, so you will need to allow more time for it to complete. 

Regardless, take a note of what happens and let me know how you went.

Calibrate your MBP battery

If you're still concerned, you can always calibrate the battery. This isn't officially required on MacBook models with built-in batteries, but some users report an improvement after doing so.

Before following these steps, read over them fully as you will need to leave your MBP in a flat and powered off state for at least five hours, so you may want to time things to suit you (for example, so that it's in that state overnight while you're sleeping). 

Follow these steps to do the calibration:


Fully charge your MBP - Since we're unsure about how the OS is reading the battery percentages, make sure you go by the charging indicator rather than the battery icon in the menu bar.
Now use the MBP for a minimum of two hours while it's still plugged into AC power. (Note: You don't have to actually use it, but have it logged in and running off AC power for two hours).
After a minimum of two hours has passed, unplug the MBP from AC power, but do not shut it down. Leave it logged in or use it as normal during this time.
Keep using your MBP (or just leave it running) until it automatically goes to sleep because it's diminished its safe power reserves. If you happen to be using it, you should see a warning that the battery is low. Ignore this.
Eventually your MBP will go to sleep. When it does, press the power button to shut it off fully.
Wait at least five hours and then plug it into AC power and give it a full charge. Once again, go by the charger to know when it's fully charged.