infra_red_dude
Wire muncher!
As such, there is no way to know the charge/discharge rate of the battery. If its under warranty, I'd recommend you take the phone and battery to the SE service center otherwise you may try calibrating it.
Calibration is not always helpful (sometimes it can be harmful too!). To calibrate the battery, first charge the battery to 100% and let it remain on AC for an hour or two. Then completely discharge the battery till the phone turns off by itself and leave it turned off for 2 hours. Now again charge the phone to 100% and unplug the charger. Try it only if the warranty has expired and do not interrupt any of the processes when calibrating.
Calibration is not always helpful (sometimes it can be harmful too!). To calibrate the battery, first charge the battery to 100% and let it remain on AC for an hour or two. Then completely discharge the battery till the phone turns off by itself and leave it turned off for 2 hours. Now again charge the phone to 100% and unplug the charger. Try it only if the warranty has expired and do not interrupt any of the processes when calibrating.