Seems like you need to recalibrate.
Turn off your device
Charge it until its full while the device is off
turn on your device while still connected to a power source and enter cwm
wait for 5 mins
select wipe battery stats
wipe dalvik and wipe cache
disconnect device from power source
reboot phone as normal
Try not to leave your phone connected to a power source for long periods when the device is fully charged as this may de-calibrate your battery/device.