Issues in android smartphone battary after charging the phon

Hi...I own a Sony Xperia S with ICS(bought a year ago). I ve observed that after using sony portable charger once or twice, the phone charging time has significantly increased from 1.25 hrs to 2 hrs and the 100% battary is not even giving 18hrs of backup if used normally.

Please let me know if my observation is true or it is happening jst coz ma battary has Become old. Any suggestion on this please.

Regards,
Sagar