Problem with Sony Battery

I am using a Sony HDR AX 2000 Handycam, with a Sony Infolithium L NP-F970 battery. I have had the camera and battery for about a year, but it has had little use. I am now finding that the battery has a short life. After fully charging about 20 minutes or less shooting time. After what appears a full charge the battery indicator shows over 300 minutes available. (I would expect more than that.) But then the battery runs down in about 20. (I am not doing much zooming.)

HOWEVER!! After a short re-charge the battery once again was up to over 300 minutes on the indicator. I ran the camera on record and it was soon down to 88 minutes. BUT when I switched over to playback without a further re-charge the battery indicated full with over 500 minutes available.

Is this a dud battery or a camera fault. Any ideas?

Palani