He enviado un correo a sony y me remite a un foro de asitencia, la solucion que le ven alli, caso de que este todo actualizado es esta:
"Having problems with low battery life time? Here are a couple of tips that will increase your battery time.
First of all make sure that your phone is running on the latest version of the firmware. You can use PC Companion to check for software updates for your phone.
http://www.sonymobile.com/gb/tools/pc-companion
You can also check battery consumption via the menu; Menu > Settings > About phone > Battery use.
Features that can be disabled or reduced when you dont need them:
Dim the screen
Turn off 3G
Turn off data traffic
Turn off Bluetooth
Turn off Wi-Fi
By doing this you will increase your battery time. But if you're still experiencing a low battery time you can try the following as proposed by many users on the forum.
When phone reaches 50% in battery level turn it off and pick out the battery for a few minutes. Put it back and connect the charger, now charge the phone turned off for about 5 hours or overnight. Start the phone as normal and check if the battery life time has improved."
la verda es que no entiendo como sacar la bateria ( al menos eso traduzco yo)