Mugen Power, the third-party battery specialists for mobile phones, has announced a 4000mAh Battery Case for the Sony Xperia Z Ultra [CC-6802]. The case provides 4000mAh of battery capacity over and above the 3050mAh internal battery found in the Xperia Z Ultra, which should double the battery life of the handset.
From what we can tell from the pictures, the battery case has an on/off switch and it also looks like there may be a stylus attachment too. The Sony Xperia Z Ultra 4000mAh Battery Case costs $98.50 plus shipping on top (free express delivery applies to orders over $150). It will be made available from 17 January 2014.