Why on earth did they take away such a useful feature - or at least give developers the ability to create an accurate live tile themselves??
Real time battery information is simply not possible. This is a limitation of the OS. You'll get the update every 15 minutes on both apps, PhoNetInfo and Battery Tile.
And why does Battery Tile need an internet connection for getting battery information? There's simply no reason for this... PhoNetInfo is more transparent and shows additional information.