I have seen many extended batteries on the net that don't increase the phone thickness. One of them is this one: "http://cgi.ebay.co.uk/1930mAh-BL-5J-HIGH-CAPACITY-BATTERY-NOKIA-5800-N900-/200471742882?cmd=ViewItem&pt=UK_MobilePhones_Mobil ePhoneAccessories_MobilePhoneBatteries&hash=item2e ad0c09a2"

But does it work?