Have you ever opened a tablet? The battery uses by far the most space, it is flat and pretty large. Even if the battery was changeable you would have to lug a huge flat battery with you. You are better of buying an external battery pack. I have 15000 mAh anker battery pack which brings me through a full day of continuous usage. A battery pack is transportable the internal battery used in tablets is not, just because of its dimensions. If you meant if the battery is replacable, on samsung devices you can open up the back panel with some tools and change the battery, it has a connector and is easy to replace once you have opened the device.
Galaxy Tab 7.7 P6800 | Note 10.1 2014 P600 | Note 12.2 P900 | Nook Touch
[GUIDE] Speed up Note 10.1