I have read different things from different websites - one said that it takes 15v from the adapter and another 12v. Which one is right? Just to make sure, I am talking about the one that has the detachable keyboard with, I believe, a battery also built into the keyboard (but I could be wrong about this). I have not bought it yet but hope to in the next 2 weeks. I am planning some long train trips in the next 18 months and would like to find/make an external battery that is approximately the same length and width as the tablet that will power the tablet for about 20 hours.