Not exactly. I know it's running better tech then a 3DS (if this is true) and the displays on those are only slightly smaller then an average smartphone display (which is usually between 4-5 inches, my Note 4 as a 5.7 inch display and that is the biggest out there for smartphones) but I'm expecting battery life to be around that of a 3DS if not better (you gotta remember a 3DS runs two screens, a 3D and a normal one. I get roughly 8-9 hours of continuous play on my 3DS when I actually play it (it's kind of neglected, the poor thing)). Plus my 3DS never runs hot (then again different tech here). The one thing Nintendo is good at, which you said, is handhelds and they've always had great battery life once the Pocket Generation of Gameboy's hit the market (the original "brick" could probably get you around the same battery life as a 3DS if you were lucky and that was only running one screen). Plus, in all honesty, the graphical quality of the new Zelda game for the NX isn't all that better then a 3DS (I equate 3DS graphics to PS1/N64 era while the Wii U is still sitting at roughly Gamecube era, maybe slightly better) so I don't see the games that might be coming out of the NX being all that graphically taxing.
What it will come down to is the on board O/S and whether or not the console will support features like the Nintendo Store, Virtual Console, and everything else the Wii U currently has. If it does then yea battery life is going to be atrocious on this machine especially with detachable controllers (which I'm thinking, for practicality, will only be usable when the system is docked). Honestly, if this proves to be true, it could either kill Nintendo's future in the industry as a console developer or it could save them. I'm leaning more toward killing their console division because the only way I see the NX working (in this format) is if they kill their DS division and make this the new handheld which they would be stupid to do as it's the only thing making them money (outside of those few gamers who own Wii's and Wii U's). As a huge supporter and fan of Nintendo's handheld division (I've owned every incarnation of the Gameboy (with the exception of the Pocket and Advance SP) and two incarnations of the DS (the DSi XL and the 3DS XL)) I don't think I could support Nintendo's decision to make the NX a portable system, it makes no financial or practical sense as one thing in this equation is going to have to die to make things work and my feeling is it would be the DS platform.