Hopefully at least 6 hours.

Hopefully you're not serious. 

As far as we know, there's no such thing as "NX handheld", or NX anything.

I agree with everyone about a 6 hour battery life and it better be with a decent amount of brightness too under the sun ...

http://www.techlicious.com/guide/six-android-phones-with-great-battery-life/

The HTC one Maz can do 25 hours and is $230 and pretty powerful. The 3ds launched with $250 price point. I could see Nintendo adopting the parts from phones for the NX handheld. 

Except it's huge(not for me, but for most people). Then again, even I'm not fine with a device that's 6.5 inches. Also, that phone wasn't very enjoyable. It was a huge disappointment from HTC.

3DS needs nowhere near that amount. Even 3500 would be insane. 

Look at google's Nexus 6P. They have technology that can make the phone charge in an hour(or two. I forgot). 

Nintendo just needs a good battery that'll last a good amount of time(my 3DS currently lasts about 4-6). Going to up to 10 hours would be nice. Faster charging wouldn't be bad as well.

Well its also stationary the NX. So they can charge it while playing at home then take it on the go. Not only this but Nintendo likes to use existing technology so i can see them implementing this. 

Maybe it won't need a handheld

It depends entirely on the hardware needs too. If it has a powerful processor and the goal is to push better graphics because they need unification between the portable and console (therefor the portable cannot be weak otherwise you can't have scalable games) and is something along the lines of a Tegra X1 or Apple A9X chip ... absolutely, they'll need something that can power a device at 7-10 watts/hour for 3-4 hours. So that means a large battery.

The good thing though is batteries these days are dirt cheap. The batteries in even high end phones for example cost like $5-$6 at cost.

6-8 hours would definently be nice. I'm just hoping its at least decently spec'd, like a mid ranged phone.