The reason why batteries last a decent amount of time in cellphones and smartwatches is because the OS tries to turn everything off as much as possible. They only works because those devices works for a few minutes at a time and sleep the rest of the time.
If your app runs, it’ll keep the CPU awake. Not to mention, the NFC itself. Even with the screen off, it’ll drain the battery fast.
Also, I’m 100% certain you can’t control the EM field directly without rooting the device. And even if you do, there’s a very good chance you won’t be able to anyway, because the chip doesn’t support it or something.
If you want to investigate the technical side of it, this might give you some ideas.