iPod nano (7th Gen) Battery Drain Problem

I'm wondering if anyone out there has any solutuons or suggestions for my iPod nano (7th generation) problem.

Problem:

- The battery drains very quickly.

- Fully charged, the iPod only plays about 2-3 hours of music before it dies.

- The battery drains when not in use as well. After letting it go to sleep or turning it off completely, when turned back on, the battery goes from 100% to what looks like the 80% mark on the battery icon. If at 50% and letting it go to sleep or turned off, turning it back on after a full day it goes to abiut 30% of battery life left.

What I have tried:

- I've made sure that I've charged it fully by using both a wall outlet and through the USB port on my Macbook Air. The results were the same.

- The iPod software is up to date, version 1.0.2.

- As per Apple's instructions, I've tried the 5 R's repeatedly... reset, retry, restart, reinstall, or restore.

- I've made sure that my iTunes is the most current version, 11.0.1 (12) and restored the iPod from my MacBook Air which is running Mountain Lion OSX 10.8.2.

- I've returned my iPod back to factory settings and synced it to my older MacBook (white colored) which is running 10.6.8, and it still had the same problems.

- I've taken my original iPod nano to the Genius Bar and the tech verified that the battery level was visably draining while we were talking. He replaced it with a reburbished unit.

- I synced this second one to my iTunes and had the same problem. I did all the above steps again and the battery is still draining way too quickly.

- I again took it back to the Apple Store and a different tech said that maybe I had too much stored in my iPod which is rated at 16GB, I probably has about 13GB in it (only music, no videos). But he added that it was unlikely that the iPod would drain the battery that much even with a full drive. Like the other tech, he saw the battery bar drain and replaced it with another refurbished iPod nano.

- I'm now on my third iPod nano. I synced it, but this time I only put 8GB of music on it. I am having the same problems. I tried all of the above again and still the battery drains super fast.

Questions:

- Anyone know what else I can try?

- Anyone have similar problems?

- Is it the kind of music files that I'm using? Most are AAC, some are MP3.

- Am I just getting bum iPod nanos?

If anyone can chime in with any useful info, I'd appreciate it. Thank in advance.