Ipod Nano Battery Life Weirdness

Ipod nanoNot all that long ago, like a month, I bought my wife an iPod Nano 2 GB. Both of us have PCs as our primary computers. I have a Mac but its a heavily upgraded blue and white G3 (now a G4 500 MHz thank you!) so I never expected to use the Nano on it. The Nano was un boxed (nice packaging) and instructions were read. It was charged over night and the next day she used it. The first day she only got three hours of life before it was saying ‘feed me!’. I figured that it needed to be run all the way down and the charged to set the battery life chip in it but the next time she used it it had the same life span. Weird. Ok, we took it back and exchanged it for a different one. We did the same thing with the same results. Three hours of use and then flat. Not finding any info on the web about this i decided to give it a shot on the old Mac. I hooked it up and it did it’s thing. I’ll say this, if you have a Mac the Nano integrates much better with it than the a PC. Not all that amazing but I figure I’d mention it because after I did that I was able to run it for 10 hours before it did an auto shut off. The music was played at 70% volume through the stock headphones with the same music as my wife had originally loaded on it. Did being attached to a Mac somehow jolt the energy management settings into life? Just weird luck? Has anyone else had this problem? Could this be a good reason to return it once again and just wait for the new Nanos to hit the market?