What exactly drains batteries in a CD Player?

[Braves]

I just purchased a SonicBlue RioVolt SP100 Mp3/CD player product link (for anyone that cares)
I noticed that the company was reporting an average battery life of 15 hours while some user opinions were stating that they only got 9-10 hours. I read some reviews saying they got 9-10 hours, but other reviews claim that they got the full 15 hours. So what exactly drains the battery? I always thought that having higher volume did, because someone told me while I was playing a gameboy that if I turned the sound lower the batteries would last longer. I know that choosing 10 second shock protection over 40 second shock protection conserves battery, but what else in the MP3/CD player uses/saves batteries?

On a side note, since this is the first time I'm going to be using my CD player with an AC adapter, does the adapter charge up the batteries or just prevent the RioVolt from using the batteries when it's plugged in? And if I went down to my local radioshack how much would I have to spend to get a pair of 2-AA rechargeable batteries and a charger. Or if there are any cheap sites online

 

[yomega]

I would not be supprised if decoding mp3s take more power than a regular CD, but this is only a guess.

The AC addapter will not charge the batteries in the unit, it will simply bypass them, that is unless being able to charge batteries is a feature of this player, but I don't think it is.

 

[deftron]

Quality of the battery could play a factor....