Try the Lithiums. It isn't unusual for them to last 100 hours.
Hi! I always use Energizer E2 titanium batteries. They're a bit more expensive, $5 or so for a pack of 4, but at least in my experience, they last far longer than any other type of battery I've ever used, so now I simply won't buy anything else. I just put the first set of them in my BP this past Saturday, so am not sure how well they'll perform, but in my MP3 capable CD player, I can typically count on a minimum of 50 hours playing time. If you want a really accurate gauge of how long a set of batteries can last, I'd be happy to let you know when I do my next battery change for the BP.