I also have an (oldish) IRiver. Runs for ages on AA batteries, an amazing long time even when the "level" pictogram says ultralow. I use rechargeable batteries mostly, which hold very well. Bomproof, but a bit bulky and rather heavy.
My other (tiny) player with USB charge has only about 9 hours of useful power. Crap, when travelling.
The first player I got (an collection piece now!) was almost unstoppable on one AAA, but had no LCD, only forward reading, 2 buttons...Primitive, but efficient, and as light as any recent model.