With the lithium batteries, I have noticed that sometimes, I can find out that they are about to go by checking the battery percentage in the middle of an mp3 file. It will have gone down somewhat from its usual 100%. Then, however, if you stop the machine and start it later, you'll be at 100% again--at least, temporarily. But checking in the middle of the mp3 can give you some indication of what is going on. I just bought some lithium batteries so I wouldn't have to worry about changing them at all during convention. But over all, I prefer the NIMH batteries, which, as someone pointed out, are a nice compromise between longevity and environmental friendliness. Angie