Since I know so little about this, I don't know if this matters, but it might. One of the speakers attaches to the MP3 player via a single prong earphone type plug. The speakers attach to each other using a different type of plug. They charge through a USB plug, which is not connected during normal play---although possibly it could be. The speakers work when the MP3 player is not connected to anything else. Previously, using speakers which did not have batteries, my wife would frequently use the speakers while the MP3 was plugged into a 120 volt outlet, as for recharging. In this configuration, the speakers did not drain the MP3 player's battery so quickly---maybe not at all. I hope that's clear. I have a feeling that if I didn't have everything where I could see it, it wouldn't be clear to me.
My wife's concern is that nothing in the literature accompanying the speakers mentions whether it is safe to use the speakers while the MP3 player is plugged in for recharging. We wouldn't expect them to recharge in this case---only to not discharge the MP3 player's battery.
Does any of that change anything?