Good question. I looked this up when I got mine 881U and the Sierra site claims that the internal battery supplements the power from the USB port when extra power is required. I assume that the power from a USB port may not be constant and so the battery.
Message Edited by bsmith15 on 03-10-200804:56:54 AM
The answers provided are mostly correct. What Excaliber meant to say was that typical USB ports do not provide enough amperes. (Nnot voltage.) Devices are only allowed to draw up to 100mA initially. You might be able to get more amperes if there aren't other devices drawing power on the same USB bus, but in my experience, this often causes the computer to crash or at the very least, for the OS to turn off that particular USB bus to prevent damage to the motherboard.
BTW, the information was also included in the manual for my 875U. I don't know if it's still in the manual for the 881U, but as bsmith stated, it is available on Sierra's website.