I think the standard battery is 1100mAh (1.1 amps at 3.7V for one hour), which is generally good enough for me, but there have been a few times on travel when an extended battery would have been useful. USB provides 5 (FIVE) volts to the unit, which is regulated down to charge the internal 3.7V single lithium cell. It should be trivial to build a battery pack to recharge any product that is so powered. But you cannot use external lithium because that would also be 3.7V. I'm thinking of NiMH (nickel metal hydride), each cell of which provides 1.2VDC. Four cells in series would be 4.8V, and there are now 3000mAh (three amp!) AA cells out there, so NiMH AA cells should more than triple battery life for the Eris. Of course, you could use five such cells to get 6.0VDC, then regulate down to 5, but that would add complexity. I didn't find any posts on building such a thing, and wonder if anyone's done it.