I did a test today of the four parallel batteries that power all the LEDs. I turned MiniMe on at full brightness at 8am, and to my amazement, at 4pm it was still going strong. By 6pm when I got home, some of the LEDs were off. That indicates that the voltage had dropped below 3.6V. I measured it, and after 10 hours it was 2.8V, well below the threshold where things go to hell. But frankly 8 hours of life at full brightness is way more than I need. That whole time the Arduino was spinning away, but it wasn’t really a fair test – for one thing I set the LEDs to full brightness and just left them – there was no ongoing computation or I/O that might have used more power. So I don’t really know how long the Arduino will run on its two series batteries yet – but after 10 hours it was still at 7.6V (it started at 8.2V this morning).