An accessory for the "POWERTUBE" battery pack which has been presented by me in another instructable or any other battery between 2.7-11.8v.
What happens when the battery of your phone or mp3 dry out while riding your bicycle?
POLOLU S7V8A will do it's magic! For specs see here.
The S7V8A switching step-up/step-down regulator efficiently produces an adjustable output between 2.5V to 8V from
input voltages between 2.7V and 11.8V. Its ability to convert both higher and lower input voltages makes it useful for applications where the power supply voltage can vary greatly, as with batteries that start above but discharge below the regulated voltage. The compact (0.45″×0.65″) module has a typical efficiency of over 90% and can deliver 500mA to 1A across most combinations of input and output voltages.
See also the S7V8F5 with fixed 5V output if you're interested.