The flowlight features a dc-dc converter that accepts anything from 0.9-6.0v and puts out a regulated 4.0v.
The battery does not affect the brightness, unless it is about to die/dropping below 0.9v. Because NiMH rechargeables can handle a higher power draw for longer, they work better in flowlights than disposable alkalines, even though the voltage is slightly lower!
We recommend using rechargeable NiMH AAA batteries with flowlights and you can find them on our website here: http://www.flowtoys.com/powerup/batteries/
If you have any questions, please feel free to contact us!