It can be done with a 2 cell battery in fact I lent one to the photographer and reporter who did the bike radar interview .
only problem was she used it on hi all the time and depleted the battery in 50 minutes .
it would have lasted a lot longer using med and low power and hi for the fast bits .
If I had detuned it a little she would have got 1 hour 30 mins on hi
or 4 hours on medium .
so a good length ride would have been achievable with a 2 cell on the back of the lid .
Smudge is looking into a Lithium polymer set up
How heavy is acceptable for the battery pack as one of smudges 4 cells is 203 grammes ideally a 3 cell pack would be the best for the Troutlet so that would be 150 grammes plus 100 grammes for the light head