I think 5 years is very optomistic. 1000 cycles might be possible with a lab quality cell and lab quality charger, but 300-500 seems more likely. Assuming a 3hr pack, that means you are using the light about 3hrs per week - very little compared to commuters, year round riders and dedicated night riders who might need that much every other day in the dark times of the year.


The major flaw in equating 300 cycles to 300 weeks is that LiIons loose capacity over time, so 2 years out you will need to cycle it more often thereby aging it even faster. LiIons can loose 10-20% of their capacity each year (depends on starting cell quality and which source you go by). At best, after 2 full years a 3 hr pack can only be expected to run 2.5 hours and perhaps only 2 hours with lower quality cells. By the second winter, the commuter or regular night rider is going to have to charge it more often, thus aging it faster.


By the third winter there are 2 or 3 new generation of lights out there, so a whole new light is a consideration, making replacement cost almost moot.