Manufacturers usually in their datasheet declare lifetime of 500 full charging cycles until the capacity drops under 80% of capacity of a new cell. That may equal to 4 to 10 years of use. If we assume that common user would use headlamp in average one time per week and that he discharges half of the capacity, simple math tell us its 26 full cycles a year. So this user would reach 500 full cycles after 19 years of use! 19 years of lifetime is probably too much, li-ion cells are ageing some even when not used.
Li-ion ages naturally from the time it was made thanks to reaction of electrolyte with an anode material. Many things can this ageing accelerate. During ageing, internal resistance rise and usable capacity drops. Because of that, battery can also exhibit more heat during high current load than the new battery.
Li-ion cells as any other cells do not like cold temperatures, especially temperatures under freeze point. Low temperature cause higher internal resistance and lowers capacity which could cold battery provide. Li-ion batteries generally work even deep under freezing point (0°C), their capacity is degraded but still reasonable even in -20°C, but this use accelerate ageing.
Ageing also depends on charging and dischraging currents. You can influence charging current by using e.g. our 0.5A mains charger which charges battery slowly, like C/5. Time of charging of depleted 23Wh batterypack to its full charge is 7.5h. Mostly you dont charge so long as the battery is rarely completely discharged.
Discharging current is hard to influence. When you shine at highest mode, discharging current would be higher and so the ageing.
Do not use li-ion uncovered when outside temperatures are under 0°C. In that case you must put battery under outter layer of your cloth, into any pocket or insulating material which will help battery to stay in decent temperature. Battery itself produce little heat during use, if you insulate it, it should keep the stable temperature. We make neoprene pouch for our 23wh batterypack just for that reason.
It is better partial discharge than complete discharge of a cell. At best you should never allow discharge so much that the battery protection trips off your lamp. Our headlamps gives frequent warnings that you have 33%, 10% o 0% remaining charge. If you happen to completely discharge li-ion battery, charge it again as soon as possible to prevent any damage.
Long-term storage, say multiple months storage, is best done in partial charge (ideally 70% charge) and in cold place (5°-10°C).
Charge li-ion batteries just in room temperature (15-25°C).