I have not used a 14500 long-term in mine, but have used one off & on. I tend to think that on a limited basis it'd be ok. I lost interest using the 14500 in it though because you lose all of the other levels. With a Li-ion it just runs "wide open," no low medium or even high. The LD10 only has a boost circuit, and does not have a buck circuit which would buck the extra power down so the other levels would be useable. Fenix lights are really efficient since they only use one type of circuit, but the price they pay for this is not being able to effectively use the 4.2v Li-ion batteries. Quark lights for instance have both a boost and a buck circuit which is why they can use any type of battery. If you put a weak 1.5v alkaline battery in it then the boost circuit boosts the power up as much as possible so that other levels are useable. If you have a powerful 4.2v Li-ion rechargeable 14500 in the light then the buck circuit kicks in and bucks the power down so that the lower levels can be useable. The big benifit of the 14500 is being able to achieve the much higher outputs that alkalines are unable to produce. Unfortunately in the LD10 the only useful output with a 14500 is max...it's over 200 lumens though.