I approached using 14500 cells with a degree of concern. A third the capacity of a good NiMH AA, even less the capacity of a Lithium Iron AA... but of course the cell voltage is over twice that of these AA's, so, from a VA standpoint, the energy is actually more comparable (but still less). Of course the big advantage is that higher voltage potential that provides more drive voltage headroom to the electronics, allowing the dramatic increase in output of lights whose electronics can handle that higher voltage. So I have bought a few lights that can use the 14500 in addition to the ubiquitous AA cells, and have enjoyed that increased power (and benefit of being rechargeable). Now my concern is that I might accidentally mix the cells and use a 14500 where it should not go - so caution on how you store these!
Most 14500's with button tops are all about the same capacity. The name branded cells are more expensive, but chalk that up to being certain the name brand cells will work/fit in that maker's flashlights without issue. The Xtar cells I've bought work and fit in Jetbeam and Olight lights I've bought, but I bought an Efest that did not work in a Jetbeam Jet I Mk, but did in a Jetbeam H10A... go figure.
So my experience with the Xtar has been good so far, but I can only report that Turbo Modes are active, not the output level achieved. Brightness at Turbo "seems the same" as with name branded 14500 cells, but this is clearly subjective and a poor test process when swapping cells in a light and gauging by eye. And only time will tell about the Xtar's ongoing service and cycle life as I recharge and use them over the next few years. My opinion is that only a handful of manufacturer's have the knowledge, equipment, processes, controls, and staff to make these and other Lithium based batteries, and that being true, the Xtar is likely every bit as good in capacity, service, and cycle life as the branded cells.
If you're worried about this, then you can adopt a more conservative approach. You can, of course, use Lithium Iron primaries for those lights you intend to set aside in storage for "just when I need them"... you can also decide to use name branded cells for lights you intend for occasional and brief use... and you can try the Xtar brand in lights that have heavy use to gain experience and confidence with their ongoing service capacity/life and cycle life. That's what I'm doing.

For roughly the cost of 3 Lithium Iron primary cells or 9 Alkaline cells you can have this rechargeable cell which, for capable lights, can often deliver more than twice the High level of illumination due to its higher cell voltage. Of course, the capacity is roughly a third of those cells, making the break-even for general use (use at Low or Medium illumination levels) more like 9 Lithium Iron or 27 Alkaline... but given a cycle life that approaches the cited 500, savings is assured.
Of course the lower capacity of this Lithium-Ion cell, 800mAH vs 3000mAH for Lithium Iron AA or 2500mAH for NiMH means less service-life, or run-time, at any illumination level. So, then, these Lithium-Ion cells are great for those who use their lights either rarely at low drain or very frequently at high drain. In the first case, about 1500 30 second uses on a typical low level of illumination will see many people through a year of EDC, and, in the second case, perhaps a dozen 2 minute uses on a typical high level of illumination will see these users get through a day with this cell to recharge overnight.
Unfortunately not all AA Lights are capable of running the 14500 cell, and use of the cell can damage such devices; the cell size is so similar to the standard AA, that care must be taken to use the right cell with the right Light. Also, not all Lithium-Ion chargers are capable of charging this cell, so check your charger specifications.
As technology improves, we may see higher capacity secondary cells in this size, but for now the Xtar's 800mAH is near the highest touted. As to durability (cycle life and retained capacity), that will need time in-service to prove.

Seems to last forever and very quick to recharge. Love this battery.

I use these in my flashlights and it makes quite a difference in their brightness.

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