As far as LED flashlights go, most users will tell you that the 18650 li-ion battery is often the preferred choice.
All lithium-ion batteries get their numeric identifiers from their size. In the case of a 18650 battery, it’s 16mm in diameter and 65mm long. The zero at the end simply means it’s cylindrical. Some 18650’s will differ in length, but only slightly. The difference is in those that are protected. This means that a protection circuit has been physically added to the cell. This is a good thing, since it gives the battery added protection from being accidentally over-charged or over-discharged. If given a choice of protected or unprotected, think of the protected cell as being an insurance policy against possible charging failures. Nominal voltage is 3.7 and shouldn’t be discharged below 3.2 in most cases. The added circuit will make certain that doesn’t happen. On the high end, a freshly charged 18650 li-ion battery will normally top at 4.2 volts.
A 18650 NiMh battery is a very powerful choice when it comes to operating an LED flashlight. A lithium-ion’s available power is measured in milliamp hours. The higher the mAh capacity, the longer the battery will last, but there are several variables that come into play. Cells are marketed with capacities anywhere from 2200 all the way up to 3400 mAh.
When shopping for a 18650 li-ion battery, you may be surprised at the price tag on some. But what you’re paying for is an extremely powerful piece of technology. Many of the LED flashlights made today can crank out several hundred lumens of light. To operate as much as 700 lumens for up to 90 minutes of continuous light, you need a powerful NiCd rechargeable battery. And most 18650 lights will only take one cell. Naturally these batteries are rechargeable. But you’ll definitely get your money’s worth from them. Even though prices can reach up to $20 per cell, you’re getting a long-lasting battery capable of hundreds of charging cycles.