Here are some of my flashlights. These all use rechargable lithium cells, 3.7 volts each.
The one of the left is my most used light, as it is the one that I carry almost all the time. It is an Olight S15R, R for rechargable. The cell is pictured just to the right of the light. It is a size 14500 lithium cell; capacity of 750 mAh. About the same size as a AA alkaline. In fact that light can also operate on a AA alkaline or eneloop, but at reduced output. The little stand is the charge base which connects via a USB/micro cord to a charge source. Maximum lumens output about 280. Also a medium and low plus a moonlight (extra low). It is a favorite not only because of the small size, great pocket clip and resonable brightness, but because it has a side switch. many li-ion powered lights have a switch on the tail cap, like the third and fourth lights.
The next light , just to the right is also an Olight S15, but does not have the rechargeable tailcap. The cell must be removed for charging.
Light #3 also uses a size 14500 lithium cell, but will not operate on a AA. It has 7 output levels, moonlight to 1400 lumens. I carry it a lot when I want more brightness, but a smallish size, light weight.
Then pictured are two different brands of size 18650 lithium rechargeable cells. They are both 3400 mAh capacity. They are used in the other lights pictured.
The light with the orange tailcap switch cap is a cheapy Chinese light bought on ebay for about $4. It has $15 worth of better parts which were removals from other lights that were improved/modified, so it's a more or less free upgrade, and is my knockabout light. Usually kept on the tractor. Maximum output about 800 lumens and it has an aspherical lens that provides light zooming.
The light laying down has 3 LED's and an optical lens, no reflector. I built this one from a shell and some parts. The optic throws an elliptical beam, very nice when clipped to a bicycle. On high it is over 1200 lumens.
The light that is 3rd from the right is a Nitecore MH20. It is short for a light that uses the 18650 cell. max output about 1000 lumens with 5 levels from a moonlight low, up to turbo. I can also be recharged with the cell inside. There is a rubber plug covered micro USB port on one side, opposite the side switch. It also has a good pocket clip. This is my choice for a brighter light to carry, with smallish size, and the longer life of the higher capacity 18650 cell.
The last two lights on the right started out the same. Both have had the LED upgraded. Both also have had the electronics, the driver, upgraded to push many more amps to the LED. Each has 6 light levels. The shorter one uses one 18650 cell. The longer/taller one uses 2 of the 18650 cells in series. It has a "special" Cree LED that has actually four LED emitters clustered touching each other in the center. The combination of 4 emitters and the higher voltage produces over 2500 lumens. And a lot of heat; it can not run on turbo for more than about 60 - 75 seconds in warm weather. The longer light uses two tubes. One tube can be removed and two of the short 18350's that are pictured used. That makes for a light that can fit a jacket pocket and still have the high output, but for a shorter time.
I have a couple of project lights waiting to be finished. One is fitted inside a kerosene lantern. The other will be a camp light fitted inside a mug.
If you think I'm crazy with so many flashlights, okay. But no more crazy than a guy with a bunch of assorted Coleman lanterns.