This is an adapter you can put an 18500 rechargeable battery into so that it's snug in the place of a 3 x AAA cartridge, like in a lot of LED flashlights. The lower battery voltage (3.7 for Li-ion vs. 4.5V for alkaline) is pretty much offset by the lower internal resistance of Lithium electrochemistry, so the flashlight appears just as bright. Electrical safety is up to you; don't use this if you don't understand the risks. That being said, I haven't had any issues with this in a Utilitech flashlight I got at Lowe's Home Improvement, similar to the one in the photo. Edit: 3D print pictures up, check 'em out. Note: It is designed for a 22mm inner-diameter cylinder; I found that it is a bit snug for precisely 22mm (difficult to remove, and you want to remove a rechargeable battery); I might redesign it for a little more flexibility. I added a new version; it's 45mm tall; as always, you can adjust Z-height in Makerware to whatever you need. I also included a couple new pictures.I printed in standard PLA (well, red colored) with no problems. Having trouble printing anything in soft PLA on my Replicator 2. I just gave it 2 shells and 15% fill at a .2mm layer height. The walls are thin, so it's probably solid, as the walls are 1mm thick (.4mm nozzle, 2 shells each side). The print, at 50mm, is a little longer than the battery, so you might want to print it at Z-height 90% or so, maybe even just 10% so you have a shorty at either end of the battery, whatever works for you. It doesn't bang around in the flashlight as much now.