The BOE-3500R is a 18650 rechargeable battery with a built in micro usb charging port. We provide one in each of our handheld lights we sell, but having a backup battery is a great idea. We like to keep an extra battery in our truck in case we use it all weekend hunting, accidentally leave it on, or let a friend borrow who forgets to recharge it. It's always a good idea to have another one ready to go meaning you will never have to wait on your flashlight to be charged. This battery fits the Barebone Outdoors TPL-25 and HPL-50 handheld flashlights.