Most lights that use two CR123 batteries in-line will also accept the rechargeable 18650 batteries. The 18650's are a little bigger in diameter so they do not always fit in all lights. Most often though, the 18650's will drop right in. I have only found one light so far that uses two CR123 batteries in-line that will not accept the wider 18650 battery. The CR123 batteries are great but if you are purchasing them on a whim or last minute they can be pricey at the department store or battery store. If you plan ahead or find a place that sells CR123 batteries marketed from Streamlight or Surefire then the batteries can usually be found for around $2.50 per battery. The rechargeable 18650 is the next best thing although, in order to recharge most of them, you need to purchase a battery charger. This was until we discovered the Streamlight 18650's. They actually can be charged via a micro-usb port located on the side of the battery. That is pretty slick. Most households already have micro-usb charging cables or can acquire one for very little money. These batteries are supposed to last for sometime close to 500 charging cycles before they will not hold a charge. At the time of writing this article, Streamlight even sells them in a two-pack for a little over $30 bucks which is pretty handy. Once one battery starts to die, then you can swap it out for the other fully charged battery so there is always one on stand-by.