These 18650 rechargeable lithium batteries will allow you to get incredible runtime out of the flashlights that can handle them. Roughly the same size as two CR123 batteries, each 18650 is 3.7V and 2500 mAh. A protection circuit is attached to the battery to ensure that you do not overcharge or over discharge the battery. As with all lithium batteries, the proper care needs to be observed. Do not overheat, incinerate, reverse polarity, mix voltages, or do any other potentially dangerous activity with these or any other lithium batteries.