when you replace batteries in a flashlight or other device, it is best to use all of the same type of battery and not mix old batteries with new batteries. In particular,do not mix alkaline with nonalkaline batteries. Incorrect use of batteries can cause hydrogen gas to build inside the batteries and cause the casing to rupture. Worse, the mixing of hydrogen gas with oxygen can be an explosion hazard. Cases are reported of flashlights that have exploded in hazardous situations.