Some 1 cell 1.5v lights the boost circuit basically goes into a sort of direct drive making the LED driven almost directly off the lithium ion battery. Some lights the circuit will still try and boost it and it will fry the circuit and possibly the LED also. There isn't a "given" that all boost circuits allow lithium ion use it is best to double and even triple check and even at that you will find you have to limit use and may completely lose all but the highest output level till the battery is nearly depleted.Thanks for stopping me from frying my light. I guess I will use one 10440 with a dummy battery. Why is the Streamlight Microstream that is designed to use one 1.5 volt AAA able to utilize a 3.7 volt 10440?