It also has to be an IMR style battery

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Batteries - We only recommend using approved 18490 and 18650 batteries in the ProVari. All of the batteries that we sell on our website have been tested to work with the ProVari and these are the only cells you should use. There is a big difference between all the different batteries on the market and a lot of testing has been done on these various batteries. The reason we recommend using high drain cells is because these cells can output more current providing the ProVari with the power it needs to operate properly. There are a number of batteries that will cause issues including the Trustfire, Ultrafire and other brands. These batteries cannot handle higher current output and the battery will turn off while in use or cause a low battery alert much sooner than it should because the batteries have a high internal resistance. Please make sure you get the proper batteries to use in the ProVari to avoid problems.
What is the difference between regular and high drain batteries and why do you recommend I use the high drain over a standard 18500/18650 battery?
We recommend only using the IMR High Drain battery in the ProVari. These special batteries are rated to have a higher power output than non-imr batteries. Non-IMR batteries can't flow enough energy for the conversion circuitry in the ProVari. You can use non-IMR batteries in the ProVari, but since the conversion circuitry might become power starved, it might think it's dealing with a dying battery and turn off. The unit was designed to use the high drain batteries and we recommend using these for best performance
Dont use cheap/wrong batteries... Buy the AW 18490's....18500's are not IMR batteries, do not use them in your Provari! And if you want to get the best charger buy the PILA charger... It makes a big difference/never over charges and makes your batteries last alot longer(charge cycles).....