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Single Cell Li-ion and LEDs -a Perfect Match!
PFlex Pro products are designed around a single Lithium Ion Cell. We believe this is the best configuration for a small EDC flashlight. The nominal voltage of a single cell Li-ion battery is 3.6-3.7 volts -the input voltage of a high power LED emitter is in this same voltage range. If a flashlight uses 2 or more Lithium Ion cells, the control electronics (driver) must reduce the voltage before sending it to the emitter -this reduces efficiency and increases heat generated by the electronics -since heat is 'the enemy', it's best to use a power source (battery) that is close to the required voltage of the led.
In addition, multi-cell flashlights should have matched batteries and the user should employ special charging techniques to ensure the cells have the same charge level and actual capacity.
The high capacity and power capability of a single 18650 Li-ion battery makes the extra effort (and risk) with multi-cell flashlights unnecessary.

Battery recommendations: This is really a matter of taste and what you are comfortable with. IMR batteries have high current delivery, they are safe, but lower capacity. Many battery manufacturers have started calling their batteries 'IMR' to designate 'high current', so just because the supplier calls it an IMR, it might not actually be a '
lithium manganese oxide' battery. Li-ion Protected cells have high capacity, built in protection, but they have lower current, and sometimes issues fitting into certain flashlights. Some unprotected cells are available in a 4.35V version which is compatible with all of the PFlexPRO products. When testing higher power lights 3.8A and higher, I use a Samsung 30Q battery. The 30Q is an unprotected cell, but the PFlexPRO products have built in low voltage protection and a good quality charger will have algorithms that will safely charge this battery. When testing lights below 3.8A, I use the Sanyo/Panasonic NCR18650GA 3500mah battery. The GA battery has higher current delivery than the older 3400mah 'B' series and a little extra capacity to boot.

Safety: I think the biggest safety measure you can take is to buy your batteries from a reputable source. I've heard from many of my customers who have bought fake batteries, usually from ebay.. If you buy your batteries from ebay, do some research... don't buy batteries just because they're cheap..It's just not worth it!