While looking into rechargeable battery options for the Nitecore SRT5 I noticed some 18650 Li-ion batteries were marked “protected” and came with a slightly higher cost.  After some quick research it appears some rechargeable batteries come with a built in safety switch to help you get the most out of them you can.  Since I like self-correcting systems I would recommend spending a couple more dollars and let the batteries do the math for you.