Using a single unprotected cell should be fine in this light as long as you know what to look for. You can run a little test to see where you realize the cell is low and not producing as much light.
Try using the light on high and when it steps down to medium keep using it to drain the cell preferably when it’s dark. As you use it, at some point you should notice the light getting dim. When it becomes obvious it’s losing lumens turn it off and pull the cell. Wait a few minutes and measure the voltage with a DMM and see where it’s at. If it’s above 3.0v then I’d say your good as long as you pull the cell when you see the same loss in daily use. The light may even step down from medium to low that be another good indicator if it does. If you mostly use it in low mode then you probably will not get any indicator until the cell is flat.
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I prefer to have a big blast of light on hand and not use it much than to need it and not have it. I just use a lower mode for most task to extend run time.
It’s like sticking Nitrous Oxide on your daily driver. Just for the times when you need to merge into that relentless traffic. 