Welcome to the forum mathematician.
Please be careful at using Li-ion cells in series. I personally wouldn't use any cells together that don't come out of the light with matching voltages. Probably the single biggest risk with Li-ions is one cell becoming more discharged and then the other cell reverse charging it. Probability of a venting event is too high to play with it.
Second warning (sorry, really) taking cells repeatedly below there rated low end (most cells are rated for 3 volts, some for 2.5 volts, creates crystals that can puncture membranes and cause a venting event too. This was discovered by NASA, I believe).
Of course, venting in a sealed metal tube can cause an explosion.
Sorry for all the warning stuff.
Is this the light you have? If it is, I really like that light a lot. There are a couple mods threads for it here.