you beat me on that one- AAs are 14500 aaa 10440iirc etc- I went to lighthound today and there are a BUNCH of sizes most we dont ever use but its nice to know there are so many options- you can find a Li batt to fit any flashlight host and with the shorter ones you can use two for double the vdc- also some dont know that there are P cells and not protected- Ps are sometimes called button tops- and they are protected and thus a tad longer -(& mo $$ too)- occasionallly a spacer is needed in some build to make up for the shorter unprotected cells- or flat tops.
Lighthound is a bit spendy but they have the biggest selection of lights and batteries=you can also buy from them on-line- nice folks to deal with also.+
they had 10440-14500- 16340- 18350 17500 18500 18650(my favs) 26650 & a few others too.++
the best 18650Ps were 2900mAh AWs at $18.75- & AW 3100mAh at $19.75-ouch
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hak
There is also the choice of getting the standard ICR cells or an IMR. IMRs are best for small builds with super high draw on a single cell or sometimes two cells. I wouldn't say an IMR is required for any draw 1A or less on each cell.
To test the current essentially replace the tail cap of the laser with the leads to your multimeter that is set to measure current and turn on the laser. Whatever you read you divide by the number of cells you are using.