Why do manufactures make lights that take 2 CR123 batteries but won't work on a 18650

It seems strange to me that a flashlight company would make a light that works on two CR123 batteries but not a 18650 Li/ion. I was just looking at a 4Sevens Quark AA2X Turbo and it only takes two CR123 batteries. I can understand them wanting their lights to be able to use the two CR123 batteries as there are lots of people who know nothing about, don't know where to buy them, or just would rather not fuss with LI/ion rechargeables but for me, if it doesn't do both I don't want the light.

I definitely don't want all 2x123 lights to support 18670 -- that means the tubes will be much fatter than they need to be to run on 2x123. But I would like all 2x123 lights to run on 1x17670 or (2nd choice) 2xRCR, and be able to get full power on high while doing so. That said, I imagine the reason that not all batteries support that, is because you can make a more efficient driver, more cheaply, if you optimize for a narrower range of input voltages...

driver efficiency. A single 18650 cen be discharged down to 3.2V or so while 2 cr123a cells are over 6V when fully charged. That is a pretty large voltage range. Also, 4sevens lights CAN use 18650 cells if you have the 18650 battery tube but you ideally would want the 4.2V max head (AA quarks, 2AA quarks, and single cr123a quarks) as opposed to the head that can use 2 cr123a cells.

Efficiency can work the other way too. Zebralight makes some lights that take a single 18650 but not 2 cr123a primary cells.

Lastly, battery tube diameter is a consideration. An 18650 cell is wider than a cr123a cell so the tube would need to be wider. Perhaps some don't want to make their lights that big and the driver voltage ranges that big.

I think for companies like Surefire and Elzetta the issue is battery tube diameter as tre mentioned. I emailed Elzetta and asked if they were going to release an 18650 light and they said they had no plans to because the smaller diameter tube offers the CR123 cells protection. Wider diameter tubes offer less protection for primary cells along with battery rattle, etc. I plan on getting a Zebralight SC600w, but other then that I prefer my lights to take 18650 and 2xCR123.

4Sevens did make an 18650 tube but it added in diameter and I length and was not an elegant solution in my opinion. Some companies I won't mention names don't make their lights 18650 compatible because they want to sell their own branded primary cells and soon tone released rechargeable lights. Others for the size and form factor others for driver reasons.

TyJo said:

I emailed Elzetta and asked if they were going to release an 18650 light and they said they had no plans to because the smaller diameter tube offers the CR123 cells protection. Wider diameter tubes offer less protection for primary cells along with battery rattle, etc.

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Well, that's easily addressed with a simple sleeve. But the questions of driver efficiency and battery tube diameter remain