I did not like 18650: batteries for ca. US$ 2.50 measured only 2100mAh (e.g.: EBL), only > US$ 5.00 NCR18650 gave me real ca. 3200mAh. With EBL 26650 i get 5000 mAh for US$ 5.00 (all measured with Opus BT C3100).

With single battery, you can not get full 10W from a Cree XML T6 without a step-up converter AFAIK because it requires somewhat higher than 3.7 V for 10 W. So i wanted a 2 * 26650 flashlight:


Unfortunately, the LED is blue-ish, and the driver only makes the LED go up to about 6 W. Not very bright. But at least it never gets warm.

Replace LED and chip with this here, piece of cake, done in 10 minutes, just a bit of soldering:


Nice warm LED, and driver with three brightness settings. Highest one seems to be close to 10W, somewhat uncomfortable warm with bare hand longer time, but no problem with gloves, medium setting cozily warm, lowest one maybe 4 W. When set up with just one battery, it maybe goes up to 7 W. Haven't had the time to really measure it and check how efficient the driver is though.

E voila: 1 or 2 * 26650, 10 W flash light for < US$30: $9 for the body, $10 for batteries, $10 for driver and LED.