In a FT03, using a 26650 would make more sense since you can get more capacity out of it. The Vapcell 26650 5500mah and 5900mah is the same cell. The 5500mah is the more recent rewrap where Vapcell prefers under rating it than over rating. I tested about 5900mah in several of the Vapcell 5500mah. They are actually the same cell as the famous Shockli 5500mah 26650
what part of the country are you in? In the Western U.S. I order exclusively from Illumn.com or mtnelectronics.com in San Jose and Salt Lake Exclusively with never an issue.
It runs a little fat but fits most lights. In my humble opinion it’s the best of the 26650 cells out there with the highest capacity and a 30 amps discharge. As far as cell age, I’m still using 10 year old batteries. You’ll probably never tell the difference, it’s still a fairly new cell. It’s couldn’t be more than 2 years old at the worst.