+1 to djozz, for me too! I've got a couple F13's and other similar lights, all in the 26650 40mm head size family, so a lot to compare with.
For batteries, I've been very impressed with the new Liitokala 26650 5000 mAh cell. Tested a few for capacity, and all tested above 5000 on my BT-C3100, and have great performance with DD FET drivers - on par with the EFEST 4200 IMR cells. For the price/performance, can't beat it. First reported on here: https://budgetlightforum.com/t/-/39541, then Mitko with testing here: https://budgetlightforum.com/t/-/39804. I bought a dozen from GearBest and have been using/testing them in my DD FET lights, and they perform as Mitko describes.
BangGood link: http://www.banggood.com/1pc-Liitokala-26650-5000mAh, and Gearbest has them, plus RIC at CNQ has them under no brand label here: http://www.cnqualitygoods.com/goods.php?id=2250.
The BangGood listing shows capacity tests and other test results, while the CNQ listing shows discharge curves -- both very detailed listing of battery specs and performance. GearBest's listing, of course, is simple, but the best price. There's apparent other sources for them, and they may be sold under different brand names. It appears Fasttech sells then under the Soshine brand. The Liitokala name on the battery is a simple sticker imprinting, so easily substituted.