I was interested to read about this. I recently bought a 1600lm Cree XML T-6 flashlight (incl two 18650 batteries & charger) off of eBay, and one of those el-cheapo universal handlebar mounts, and while it is BRIGHT (I am not set up to measure lumens; I don't have a lot of faith it is actually 1600, but it seems about as bright as a car headlight), it doesn't last long. At full charge, it is super-bright and the pool of light has sharp edges, at the end of my 1/2-hr commute I'm pretty sure it is less bright, and definitely the edges are starting to soften. After another 1/2 hour it is basically dead.
It's not a big issue, I only have a 1/2 hour commute, only one way is going to be in the dark, and I can recharge every night. And I had low expectations for a chinese light off of eBay, I just wanted to pay only like $25 instead of hundreds for a high-powered light specially-shaped for a bike.
Any idea would I get better runtime out of my cheap light if I paid a little more $$ for some higher-quality batteries? I assume any replacement 18650s could use the same charger? The ones that came with the light say on the side "UltraFire LC 18650 2400mAh 3.7V" full/cutoff 4.2/2.75V
thx for your expertise...