I am also a big Fenix E01 fan. I use lithium batteries and I use the flashlight multiple times a day. An Energizer lithium 9x type lasts me about a year. The 9x supposedly lasts 9 times as long as an alkaline battery.

The Fenix has an o-ring under the cap so it doesn't come loose on it's own--at least it has never happened to me.

When I first started carrying the Fenix, I thought I would only need it for emergencies, but you don't realize how handy it is until you always have one in your pocket.

For an EDC I would stick with a AAA flashlight as it is much smaller than a AA one.