On Wed, 22 Feb 2017 20:47:57 -0500, micky
wrote:

I have one of thoe little 2.5" LED flashlights, that have 9 small LEDs
and use 3 AAA batteries. It's like the ones that Harbor Freight
sometimes gives away free, same size, but nicer.

Did you notice that almost all such flashlights are made from
aluminum, not plastic, which would be cheaper? That's because
aluminum makes a good heat sink to get the heat away from the LED's.
LED's loose output and efficiency when hot, so it pays to keep them
cool. The COB style of LED mounting is quite common in larger
flashlights, which offer a heat conductive path from the LED to the
aluminum heatsink case.

The guy I bought the first one from said it would run for days on one
set of batteries, but I forgot that it was still on and i put it in my
pocket and an hour or two later, I noticed it was on because it was
warm.

It's still really bright, but if it's noticeably warm, how long can the
baterries really last? Certainly not two days. ?

No, it won't run forever. AAA alkaline batteries are good for
1000ma-hr each. At something less than 4.5V for 3 batteries, that's
4.5 watt-hrs. Commodity LED's generate about 50 lumens/watt, so if
your Harbor Freight flashlight managed to belch 15 lumens, it will
consume 0.3 watts. Runtime is therefore an optimistic:
4.5 watt-hrs / 0.3 watts = 15 hrs.
It's probably less because I didn't bother throwing in losses in the
current source, decrease in battery voltage as it runs down, and
heating effects. But, it should give you a ballpark guess as to how
long it will run. My guess(tm) is it will run about 8 hrs with a new
set of batteries.

I once, recently, forgot the screen was lit on my 5.5" smart phone and
only noticed because it felt warm in my pocket.

If your smartphone as a battery usage graph, like all Android phones,
you'll find that it also has a list of which applications are sucking
the most power. The backlighting for the OLED screen is invariably
the highest. If you don't talk on your phone, and don't run any apps,
the battery life of your phone will be totally dependent on the
brightness setting and efficiency of the OLED display, which is about
the same 50 lumens/watt as your white LED. (Yes, I know there are
press releases for 100 lumens/watt, but those are under laboratory
conditions).

It's got a proximity sensor but I don't remember what it's supposed to
do.

Never heard of it. Maker and model number?

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558