On Wednesday, February 22, 2017 at 5:48:12 PM UTC-8, micky wrote:
I have one of thoe little 2.5" LED flashlights, that have 9 small LEDs
and use 3 AAA batteries.

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

The internal resistance of the batteries, and the internal resistance of
the LEDs used, is very much a part of the design of those flashlights.
Both the LEDs and the batteries do get warm in normal operation,
and (as you suspect) it's likely that a fresh set of batteries will last
well under a day of ON operation; Duracell AAA claims 3.5 hours at 1/4W
output, so a three-cell light could deliver over half a watt for a useful
amount of time; hours, not days.

https://d2ei442zrkqy2u.cloudfront.net/wp-content/uploads/2016/03/MN2400_US_CT1.pdf