how long can a AA battery last running 1 led?

Re: how long can a AA battery last running 1 led?

It depends...

For example, if you have a 100mW (0.1 watt) 5mm white LED, and assume that each AA alkaline battery can supply 1 watt*hour (approximately 0.1 amp for 10 hours = 1v*0.1a*10h=1Watt*Hour)... Then

2 batteries * 1 Watt*Hour / 0.1 Watt = 20 hours

If you can use less light, for example 50 mWatt, then the batteries will last about 2x longer.

Also, it gets more complicated. LED's are not linear devices... They typically need some sort of electrical circuit to limit current draw--especially with larger batteries (which can supply too much current and burn out the LED).

So, for example, many of the cheaper LED flashlights will use 2-3 1.5 volt batteries and a resistor (in series with the LED) to limit the current through the LED. The resistor costs useful battery life because it prevents excess current by simply converting the current/voltage into heat.

Better LED flashlights will use some sort of electronic circuit (technically, a "buck mode" or "boost mode" switching power supply). This gives you better LED light output regulation and is more efficient (and gives you longer battery life) than a simple resistor. Also these electronic circuits can give you other options, such as variable light output and SOS flashing.

There are many small, high quality flashlights out there that you can buy--and some companies that can sell you ready made power supplies, kits, or you can design/build your own out of discrete parts or with IC's.

Some flashlights:

Arc 1 AAA flashlights
Peak LED flashlights (multiple types)
Fenix (very bright/multiple output AA and CR123 LED lights)

For example, a Fenix L2D (2x AA battery with Cree white LED) can output:

General Mode: 9 lumens (55hrs) -> 40 lumens (10.5hrs) -> 80 lumens (4hrs) -> SOS

Turbo Mode: 135 lumens (2.4hrs) -> Strobe

9 Lumens is equivalent to a small 1 LED flashlight (1-2 Lumen is a dim LED flashlight--useful for dark adapted eyes--such as the ARC brand AAA light) (bright enough to walk/run around with dark adapted eyes, read a news paper, etc.).

135 Lumen is a very bright light--you can point it at a tree a 100 ft / 30 meters away like a small spot light and see raccoons/etc.

Note that when you have a very bright/high powered light, you will probably need to use NiMH rechargeable batteries--Alkaline Batteries have high internal resistance and will not last as long because of the high current draw.

-Bill