Cheap flashlights

I bought some cheap flashlights recently. They were $5.99 for two on sale. After shipping the total was $8.06, and they came with batteries.

I’m happy with how bright they are - they have 9 white LED’s on them, and are powered by 3 AAA batteries, which clip into a holder that wires them in series. The majority of the weight is in the batteries, but even with them installed it’s a very light unit.

I wanted to see how long they would run, so… here’s that math:

1 AAA battery alkaline battery = 1.5V, 1.25AH (from http://www.pacificsites.com/~brooke/batt.shtml)

3*AAA = 4.5V, 1.25AH, thus 5.625WH

9 LED flashlight uses 0.28A (280 milliamperes) continuous (as per my measurements), or 1.26W at 4.5V

5.625WH/1.26W = 4.464 hours (I could have left all the measurements in amps)

Assuming the LED’s will light for 80% of the life of the battery…

4.464*0.8 = 3.57 hours runtime.

Pretty good for a cheap little $4 flashlight, although the vendor claims they will run for 24 hours. If I knew the cutoff voltage for the LED unit, and at what point the batteries would drop below that voltage, I could probably get a better estimate of the total runtime.