Compare dive lights’ power consumption

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XTAR

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Based on the different methods of producing light, you know the LEDs use less power but give the same intensity as halogen lamps. Except from the dive lights’ bulb type, to test the power consumption, the battery type, capacity are also important factors. Particularly in amperage per hour, this will provide fundamental information for understanding the power consumption in relation to the burn time of the light source.

Battery types used in dive lights range from Li-ion to Nickel cadmium. Larger-energy battery systems are shown to have higher amperage per hour ratings. To calculate the total power that is available inside of a battery unit, multiply the voltage rating by the amps/hour rating. This will give a total energy density output per hour in wattage.

There is an example showing how understanding power helps to understand how a light works: If your light uses one Cree XM-L LED, and runs off one Li-ion 18650 cell. The cell is rated at 3.7 V, 2900 mAh. This battery could provide close to 2.9 A for close to an hour. The battery voltage drops continuously as it depletes, but let's take 3.7 V as a fixed and approximate value. The battery can supply power equal to 3.7 V x 2.9 A, for close to an hour. This is the cell's Watt-hour rating: 3.7 x 2.9 = 10.7 Wh.

Then, take a look at how the LED consumes the power. The LED has a "forward voltage" (Vf) of about 3.5 V. That is, as long as there is enough current through the LED to turn it on, the voltage across the LED is about 3.5 V. If we drive the XM-L at 2A, the LED is consuming 2 x 3.5 = 7 W. If we estimate the driver efficiency to be about 80%, that means we have to supply 7 / 0.8 = 8.75 W of power from the battery. An 18650 should be able to drive this light at 2 A for a little over an hour.
 

inquisit

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The tricky part is finding the current draw for a given LED before purchase. (A simple test in a bucket of water gives runtime if you've already bought it.)

Many LEDs can be run at several current levels (differing output levels), so cross your fingers that info is in the specification. Trying to infer that from quoted lumens can be questionable given the tendency of some brands to inflate that value.
 

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