Runtime Formula Help

Can anyone help me? I'm trying to figure how to determine the runtime for my various setups. Is there a formula i.e. : 4 AA batteries at 1.5 volts each with a Malkoff M61 with a drain of such and so will give you a runtime of about 5 hours at the advertised 225 lumens!

I'm specifically trying to figure how much runtime I'll get with 4 AA Eneloops (and/or 4 alkalines) running a Malkoff M61 LL but I would also like to know how to figure this out on my own with some kind of simple formula if there is such a thing. Thanks

ash211 said:

Can anyone help me? I'm trying to figure how to determine the runtime for my various setups. Is there a formula i.e. : 4 AA batteries at 1.5 volts each with a Malkoff M61 with a drain of such and so will give you a runtime of about 5 hours at the advertised 225 lumens!

I'm specifically trying to figure how much runtime I'll get with 4 AA Eneloops (and/or 4 alkalines) running a Malkoff M61 LL but I would also like to know how to figure this out on my own with some kind of simple formula if there is such a thing. Thanks

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You need to figure out the watt/hours of the batteries so for 4 AA alkaline it would be 1.5 V * (4*2aH) so 1.5* 8aH =12wH

Now you need to figure out the consumption of the light including how hard it is driven. For this you need to know the vf of the emitter (let's assume 3.6 for XR-E, XP-E) and lets assume it's being driven at 1 A. You need to know the specs of your light to know for sure.

3.6v * 1A =3.6 wh

Divide battery capacity in wh by the light power consumption in wh...so 12/3.6 = 3.3 hours runtime

I didn't take into account power consumed by the driver. Many people use a figure of 85% efficiency so reduce that runtime by 15%.

This is all an approximation anyway assuming that it's regulated and actually drawing 1 A for that long. In all likelihood it will run longer at a greatly diminished output.

THis is still a good way to compare various flashlights as long as you realize the underlying assumptions.