How do you measure runtime?

Please excuse me if this is a stupid question, but how do you measure runtime? I recall someone saying it had to do with maH of the battery or something.

I plot the output of a flashlight over time untill the battery dies. I have a light meter adapter that pluggs into a DMM which has a serial interface which is plugged into my computer. The computer generates a plot of mVdc vs, time. The system is calibrated such that 1 mVdc = 1 Fc. My measurment is made in the brightest part of the beam at one foot away from the the light source. The reason for measuring at one foot is that 1Fc@1Ft = 1Lux@1M. The reverse is also true...1LUX@1M = 1Fc@1Ft if measured at one meter. I define runtime as the time it takes to reach one half (50%) of the maximum brightness of the light.


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If you know the current draw of your light and the amp-Hr rating of your battery, you should be able to calculate the life of the battery. Example...if your light takes 500ma and the battery has 2000ma-H then 2000ma-Hr divided by 500ma will give you 4 hours of light output.

Could I ask you Roy... what light meter do you use that plugs into a serial port?

Here is a picture of my plotting ste-up/


Information about the light adapter can be found here. . The DMM is a Radio Shack model that is no longer caried, but they have new equivalent DMM's. If you look at the top of the DMM you can see the serial connector. The light adapter is an Extech Mdl 401021 Light Adapter.