battery capacity calculations metal nickle halide

Most cells are rated based on a 10 hour rate which means if your cell is rated for 3500mAH, you should be able to draw 350mA for 10 hours. In my experience, when you draww less than the 10 hour rate you usually get more than rated capacity. Since you say you are only drawing 20ma, your calculations appear to be correct and come out to a little under 7.5 days and I would expect a little more than this based on what I said above concerning more capacity at loads under the 10 hour rate. If you are not getting that then you must either have more current draw than you think you do or the battery is not delivering rated capacity (which would not surprise me).

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What is the rated voltage drop across the LED with 20mA going through
it?

The OP didn't describe the circuit (single LED? series resistor?).So another reason for the apparent discrepancy could be that the voltage across the battery is falling below the level at which the LED illuminates.

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Terry Pinnell
Hobbyist, West Sussex, UK

circuit is simply a led across the + and _ pole of the battery i think what is happening is the voltage is dropping such that the led cannot light

thanks for all the info

ilaboo wrote: [...]

That is a small device which should keep you going some longer... Good luck with it:)

Indeed!

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Terry Pinnell
Hobbyist, West Sussex, UK