estimating battry life

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Hello
i have a problem with this estimation :
i have a power LED that draws about 0.2Amp and uses 4.5 volts
I supply this LED with 3xAA batteries that gives 1.1Amp and the voltage is 4.5
so how long does the batteries last if you turn on the LED constantly?
how to do the math here?

I don't need a Handkerchief; i have no more tear to shed anymore - Solid Snake

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see also the Sept. 2013 issue of Circuit Cellar (at Barnes & Noble, etc), Novacek's article, or get a well worthwhile subscription.

Batteries have an amp-hour (Ah) rating such as 1500mAh- that's 1500 mA for one hour or half that current for twice the time, etc. So compare your load to the mAh rating of the battery. Batteries in series do not have additive mAh rating. Batteries in parallel are not valid (without added electronics to load-level). BEWARE Ah ratings need to consider the battery voltage decline.

Each battery chemistry has a different curve of voltage vs. depletion vs. time. Google around for the curve for your battery type (NiCAD, Lithium-ion, etc.) What's important is the voltage droop in time vs. what your circuitry needs, e.g., a low dropout regulator (LDO) minimum headroom, say, 0.3V.

And, temperature affects the capacity.

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See:

http://en.wikipedia.org/wiki/AA_...

So the mAh value depends on the battery chemistry. If you take the mid-range there it's about 1500mAh as Steve said. Your load is 200mA so the batteries will last 1500/200 = 7.5h.