Runtime question: 5W vs 1W

[Phaserburn]

Ok, hoping someone will clue me in on the detail I'm missing. If the Arc LS will get 2 hours from 1 123 at 350mah, why wouldn't a 5W drawing 750mah from 2 123's also get 2 hours? Twice the draw vs twice the batteries. It was quoted as 1 elsewhere in the forum, and I'm not seeing why. ?

 

[Tweek]

Because the 5W also takes roughly twice the voltage to power it, so you add another battery in series to get more voltage. This doubles the amount of voltage, but the available current is the same. Dont forget that the load has also doubled, so now out runtime will be cut about in half. Another factor, is the internal resistance of the batteries. At higher current loads, the voltage will droop more and more, limiting the amout of power that you can extract from the batteries at that current level. This is why you can run a regular incandescent AA light down to where it's almost useless, and still get 6 months of time running a remote control off those batteries you just killed in the flashlight.

If you wanted to double the runtime, you would need 4 123's, two sets of two in series, paralleled together.

Hope I cleared that up some... /ubbthreads/images/graemlins/smile.gif


Chris

 

[McGizmo]

To add to Tweek's comprehensive answer, it may help to consider that the 5W is essentially composed of 4 1W dies; 2 parallel circuits of 2 die in series.

- Don

 

[Phaserburn]

Yes. Thanks!

 

[Phaserburn]

Additional question, then: What would be a brighter combo in a .5 ohm resistored 5W Blaster VI - 3x123, or 6 AA NiMH? The 6 AA are 7.2V vs 9V, but the NiMH release more current?

 

[Bullzeyebill]

I am not sure, but from reading some of the CPF experts in this area, the 3x123 might be initially brighter, but NiMH with greaterr capacity will last longer, brighter. Bill