Help for a Newbie

Hi, this is my first post but I have been lurking in the forum for a while. Trying to get my head around the numbers, found some explanations on the forum but still can't fit it all together. Say if I had a 50 LED torch each LED running at 20 maH, so presumably, that's 1,000 maH. Say the torch was powered by three AA's and I was to use three NiMH 2,500 maH batteries, so a total of 7,500 maH, does this mean I get 7.5 hours of light? More or less, same brightness, diminishing brightness? What if the batteries were 2,500 maH alkalines, so 1.5V rather than 1.2V, any difference or is that factored into the installed resistors? What about regulated or non regulated, what difference does this make? What about all of the inefficiencies within the torch, does that impact the runtime? Any other numbers to consider?

Apologies if these are dumb questions.
Thanks

waddzz said:

Hi, this is my first post but I have been lurking in the forum for a while. Trying to get my head around the numbers, found some explanations on the forum but still can't fit it all together.

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Welcome to CPF!!

waddzz said:

Say if I had a 50 LED torch each LED running at 20 maH, so presumably, that's 1,000 maH. Say the torch was powered by three AA's and I was to use three NiMH 2,500 maH batteries, so a total of 7,500 maH, does this mean I get 7.5 hours of light?

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I'm certainly no expert, but I don't think it works that way. Most lights use the batteries in series, so 3 1,2v 2500mah AAs would give you 3,6v 2500mah (IIRC). So this would definately not get you 7.5 hours of light.

waddzz said:

More or less, same brightness, diminishing brightness? What if the batteries were 2,500 maH alkalines, so 1.5V rather than 1.2V, any difference or is that factored into the installed resistors? What about regulated or non regulated, what difference does this make?

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To keep the brightness on +/- the same level from beginning to end, you'll need regulation. Take a look at dome of the runtime graphs at flashlightreviews.com and you'll see the difference regulation makes. Without regulation, the light will start to dim from the moment it was turned on.

waddzz said:

What about all of the inefficiencies within the torch, does that impact the runtime? Any other numbers to consider?

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Yes, those will influence runtime and brightness of the light. Deox-it and progold are your friends

As far as alkalines vs NIMH, yes the alkaline has a higher voltage (under NO load, of course).

NIMH and lithiums hold their voltage better under load (less internal resistance).

For example, if you have a 2500mah alkaline at 1 amp draw, you would think you would get 2.5 hrs worth of power, right? But the internal resistance is such that you drop some of that power across the battery ITSELF, so you might only output 2000mah or whatever. I'm not an expert, maybe someone has the details of the internal resistances of typical alkaline batteries (Silverfox?)

A 2500 mah NIMH or lithium has much less internal resistance, so less power is dropped as heat in the battery and more makes it to the load (the bulb). Also, NIMH/Lithiums tend to discharge flatter, which means they hold a higher voltage longer through the discharge cycle.

What all this really means is that even though a NIMH is 1.2 V, due to higher current flow and better voltage characteristics under load, a NIMH will probably run your light brighter and longer vs a regular alkaline.

Other advantages include smaller amount of self discharge, and in lithium's case, better temperature range of use (can be used extremely hot/cold temps without affecting battery).

I am no expert again, and I'm sure some others can chime in if I am wrong or just mangling this explanation.....

dragoman