3 cell vs. 4 cell LED lights

I'm wondering if 3-cell resistor-limited unregulated LED lights (e.g. Lightwave 2100, Trek 4) get as much energy from their batteries as 4-cell lights do. The idea is the Vf of white led's is 3.5 volts or so, right? So if that Vf is constant, once the batteries are down to 1.2 volts or so, there's almost no voltage across the resistor and the light would get very dim. If you use rechargeables, it would start out dim and stay that way. With 4 cells (Streamlight 4AA) you'd use more resistance and have more voltage drop to work with--you're still getting reasonable current even at 1 volt per cell.

What happens in reality? 3-cell lights sell pretty well and get decent reviews, so they must at least work ok. Is the effect I'm describing real at all?

Paul, I have a pet peeve with items that take three cells. You're right about the improvement just one more cell would make. What's more, four cells would allow for regulation over most of the range of alkaline and all of NiCd or NiMH discharge.

I think economic myopia is responsible for three cell LED torches- the case costs at least as much as the electronics.

BTW Vf of LEDs varies with current. There are resistive losses as well as the junction's logarithmic VI curve and some variations between devices.

Three cell lights are bad if your battery charger can only charge pairs of batteries.

Plus you can only find packs of batteries containing even numbers. What are you supposed to do with the remaining battery?

Most of the three-cell LED lights are direct driven. It's just laziness on the part of manufacturers, since three batteries is the minimum to run an LED.

Well, if the three cells were packed together to reduce the length of the light, thats a good reason.

Those of us that buy NiMH by the (one or two or three) dozen ( they are only 80 cents each in bulk!) and have a charger that will charge individual cells ( and you really want one of those anyway so you dont have to pair up batteries for life) could really care less about the odd-cell argument.
Otherwise, three cells allows manufacturers to create a cheap luxeon light (like the three aa cmg reactor 3 or the three aa collimator). These are dirt cheap lights that simply could not be that cheap with a regulation module. They do have a place, even if most everyone in this forum with rather have a white regulated luxeon. I personally chose a BB400/QL3 for a minimag over the 3 cell alternatives. Now if my BB400/QL3 would only ship /ubbthreads/images/graemlins/icon23.gif /ubbthreads/images/graemlins/mecry.gif

[ QUOTE ]
shrap said:


What are you supposed to do with the remaining battery?



[/ QUOTE ]


That's why God created Arc AAA's and Infinity Ultras.

Streamlight found out that a four AA light sometimes isn't a good idea. After a number of changes in resistor set ups, I don't think it will fry the LED's now.

Last night, I was kind of bored. So, I took two PR bulbs, from 4 cell and 3 cell lights, and swapped them. I didn't have them on for long, but this is what I found. The 4 cell bulb was considerably dimmer, in the 3 celler. The 3 cell bulb was fairly bright in the 4 cell.

I didn't leave either on for long, but if I were to have left the 3 cell light in the 4 cell flashlight, would it harm it? Does this decrease bulb life?

I left a 3-cell bulb in my 4-cell Dorcy for about 1/2 hour, and it melted the reflector.