For my headlight I use a regular flashlight, fixed to the handlebars with interlocked hose clamps. It takes two 18650 batteries. I recently bought a similar flashlight that uses only one 18650 battery. They both have the same CREE LEDs (XML T6).

I've never been a whiz at electronics. Does the fact that one of the lights uses two batteries mean that it should be brighter? Or that a full charge should last longer? I gave a try shining both batteries in my dark garage, and the brightness seemed about the same (2-battery one maybe having a little edge).

If it's a high end flashlight the brightness will be the same either way, you'll just get double the time. It will have a high efficiency constant current regulator driving the LED and what matters is watt hours.

If it's a cheap flashlight you might get almost anything. It's not uncommon for a cheap flashlight to be brighter with two cells because it doesn't have a boost converter built in so the higher voltage affects the brightness.

But OTOH the cheaper flashlight might result in just more heat, if it has a really junky regulator it might take the excess voltage and just bleed it off as heat. It would have to be real crap to do that, even $10 flashlights are better than that.

hopefully it gives you more life. I have a fairly good light that blinks a warning if the current draw is unsustainable. The high beam makes it blink -- they just have that mode for special circumstances. So if it had another battery, it probably would be able to do that mode without drawing too much current from either battery.

Well, it depends.

If the two batteries are wired in series (the negative of one to the positive of the other) then the "power source" for that light is double the voltage with the same capacity. That simply means that it was most likely wasting more energy in regulation than the other one was. If the batteries were wired in parallel (positive to positive, negative to negative.), then it'd be twice the capacity and the battery should last roughly twice as long (assuming the same losses in similar circuits.)

I'd guess that the older flashlight that used two batteries used cheaper parts which had larger voltage drops which means it required a higher voltage power source to begin with.

Meaning that they should last similar amounts of time.

Also, no, if it's the same LED I doubt one light would ever be brighter than the other. It would be dumb for a flashlight to be designed not to provide the LED with the most efficient amount of power. (Provided ample heatsinking ability.) Sure, one light MAY be slightly brighter (very slightly) but that's only because of different design parameters/choices and not because of the batteries.

So I got two votes for more life, one for same life, and none for different brightness.

The older light with two batteries, I put them in as you describe "in series", both down the same long tube, buttons (positive?) inwards, can't imagine doing it otherwise. Could it be possible that I've been putting my batteries in wrong?

I guess I could do a rundown test, charge two batteries and see how many hours the 2-battery light gets on high, then charge them back up again and see how many hours the 1-battery light gets on high (measure twice and average?); use the same batteries to eliminate any difference in battery quality

if you put the batteries in backwards, there is a very good chance that you will let the magic smoke out. And in any event, it wouldn't light up. So if you have been getting light, they are the right way.

I think you got 3 votes for "nobody knows without seeing the schematic." You could get either more life, brighter light, or both, and there probably is no simple manipulation that you can perform that will change the operation of either light.

Agreed. However, I don't think the lights would blow up (release the magic smoke) if he put the batteries in backwards. I'm almost positive that any commercial device has to have some sort of reverse battery protection. (Especially for batteries that slide in, murphy's law dictates that I ALWAYS put those in backwards the first time.)

With that said, if the lights work with the batteries in series, it will most definitely NOT work with the batteries put in in opposite directions. If the batteries slide in, there's no way they can be in parallel.