Does running a light on 2 18650's give you more run time than...

running the same light - say a thrunite catapult on 1 18650 but having a back up battery. All things being equal - output etc. I know that makes the Catapult a bad example as it runs at lower output.
Take the new XML Deree DBS - it can run on 1 or 2 I think at the same output...

Hi ricky. The answer to your question is generally, yes. That does not mean that you will always achieve twice the runtime, as is likely with your Catapult example, but the light will pretty much always run longer on two cells.

Now if you're suggesting running a light on one cell until that cell is drained, and then installing a second backup cell and then running it down, the combined runtime of both cells will sometimes be longer than if the light were run on two cells from the start. This is because some lights draw less current when running on one cell (as is likely with the Catapult), than when running on two cells. This depends on the driver used in the light.

If the light has less output on one cell than two cells, then the light is drawing less current from a single cell, than when two cells are used. Again, in this case the light would run longer running one cell at a time, one after the other, than running two cells at once, albeit output will be lower when running on one cell.

If the light output is the same, running one cell, or two cells, then the runtime will be close to the same whether you run two cells in succession, or two at once. However, in most cases the driver used in such lights will allow a slight advantage to running two cells at once, over a single cell because there is a slight advantage to the higher voltage achieved when running two cells in series.

Dave

Usually yes, but it really depends on the lights driver. Some lights will deliver about twice the current on 2 cells vs 1.

Most buck cells are will be more efficient at lower voltages, so there will be a penalty for running 2. But on the other hand 2 cells will allow you to drain them pretty much fully and at full brightness while a single cell light probably won't. Either it will shut off, or it will start running dimmer, which would in turn increase run time but not at full regulation.

Thanks for the answers guys! All other things being equal I'd rather have a smaller light and carry a back up battery.

I was interested in getting a custom Dereelight DBS XML. Deree says the XML has to be run on 2 batteries such as 2 18500's or 2 18650's to reach full brightness. I don't mind the length of 2 18500's but would prefer the length of 1 18650. I had contacted Milky about making a custom drop in with modes spaced farther apart - ie 1 lumen, 30 lumens and 800 lumens. He not only said he could do that but unlike Deree's drop in could make it run on full power with just 1 18650.
I prefer the smaller size but was just wondering if there was any advantage to having a light run on 2 18650's as oppossed to carrying a loose 18650 along with your light...

Yes, total runtime will be longer using 2 cells at once:

Cell capacity is given using a 20 hour discharge period, sometimes 10. Running them at a higher discharge current cuts battery output, and you will not get the rated capacity.

Using 2 batteries (serial or parallel) cuts the load on each battery and they come closer to the 20 hour discharge capacity.

But if the regulator is less efficient at a higher voltage (running in series) that will negate some of the gain. If running in parallel regulator efficiency will be the same.