18500 and 18650 confusion
- Thread starter vio765
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it depends on the driver.
if the driver for example: 2.7-8.4v and regulated, then it should perform same with 1 or 2 cells.
however, sometimes 2 cells (less capasity) will be tad brighter but much shorter time.
i don't think there is a 100% answer, as different drivers provide different options.
hoongern
Enlightened
2x18500s contain more energy than a single 18650, and also a higher voltage which means that the circuit would probably be more efficient.
I'm pretty sure that 18500s would run longer than a single 18650.
However, I don't own that particular light.
(For reference, 1x18650 has about 3.7V*2.6Ah=9.62Wh. 2x18500 have about 7.4V*1.5Ah=11.1Wh)
Hoon, that is the same math I had. The doubling of voltage indicates higher output, but the result of our math makes me wonder if the added brightness is because of the higher energy of 2x18500 cells. Thus, is the higher energy going to burn off as more output, longer runtime, or some combination of both? Assuming a perfect world, I would be thrilled at ~20% increase in brightness and ~10-20% increase in burntime.
mdocod
Flashaholic
Almost all lights that are compatible with ~3-9V input are using a low-overhead buck regulation circuit. I would have to assume that this light uses a similar configuration.
In most of these types of lights, the brightness fresh off the charger with 1x18650 or 2x(any li-ion) will be about the same. Sometimes the single 18650 will be 10-20% dimmer off the line but it's not noticeable. However, when you actually run these 2 configurations through the paces, you find, that the 18650 will steadily taper down to ~40-50% of it's original output through that discharge, whereas, a pair of cells (18500s for example), will start off at the highest possible brightness and maintain it until the protection circuit in the cells kills the circuit.
In lower ouput modes, the single 18650 will run "in regulation" for a portion of the discharge. While a pair of 18500s would run in regulation through the entire discharge.
Since the single 18650 run-time is directly correlated to the Vf of the LED, while the pair of 18500s will operate with very little difference in run-time based on the Vf, any sort of good estimates on comparative run-times are going to be impossible to make.
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My educated guess is as follows:
a pair of 18500 cells will run this light at full output for about the same run-time that a single 18650 will run this light with diminishing output. I wouldn't be surprised if the single 18650 actually had slightly better run-time (but less average output). In lower modes, the pair of 18500s should start to have better runtime with the same output.
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Everything is a tradeoff. Good Luck,
Eric
In most of these types of lights, the brightness fresh off the charger with 1x18650 or 2x(any li-ion) will be about the same. Sometimes the single 18650 will be 10-20% dimmer off the line but it's not noticeable. However, when you actually run these 2 configurations through the paces, you find, that the 18650 will steadily taper down to ~40-50% of it's original output through that discharge, whereas, a pair of cells (18500s for example), will start off at the highest possible brightness and maintain it until the protection circuit in the cells kills the circuit.
In lower ouput modes, the single 18650 will run "in regulation" for a portion of the discharge. While a pair of 18500s would run in regulation through the entire discharge.
Since the single 18650 run-time is directly correlated to the Vf of the LED, while the pair of 18500s will operate with very little difference in run-time based on the Vf, any sort of good estimates on comparative run-times are going to be impossible to make.
-------------
My educated guess is as follows:
a pair of 18500 cells will run this light at full output for about the same run-time that a single 18650 will run this light with diminishing output. I wouldn't be surprised if the single 18650 actually had slightly better run-time (but less average output). In lower modes, the pair of 18500s should start to have better runtime with the same output.
------------
Everything is a tradeoff. Good Luck,
Eric
357mag1
Enlightened
What mdocod said above is almost exactly what I experience with my TK11 R2. With a single 18650 the TK11 is not quite as bright as with two 18350s or 16340s. Very hard to tell by your eye but easy when compared to my Olight M20. On 18650s both lights are about dead even with the TK11 just slightly brighter in the hotspot. Throw two Li-ions in the TK11 and the hotspot is significantly brighter than the M20 hotspot.
Tailcap current on the TK11 drops from about 900ma with an 18650 to 660ma with two Li-ions. The M20 is better regulated because there is no difference when going to two Li-ions in the light. Tailcap current measures 1100ma with one 18650 and about 555ma with two Li-ions.
So each light would be different based on the circuit used and individual efficiency of that particular light (I have multiple copies of some lights and they are all slightly different).
In the two lights above at the current draws listed and assuming you could use 18500s in them the M20 would run roughly 3 hours on two 18500s rated at 1600mah and only about 2.5 hours on a 2600mah 18650.
The TK11 could get almost 3 hours on a 2600mah 18650 but would only would only get about 2.5 hours on two 18500s.
The TK11 would stay brighter on the two 18500s for the duration of the run and the M20 would have the same brightness on either setup due to better (wider voltage spread) regulation.
Hope that makes sense.
Tailcap current on the TK11 drops from about 900ma with an 18650 to 660ma with two Li-ions. The M20 is better regulated because there is no difference when going to two Li-ions in the light. Tailcap current measures 1100ma with one 18650 and about 555ma with two Li-ions.
So each light would be different based on the circuit used and individual efficiency of that particular light (I have multiple copies of some lights and they are all slightly different).
In the two lights above at the current draws listed and assuming you could use 18500s in them the M20 would run roughly 3 hours on two 18500s rated at 1600mah and only about 2.5 hours on a 2600mah 18650.
The TK11 could get almost 3 hours on a 2600mah 18650 but would only would only get about 2.5 hours on two 18500s.
The TK11 would stay brighter on the two 18500s for the duration of the run and the M20 would have the same brightness on either setup due to better (wider voltage spread) regulation.
Hope that makes sense.
This makes sense to me now. I am focused on a balance between runtime and brightness. Some prefer brightness even if a particular light will only short for a short while. Case in point: I decided to switch from the Eagletac P20A to the Quark AA2. The Quark is not as bright as the Eagletac in High, but the Quark offers more "light value" in terms of light delivered versus burn time. Anyone would agree that when it counts, having some light is better than no light. My point is that I am looking for the best combination of battery in this light to maximize light value. 2x18500 of 1x18650? I guess I could take on the Flashaholic approach and do both routes! 
