The light is Nitecore P36 and the batteries runs in series. Fully charged 2 x 18650 produces 8.4 volts and 4 x CR123 produces 12 volts.
Question: Will the 12 volt translate in more output than the 8.4 volt?
I don’t think CR123 cells have the output in current to out do 2x high drain cells. I think Nitecore have allowed CR123’s for certain scenarios
CR123 cells aren’t rechargeable so it’s not the ideal choice
The Nitecore P36 is only 2000 lumens which using 2 X 18650 will give you full power, and much longer runtimes.
If you have ’em or have cheap/free access to ’em, ’123s last nigh forever so can be great as reserve/emergency batteries. More than offsets concerns about peak output, etc.
Eg, most/all my Nitecore lights pretty much always can take an 18650 or pair of ’123s, and the manual says flat out that you won’t get peak output with ’123s as you can with an 18650, but if I had to keep a light in a BOB or even a car, I’d stick with ’123s.
I don't know if this light is voltage or current driven. Just a thought. The manual says it can use CR123s but doesn't go in details about it.
By the way, there are rechargeable CR123 batteries around. Seen em on Amazon.
Probably not. Looking at this test, making 2000 lumens from an MT-G2 requires just over 6 volts, which two 18650s in series can put through a buck driver until they’re near-dead. The P36 isn’t a hot-rod light, so its regulation will probably keep the output very close between different battery types.
That aside, running disposable batteries is pretty wasteful.
Are they 3 volt or 3.7?
There are three types of batteries sometimes described as rechargeable CR123A:
- Standard Li-ion 16340 (4.2V full - often incompatible with things that want 3.0V CR123A). These come in up to 800 mAh for 2.8 watt-hours - 10.8 for four of them, which is less than half the 24.4 Wh you’d get from two 12.2 Wh 18650s
- 3.2V LiFePO4 16340 (3.6V full). Many Li-ion chargers can’t handle these, and the capacity is even lower - only recommendable where absolutely required for compatibility.
- Diode-limited, where a standard Li-ion cell is used, but voltage is limited to around 3.0 during discharge. These are… kind of janky, and low in capacity as well.
The P36 can actually use any of these, but you probably shouldn’t.
There are also 18350 cells, which you could use four of. Those have a bit more capacity but still less than two 18650s.
You should use protected cells if you use four rechargeables because the risk of over-discharge and reverse-charging (a fire hazard) is high with a series of four.
But really, it’s a regulated buck driver. Just use 18650s. The only difference you’re likely to see with four smaller cells aside from shorter runtime is that it won’t dim as much just before the batteries give out.
All good points guys. Thanks. 
Given that the led needs about 3.5 amps at around 4 volts to make 2000 lumens from the test data.
2 series 18650’s in a buck driver would easily do that with given its half the supply volatge of the 18650’s (8.4v) converted down to 4 volts the current needed would be about half at 1.75 amps from the batterys. Even at almost depleted voltage (6v) the batteries would still only need to produce about 2.5 amps.
Cr123 at 12v would need to produce about 1.25 amps to reach 2000 lumens which they should do fine.
https://lygte-info.dk/review/batteries2012.
At a depleted voltage of 1.25 each, the voltage is at 6v total, so it takes about 2.5 amps to make 2000 lumens which they should do with a way shorter run time than the 18650’s. I didn’t account for battery sag once the load was applied because different batteries have different curves and sag but it should be close enough.
I’m not saying this light will work at the same lumens for each battery type but given its a buck driver and from the battery testing I would think it should but the cr123 would have a much shorter run time, probably less than half.
Then the CR123 only make sense for cases like camping and such. Get a 12 pack of disposables and toss in the bag. No need for recharging.
Not really as you still can recharge 18650 batteries using a USB charger in a car, or any outlet nearby. You also can carry extra 18650 batteries. I would say the CR123 batteries would be better for a disaster, or being lost in the middle of nowhere with no power.
What if you’re lost while out camping 
It would be easier to take 4x 18650’s rather than 12 CR123’s.
Moderator007 did say that CR123’s could be less than half the runtime so it doesn’t really make a lot of sense
i guess the only advantage is that the P36 can also run on Cr123s if needed be. 
Disposable CR123As are good for long-term storage. If you want to leave a bunch of batteries in a cabin as spares for a decade or so, disposable lithium is the way to go. For most other use cases, 18650 beats multiple of anything smaller.
Streamlight lists the following for one of their lights.
4x123
2500lm for 1:30
1000lm for 2:30
250lm for 10:30
2x18650
3500lm for 1:15
1000lm for 3:00
250lm for 11:30
This is a current controlled light. So we see that the 18650 can produce (in this light) more output.
And at the same brightness - more run time.
But like others have said, 123s are great to stuff into a light that is going to be left unused for long periods.
Emergency light for a car safety bag? Perfect.
And they are quite light.
All the Best,
Jeff
With 2 18650’s and it producing 3500 lumens and the 4 cr123 only producing 2500 lumens then the current to the led is most definitley higher using 18650’s but that doesn’t make much sense, since the cr123’s have a higher output voltage. It looks like its only constant current in any mode but high.
Given that 4 cr123s under a 1.5 amp load will have about 1350 mah of capacity at a load voltage of about 2.4v starting out (9.6v total) and
2 18650’s (Samasung 30Q’s for example) under a 2.5 amp load should have about 2800 mah of capacity at a load voltage of about 4.05 starting out (8.1v total) I dont see how they come up with these numbers. Now when you start depleting each type of cells things change but I tried to kindly average the current to compensate a little.
If you look at HKJ’s graphs for both types of cells and follow the curves I’m baffled at how they come up with those numbers. The 18650’s have a very similar loaded voltage through the entire discharge and would actually have a higher load voltage towards the end of the discharge yet they would still have over twice the capacity.
4 cr123’s 9.6v loaded at 1350 mah
2 18650’s 8.1v loaded at 2800 mah
Use 2 Sanyo NCR18650GA’s and the capacity would jump to about 3200 mah.
All that translate to different outputs of the light?
I found a error I had over looked or didn’t exactly get right to start with from djozz graph. I miss read the vf of the led 
. It takes 3.5 amps at 6.5 volts to make 2000 lumens out of a MT-G2 led.
That would change things a bit.
4 cr123 loaded at 2.5 amps should have 9v total at the start and less than 1000mah capacity.
2 18650’s loaded at 2.85 amps should have 8v total at start and still about 2800mah capaity.
In high mode the cr123’s will not be able to keep up with the current required to make 2000 lumens for very long.
It looks like the light output will deplete along with the cells voltage. Probably extending the run time at a lower light output.
But I still dont think the run time would be anywhere close to the 2 18650’s and most certainly at a much lower lumen output. It be interesting to test it if I had the light.
Anyway, I’m glad I got that sorted out and sorry for any confusin I might have caused.
I dont think cr123’s would be very useful in this light unless your just looking for a long shelf life battery source. Even then, they would do much better running in a lower mode to extend capacity and run time.