# Runtime at medium power with 18650 vs CR123

#### scsiguru

##### Newly Enlightened
I have a Fenix HL55 LED Headlamp and have been using two CR123 disposable batteries in it. I'm not that happy with the runtime I get. I'm considering investing in a some 18650 batteries and a charger and I'm wondering if I will get more runtime with these batteries? I generally run the headlamp at the medium power.

#### hamhanded

##### Enlightened
CPF Supporter
If we look at each configuration's power:

2xCR123 = 6v nominal @ ~1600 mAH = 9.6 Wh
1x18650 = 3.7v nominal @ ~3400 mAH = 12.6 Wh

1x18650 can provide you with more power. In most cases, you'll get a little more runtime that way. You also lose the overhead associated with managing two cells (unbalanced cell voltages).

2xCR123 can be as high as 6.2v, and fresh cells under load can drop to 5v. A li-ion can be as high as 4.2 and is less likely to sag as much due to higher current handling. So, in practice, the runtime you get will depend heavily on the way the driver handles such a wide voltage range. I'll leave a time estimate to someone familiar with the way the Fenix HL55 driver works.

#### vicv

##### Flashlight Enthusiast
Will the headlamp run on a single 18650?
If so 2x cr123s have a nominal voltage of 4.8v and 1500mah= 7200mwh
An 18650 has 3.7v x 3500mah= 12950mwh. So the 18650 has close to double the amount of available work.
Depending on the circuitry in the headlamp, it would definitely provide longer runtime. And possibly less output

#### vicv

##### Flashlight Enthusiast
Looks like I was beaten to it. The only thing I will disagree on about the above information, is the CR1 23 A’s do not provide 3 V under load for very long so I do not consider them to be 3 V nominal. But regardless of the number you choose, 18 650 still has more capacity

#### hamhanded

##### Enlightened
CPF Supporter
Looks like I was beaten to it. The only thing I will disagree on about the above information, is the CR1 23 A’s do not provide 3 V under load for very long so I do not consider them to be 3 V nominal. But regardless of the number you choose, 18 650 still has more capacity
This is true, they do. But "nominal" is the "named voltage" of the cell, which I used for consistency in calculations. If you measure a fresh CR123 you'll see it's 3.2 volts, even though it can only provide that for short periods. I guess we can call it "theoretical maximum power" under ideal current draw conditions.

This nominal voltage convention is pretty standard with primaries: AAAA and D batteries are both nominally 1.5v even though a D cell will have less voltage sag under load, all else being equal. Lithium AA batteries are nominally 1.5v even though they provide around 1.7 volts when fresh.

#### vicv

##### Flashlight Enthusiast
I agree and it’s annoying. But I have to go with practical numbers. A cr123 is never 3v at any time in it’s discharge. At least not at flashlight draw levels. Otherwise we’d have to use 4.2 volts for the 18650.
I do understand using spec numbers but if you’re looking at actual run time I feel you have to show what the cell will actually provide. Like using actual fuel mileage calculations to figure out how far my vehicle can go on a tank of gas, compared to using the numbers that the manufacturer shows. All that will do is leave me on the side of the road
That being said, a simple search would have shown many many threads already with this exact title. 18 650s have only gotten better and sierra 123 so stay the same so the gap is widening

#### hamhanded

##### Enlightened
CPF Supporter
I get what you're saying, now, yes. CR123s might be good for long storage, and resistance to cold. But they're otherwise inferior to rechargeable lithium ions in every other way. Now, if you include RCR123s, it might change things slightly... but that's not what the poster asked about.

#### vicv

##### Flashlight Enthusiast
i’m not necessarily saying that CR 123s are bad. I think they’re great cells. I have many lights that I use them in. They have quite a few advantages over rechargeables. But they have definite disadvantages as well. Environmental, run time, overall value. But in a light that you are going to use only rarely, I think they are the superior choice if your light will accept them.

#### hsa

##### Enlightened
The problem with all that capacity(numerically) is that you have to run the 18650 all the way into the ground, dead. Not a good idea. If used correctly though it is still an advantage economically.

#### scsiguru

##### Newly Enlightened
Will the headlamp run on a single 18650?
If so 2x cr123s have a nominal voltage of 4.8v and 1500mah= 7200mwh
An 18650 has 3.7v x 3500mah= 12950mwh. So the 18650 has close to double the amount of available work.
Depending on the circuitry in the headlamp, it would definitely provide longer runtime. And possibly less output
Yes...headlamp can also run on one 18650

#### DIWdiver

##### Flashlight Enthusiast
The problem with all that capacity(numerically) is that you have to run the 18650 all the way into the ground, dead. Not a good idea. If used correctly though it is still an advantage economically.
Maybe true, but you can get 95% of the maximum capacity without running them totally to the ground. And if you recharge them fairly soon, you can avoid doing too much damage.

If you run the batteries down frequently, the economics are hugely on the side of LiIon. If you don't run them down frequently, the runtime is largely on the side of newer LiIons, less so for older ones.

So the only advantage primary cells have is very cold conditions, long storage, and very infrequent use. Exactly the reasons I bought a CR123 light for my wife to keep in her car.

#### hsa

##### Enlightened
Cr 123 seem perfect for glove box duty and lots of todays lights will run on either so a win, win.
What light did you get for the car?