Battery Question: What an I missing here?

[Pima Pants]

I'm new to Li-ion batteries and couldn't find this answer even after perusing Battery University and this fine site.

I recently purchased a Ultrafire wf-139 charger and one 14500 battery. I understand that the 14500 battery is 3.7V as opposed to an alkaline 1.5V primary. I am using the 14500 in an Ultrafire no-model number aspheric and the difference with the 14500 is remarkable. What I don't understand is why an 18650 battery is also rated at 3.7V. If I use two CR123 primaries, that is the equivalent of 6.0V. I want to use an 18650 in my old Surefire 6Z with a new P60 drop-in and just figured an 18650 would really kick butt, but now I'm not so sure. What advantage would I realize with an 18650 as opposed to two CR 123 primaries? Is it just run time?

You help is appreciated.

[Mooreshire]

Yes, the advantage that you might notice with a single 18650 versus two CR123 size cells would be run-time. The electronics have to be compatible though, since an 18650 is only producing half of the voltage that would be produced by the two CR123s.

Your 14500 cell produces the same voltage as an 18650 battery does because the chemicals inside dictate the voltage they produce and they are identical in that regard. The quantity of material is what dictates capacity (potential run-time), which is why a larger sized 18650 has a larger capacity than your 14500 battery does. All rechargeable lithium-ion cells of a given chemistry produce the same voltage.

Comparing the two CR123 size batteries to the single 18650 we'll note that the two 123 cells add their voltage together but not their capacity, whereas the 18650 has double or more capacity (due to its size) but half the voltage that the two 123 size cells can combine to offer.

If your light is compatible with either the two CR123 cells or the single 18650, then the difference is potentially going to be less output for twice as long with the singe cell. Hopefully you can check what the voltage range is for your P60 drop-in is before you buy more batteries though.

[Lynx_Arc]

The biggest difference between the 18650's and the 2xCR123's is the cost. Initially the 18650 + charger will set you
back a lot more but if you use the light a lot over time it will start to save you money. 2 CR123s give you about 6.4V new while an 18650 off the charger will give you 4.2v so about 50% more voltage in the 123s or 1/3 less in the 18650.

[TEEJ]

Think of it this way...Each cell has a voltage range...the rechargeable lithium ion's are rated at 3.7v, but really start closer to 4.2v, and drop down to ~ 2.5 -3.2v or so depending on the cell and/or the flashlights own circuits.

If we use 3.7 v as a midpoint between the fully charged and discharged state it might drop down to, and say we use TWO cells, that ADDS the volts if they are in series (End to end essentially), giving us 7.4v instead of just 3.7v.

7.4v might allow the LED to produce more lumens than if limited to 3.7v (If the light's circuitry, etc, allow it).

If in series, you add the volts, but the mAh remain the same....so if the two cells are 3100 mAh cells, you still have 3100 mAh of run time.


If we wire it so the cells are parallel, then the VOLTS stay the same, but you ADD the mAh together for all the cells....so you'd essentially have a 3.7v 6200 mAh power source.

The rechargeable lithium ion CHEMISTRY is why it produces that voltage...so the larger cell has the same voltage as the smaller cell.

The NON-Rechargeable LiIon chemistry also produces a particular current, rated at 3.0v (The way the rechargeables are rated at 3.7v, etc...)

Larger or smaller primaries of that chemistry will give the same voltage, etc....same principle.

There are sub-divisions in the above for SPECIFIC chemistries, but the concept is the same, the PARTICULAR chemistry determines the voltage of the cell.


As to what your light will DO with the cells, different lights have different circuitry. SOME will be brighter on rechargeables, and some won't, depending on their regulation. My Klarus XT11 is a LOT brighter on two CR123's than it is on ONE 18650.

But, that same light only burns for ~ 750 mAh at that brighter output, as opposed to the 3100 mAh the 18650 gives me.

So I can have ~600 lumens for say ~2+ hrs, or 850 lumens for ~ 1/2+ hr with the RCR123's.

On the flip side, my SF Fury is no brighter with RCR123's than with an 18650...it has different regulation.

You'd need to know what type of regulation your light uses to know if and how a particular cell combo would change the performance.

Since you found that at least one of your lights was brighter with the higher voltage, you can bet its regulation was the type that allowed that.

[Pima Pants]

You guys are absolutely amazing with those explanations. I learned more in your three posts than I would after reading a ton of material. Thanks for your knowledge and kindness to someone just starting off with these battery chemistries.