Rechargable 18650 vs CR123A questions


Newly Enlightened
Sep 19, 2023
I just ordered a Fenix PD35. It comes with a Fenix 2600 mAh 18650 battery, and it can also be used with two CR123A primary cells instead.
I have some questions that don't seem to be easily answered by reading battery specification sheets.

Will I get much less run time with CR123A cells? Can you estimate how much less?
Does the flashlight use 3.6 volts from the 18650 as efficiently as 6 volts from the two CR123As?
The 18650 is marked 9.36 watt hours and two CR123A cell should give 9.0 watt hours. Are the capacities comparable when used in a flashlight?
If I charge the 18650 battery and don't use it, at what rate will it self discharge?
Is there an estimated shelf life for an 18650 battery? Should I expect it to be useless in five or ten years no matter how I use it?
If I leave the light and the 18650 in my car all winter where it is below freezing about half the time with lots of temperature cycling, will the battery hold up? Will its capacity be permanently reduced, will it leak, and might it burst into flames without warning?


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Aug 9, 2015
John 3:16
The 18650 won't just burst into flames and very likely won't leak. It'll store well at cold temps.

If you use it a lot the battery will wear out. If you don't it won't.

The light is what some call dual fuel. In other words it can be used with varying voltage batteries. The 18650 should give better runtime. Think larger fuel tank.


Flashlight Enthusiast
Mar 22, 2013
Southern Ontario
If you leave an 18 650 at full charge, it will degrade a bit. But it will not self discharge. It will not burst into flames. And cold will not do any damage. The only damage cold can do is if you charge it while it's very cold.
I don't know what driver is in that light, but you will probably get longer regulated runtime on the primary batteries because of the voltage overhead. But they cannot output the type of current that the 18650 will, so it will probably be less bright.
CR 123 A's perform better in the cold. It's just the chemistry it doesn't mean either will do damage, you will just get reduced runtime on the 18 650. But once it's warm, it will go back to the way it was.
My recommendation is that if this is going to be a glove box, light and barely used, use cr123 A's. They can sit there for 20 years and not lose any capacity. A rechargeable is a waste if you're not recharging it. And it will degrade overtime. If you're going to be using your light every day, then use an 18 650 as it will save you money. And will be more environmentally conscious.

Guitar Guy

Oct 23, 2016
West Virginia
I just ordered a Fenix PD35. It comes with a Fenix 2600 mAh 18650 battery, and it can also be used with two CR123A primary cells instead.
Depending on how cold your weather gets, your 18650 should perform. The problem with 123 is you usually get reduced output on the higher modes, so why limit a good light? Fenix was making some "cold resistant" batteries. Not sure if they're still available. I carry a Fenix 18650 light in my truck all winter with the 18650 in it, and keep a few 123 cells as spares, since they're cheap and last for a long time. Then you have the best of both worlds. The 18650 doesn't self drain much, if at all, but I may top of the charge once or twice a year, depending on if I use it much.

What I would suggest is getting a 3400 or 3500 mAh cell to upgrade your runtime from the 2600 mAh. Even if not a cold resistant one, you should still be good, unless you're talking EXTREME cold. You can get a good Panasonic KeepPower protected 18650 cell for around $9.

Here is what Fenix says about extreme temperature range.

Sep 30, 2020
Caught in an air duct
Thank you for the definite answers! 18650's are better than I thought, but I will use CR123As for any light kept in a car.
"I will use CR123As for any light kept in a car." I find that this philosophy saves me from having to keep track of seasons! I'm getting old, and they do sneak up on me. :rolleyes: It is worth noting that the trunk of a vehicle is far cooler in summer than the passenger area due to the greenhouse effect from the windows. Just throwing that tidbit in for free since I don't know if it ever gets that warm in Vermont!