CR123 primaries in a rechargeable 16340 light?

Batteries and Chargers Consumable Batteries
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I have a question that for which I have been trying to find a direct answer and cannot seem to find it in the archives here, so maybe it’s best to pose it in a new thread.

I have a headlamp (Fenix HM50R) which runs on a rechargeable 3.6V 16340 that was included with the light. However, in the literature that came with the headlamp, they said it was also compatible with CR123 primaries and they provided runtimes for both types of cells were listed. I love this. If I’m ever in a pinch, I can pop in a CR123 primary to get me through until I can recharge.

My question: should this be true for all 16340 rechargeable lights? I understand that the voltage of a CR123 primary is only 3V vs 3.6-3.7V on the 16340 Li-ion rechargeable cells and that the performance will be a bit different. But is there any risk involved? Are there any downsides I’m not accounting for?

The reason I’m asking is because I have another 16340 pocket light that I really like. The spec sheet makes no mention of using CR123 primaries as an option. But I would like to have that as an option in the back of my mind.

Keeping a spare CR123 primary around would be a good habit if I was ever in an extended blackout situation or an extended camping situation and didn’t have the ability to charge for a few days.

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If the light isn’t designed for a CR123 then it will trigger, or be close to triggering, LVP assuming it has that. If it doesn’t then I’d expect the light to be dim and likely only to run at moonlight level.

I wouldn’t think that there are any risks involved since the CR123 puts out less amps than the 16340.

Will your other light take an 18350? They offer more capacity and some/many 16340 lights will take them.

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should this be true for all 16340 rechargeable lights?

no, not all 16340 lights will work with CR123 Voltage range

Voltage range:
for 16340 is 2.5V to 4.2V
for CR123 is 1.8V to 3.2V

I have another 16340 pocket light that I really like.

what make and model light is it?

It will not hurt to try CR123 in a light designed for 16340 only. A light designed for 16340 only will have a Low Voltage Shutoff built into the driver of the light and it will shut off at 2.5V.

if CR123 is not compatible, the light will just be dimmer than normal, and it will shut itself off before the CR123 is actually empty.

When I carry a light that uses 16340, instead of carrying a spare battery, I actually carry a spare light with 16340 inside. That allows the spare light to act as a battery carrier.

If my primary light is broken or lost, I have a complete backup light, instead of just a useless spare battery.

Dual fuel lights like your Fenix have No Low Voltage Protection built into the driver, in order to allow the use of CR123 at voltage that is unsafe (too low) for 16340.

Therefore to use 16340 safely in your UnProtected Fenix, the battery must have a Protection circuit built into the battery, that shuts the battery off before it goes below 2.5V

IF you put an UnProtected 16340 into your Fenix (Do Not do that), it can drain below 2.5V. This is dangerous, as the battery could overheat during recharging and cause a fire.

example, I have a 16340 light that is Not CR123 compatible. It is a Sofirn SC21 Mini. When I turn it on with a 16340 inside, it comes on at 65 lumens (because that is the level I have set it to atm).

When I put a CR123 into the Sofirn SC21 Mini, the output drops to 6 lumens and the red low battery indicator light starts flashing.

Another example, I have a Sofirn SC21 Pro, that is set to start at 70 lumens when using 16340. When I try a CR123 in that light, the output drops to 0.1 lumens within a few seconds. This is totally impractical. And fwiw, the battery is UnProtected, so it would be a Bad Idea to use it in your Fenix with UnProtected driver.

One reason I like the SC21 Mini and SC21 Pro, is that they have built in USB charging. So not only are they good lights in their own right, as well as good backup 16340 battery carriers, they are also good 16340 Battery Chargers :wink:

bottom line, dual fuel lights such as your Fenix, when using 16340 should ONLY be used with Protected Batteries.

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Depends. I’ve used a 3.2V LFP cell in my GTmicro, and it works fine, albeit not at full power, of course.

Then again, it’s a flat 3.2V 'til it falls off the cliff when it’s spent. Pretty sure '123s will drop off from 3V, especially under any load, so it might barely squeak out any light before cutting out.

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Won’t hurt. Might be under-powered > pathetic. Depends on the light.
I do exactly that for my hiking headlamp.
As a trial I tried a CR123 in the light as a bedside/bedroom light. The light has a color/charge level indicator. It IMMEDIATELY went red on a new battery. But it worked fine, but not as a monster light.
I thought it might die soon. It lasted for months. It would not go to higher levels, just low, which worked fine for my use. But it sustained that output for a very long time. Exactly what I wanted.

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It’s a Thrunite W1. It’s one of my favorite lights I have ever used because it is simple, durable as heck (accidentally dropped it on concrete multiple times over the last couple of years and is no worse for the wear), and has my favorite UI ever: press for on with mode memory (I keep it in medium), long press for moonlight, double press for turbo. That makes it the only light I have ever personally used that has access to moonlight, medium, or turbo directly from off without being overly complicated and having too many modes or ramping, which I don’t care for.

Thanks for this excellent answer. It makes perfect sense and it was exactly the kind of info I was looking for. I looked at the specs for the battery from my headlamp and indeed, as you predicted, it has a protective circuit to prevent over-discharge. It sounds like my best bet is to keep one or two extra rechargeable 16340’s around and make sure they are charged. Both lights have onboard charging through micro USB, so I can use one as a charger while the other is in use.

If I’m going to have spare 16340s, I guess it makes sense to get an extra that has the over-discharge protection so it can be used in either light. There shouldn’t be a disadvantage to using a protected 16340 in the W1 that already has a protection circuit in the driver, right? I’m hoping to just simplify my setup and have a spare 16340 that will work in either light.

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I’m just starting to learn a bit more about batteries, but I have often wondered why almost all the manufacturers switched to designing flashlights around Li-ion rechargeable lights instead of commonly available primary cells. I like the ability to recharge instead of discard as much as the next guy, but I’m beginning to realize that there are other advantages.

My older flashlights that used primary cells would get dimmer and dimmer as the battery discharged and would continue to emit light until it was barely a flicker. It seems that most of my newer lights that are running on Li-ion rechargeable cells keep the same brightness until the cell is discharged to a certain level, and then abruptly stop working.

Is that a generalization that broadly applies? If so, that’s another good reason to use rechargeable cells over primaries in my book. I would rather have the expected amount of light until it abruptly dies than wonder if my mind is playing tricks on me or if my light is dimmer than it was yesterday and having to guess if the primary cell needs to be replaced.

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Depends on the light or more specifically its driver.

Primary '123s are good because they last 20yrs if not more, and a small light that takes 'em makes a good shtf light.

My MH20 works with a single 18650 but can work fine with a pair of '123s.

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there are two issues that may or may not interfere with using a protected 16340 in a light whose driver has built in LVP

  1. Some protected batteries are longer than some unprotected batteries. If the battery is too long, the tailcap may not screw down as far. That can potentially prevent the body tube from making contact with the tail, so there will be no electrical circuit completed.

  2. A second possible pitfal would be IF the protected battery trips protection prematurely. This is mostly a potential risk when attempting to use a very high output, with a battery that is less than full. If the protection is tripped, the result can be sudden darkness, which is potentially dangerous, if not inconvenient.

I would rather have the expected amount of light until it abruptly dies than wonder if my mind is playing tricks on me or if my light is dimmer than it was yesterday and having to guess if the primary cell needs to be replaced.

there are pros and cons to both types of drivers, the ones that get dimmer as the battery gets emptied (UnRegulated output), can help to visually inform the operator that they should prepare to run out of light, and plan to move to safety and or prepare to replace the battery.

It is good practice to measure battery voltage periodically, to help gauge how empty it is becoming, and anticipate remaining runtime.

otoh, I agree that Regulated Output drivers, wont make us “wonder if the light is getting dimmer”, which I agree can be bothersome in itself.

In this case it is again helpful to measure battery voltage periodically, in order to anticipate, and be prepared for the possible sudden and inconvenient loss of light.

this is the tool I use to measure battery voltage, whenever I wonder if I need to replace and/or recharge

or in some of my lights, there is a built in battery voltage readout, for example the SC21 Pro can tell me its voltage. But the SC21 Mini cannot, it only shows a green or red indicator light that is built into the switch button.

Your Thrunite has an indicating switch, and when the cell voltage is low, this switch will turn red, then blink red. If you insert a CR123, that is probably what it will do… :wink:

Your light also has a step down feature that will go to a lower output, before running out of battery power. Looking at this runtime chart helps visualize the step down:

chart from this review

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Thanks again. I’ll definitely keep all that in mind.

I knew the W1 had an indicator in the switch and a step down feature (especially for higher modes), but the chart is handy.

When I got home from work, I popped the Thrunite 16340 out of the W1 and found fine print that I almost needed a magnifying glass to read. It is a protected cell, too. So both of my 16340 lights must be relying on protected cells instead of a protection circuit in the driver. So at least I don’t have a mix of protected and unprotected 16340s to worry about.

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Some of my Fenix 16340 lights (RC09, PD25…) are CR123A compatible, but also do have low-voltage detection / action(s) as a feature. These lights check the cell voltage (to determine cell type) at power-on. If above a threshold ~=/>3V, Li-ion is assumed, and such function(s) is/are enabled. If the cell voltage is ~=/<3V, lithium primary is assumed, and thus such low-voltage detection is disabled for the duration of on-time.

I have 2 HM50Rs (which are CR123A compatible), and I believe this function is incorporated in those also. I don’t know about the newer HM50R V2 version though.

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Thanks! Appreciate the info, I also have the older original HM50R and not the v2.

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I’ve been thinking about this a lot and seriously considering adding another basic light that runs on primary CR123s to keep around for a rainy day.

But I already have an AA light that I love. Maybe it would be better to keep some AA Energizer Ultimate Lithium primaries stashed. Shouldn’t an AA lithium primary have the same shelf life as a CR123 lithium primary?

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depends who you ask, lol

google sez (trust but verify):

AA Lithium Primary: 20 year shelf life
AAA Lithium Primary: 20 year shelf life
9V Lithium Primary: 10 year shelf life
CR123 Lithium Primary: 10 year shelf life

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Got some [ AA Lithium Primary: 20 year shelf life], and they were half dead when put to use in a couple years. Couldn’t tell initially because the voltage was OK, but the capacity wasn’t there.
Only happened once though. I have not used a bunch of them, mostly sticking with NiMh. Nice for some applications but pretty pricey.
Those things have a date code? Don’t remember checking.

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I dig my Streamlight ProTac 1L-1AA for a great ‘flex fuel’ EDC. Runs on CR123A or most any AA, and is a fine EDC w/ things like a Borofloat lens, 2-meter impact rating, Ten-Tap options, etc. to boot. It doesn’t use anything 3.7V, but for a ‘3V’ secondary / rechargeable fuel (for the higher output), I use LPF123s. For a very useful light w/great features that’s very flexible on fuel, I’m not aware of anything else like it. Back to the thread title though, it does not do in-light recharging of anything, but for utility and power supply flexibility, …

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Funny you should mention that. I went to Bass Pro last night and looked at that light in person. I really like what I’ve seen both in reviews and checking it out. I may have to order one as soon as I get some discretionary funds.