Defiant 300 lumen tactical light

Flashlight Modding and DIY Parts
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Hi everyone…I’m new here, but I’ll jump right into things. I recently bought two Defiant lights from Home Depot, 300 lumens each, running off 4 AAA batteries, for a decent price. Now, I was fooling around with it and figured I could use some other battery combinations.

The light will fit 2 CR123a batteries in series. Now since the light runs off of 6 volts this is obviously not a problem. Now one of the 123s i used is dying so after some time I’m getting about 4.8-5.2 volts; one 123 is at 2.8 volts the other one is around 2-2.4 volts. When I rig it up I get 2.8 volts across the LED itself, meaning it’s not gonna be as bright as it would be fresh.

So next I tried an UNPROTECTED 18650. Now before I go into this I’d like to get an idea as to how rechargeable lithium ions work. I know that they start at around 4.3 volts fresh and then drop to 3.7 volts dead. That being said, does this mean that any device designed to use 18650s is supposed to stop working at 3.7 volts? Otherwise, wouldn’t the lithium ion be able to power something even after it’s dead, since it will still create a potential difference in the circuit? The reason I ask about this is because when I put in a dead (3.7 V) unprotected 18650 in the light and turn it on, I get full brightness. I measured the voltage across the LED and I get a full 3.0 volts. Why is this? Why is it that when I have two used 123 batteries in series which total anywhere between 4.8-5.2 volts, I’m getting a noticeably dimmer light and 2.8 volts across the LED, BUT when I use a dead (3.7 V) 18650, the light is brighter and I measure 3.0 volts across the LED? Same deal with 4 NiMH AAAs—I’m getting 5.2 volts total (4 batteries) and when I turn it on it’s a little dimmer and I measure around 2.8 volts across the LED. Why is the light fully bright with an 18650, which would range anywhere between 3.7 V to a maximum of 4.3 V, whereas with 2xCR123a @ a total of 5.2 volts, OR 4xAAA @ a total of 5.2 volts, I’m getting a dimmer light? It just doesn’t make sense to me…

Now it seems 2xCR123a is kind of useless. I’d be better off using 4 lithium primaries as it was intended. The only problem is the light is poorly designed and when using the included battery holder, if you shake the light, it will change modes (shaking the light causes the terminals to disconnect, which power cycles the light). So if I drop it, it’ll change modes and start to strobe or go dimmer, which is annoying. This is why I want to use the 18650.

So basically I need to know if it’s safe to use an 18650 with this light. I’m just worried because the light is obviously going to keep sucking juice even after 3.7 volts. Is this because the 18650 I used was unprotected? I really like how I was getting full brightness even with a discharged 18650…if this works then I essentially have a cheaper light that is going to run at full brightness from 4.3 to 3.7 volts. My only concern is that I need for the light to realize the battery is dead at 3.7 volts so I can recharge it! Get what I’m saying?

Sorry if my post is confusing. Feel free to get technical, my background is in science. Thanks very much in advance for any help!

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Most Li-ion 18650’s are fully charged at 4.2V, and dead at 2.9-3.0V. I believe that the reason your light is brighter with a 3.7V 18650 is because the cr123’s are almost dead and cannot supply the needed current to the LED, regardless of their higher voltage. Can you measue the current at the tailcap and/or LED? Also, I would run an 18650 down to just over 3.0V and test it in the light to see if the brightness is noticeably dimmed. If it is, there is your “low voltage warning”. :smiley: If not, it would be a bad idea to use unprotected cells in that light.

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By the way, welcome to BLF!

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Welcome to the forum mathematician.

Please be careful at using Li-ion cells in series. I personally wouldn't use any cells together that don't come out of the light with matching voltages. Probably the single biggest risk with Li-ions is one cell becoming more discharged and then the other cell reverse charging it. Probability of a venting event is too high to play with it.

Second warning (sorry, really) taking cells repeatedly below there rated low end (most cells are rated for 3 volts, some for 2.5 volts, creates crystals that can puncture membranes and cause a venting event too. This was discovered by NASA, I believe).

Of course, venting in a sealed metal tube can cause an explosion.

Sorry for all the warning stuff.

Is this the light you have? If it is, I really like that light a lot. There are a couple mods threads for it here.

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A sticky on Li-on use would be pretty helpful for people in this situation. Is there one? I don’t see any in the rechargeable section.

I suggest you spend some time looking up basics in this site:
http://www.batteryuniversity.com/partone.htm

Primary alkaline cells have good capacity but throughput is not as good.
Rechargeable, especially lithium, have a much better throughput, or at least can if the battery is not junk. You light would probably work better on rechargeable NiMh than primaries. Lithium has even better throughput so depending on the light driver it may, or may not be brighter.
At 3.7v your Li-on was not dead so was still able to support the light draw better than the primaries.

Unless specifically built in to it a light will not have a cut-off so your battery is at risk……so are you. If you pay attention as Legolas suggests you may be able to see the light dimming in time. After all, it’s dimming with the less capable batteries. If not, don’t do it.
Get a protected cell, be happy, be safe.

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What your experiencing with the different voltages at the emitter is the result of the driver trying to supply the correct voltage to the emitter while also supplying enough current to drive the LED. Your Cr123a’s are dead for all intents and purposes that’s why you have less output with them than with the single Li-ion. Put 2 fresh Cr123a’s in there and you will see a world of difference.

P.S. Don’t worry about running Cr123a’s in series as they are not Li_ion. Most Lithium primaries are Lithium Manganese Dioxide or Lithium Thionyl Chloride and are much safer when ran in series than Li-ion.

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Thanks for all the replies. I didn’t really want to use unprotected cells with this light anyways. If I used a protected 18650, would it “know” to stop working at 3.0 volts? I thought these cells were dead as I pulled them out of an old laptop battery that hadn’t been charged in years so I just assumed they were dead. I don’t have any protected 18650s right now but if you guys say it would be safe then I’ll definitely start using them with this light. In the mean time I’m going to run this unprotected cell down a little more. Thanks guys!

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Lol. The LED broke/melted off. I was opening it to try and measure some voltage readings and it just fell out onto the floor. I’ll return these and look for something better. Thanks again guys.