Hi - I’ve just picked up a few inexpensive flashlights that use 14500 batteries. Specifically, Acebeam Tac AA, Acebeam pokelit AA and a Thrunight Saber. They all have batteries with a usb charging port. I also picked up some backup Keeppower 14500 batteries that I can charge in an external charger.
The keeppower batteries work fine in the Pokelit and the Saber, but not in the Tac AA. I saw that the builtin batteries are not the same size as the Keeppower replacements which seems odd - aren’t all 14500 batteries the same? Otherwise, what makes them 14500?
Probably making a bad contact. Nothing to lose sleep over.
There are a few variables… Length, type of positive contact, and protection or lack of protection.
Look at the positive end, some have flat tops, some have button tops. Flat tops dont work in some lights built for button tops. Some button tops are slightly longer and may not work in lights built for flat tops.
Some batteries have a protection circuit, some do not. Some lights dont have built in protection, some do. Unprotected lights, need protected batteries. Lights with built in protection, dont need protected batteries.
Otherwise, what makes them 14500?
14mm diameter, approximately 50mm length (some are longer or shorter than others).
Some lights that use shorter batteries wont close far enough for the battery tube to make contact with the head, when using longer batteries.
You should find out whether each of your lights has built in protection or not. If they dont, then they should only be used with batteries that have a protection circuit, designed to stop the battery from being overdischarged.
Usually lights that dont have built in protection, require button top batteries.
USB rechargeable batteries are usually button tops, and they have built in protection.
Some lights that are designed to use button tops, have physical reverse polarity protection, that will prevent a flat top from making contact.
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Well said.
Thank you jon_slider. Very informative post.
Thanks for the really thorough response! So the USB-C battery it comes with is a button top but doesn’t say protected on it. The replacement I bought is also a button top and is protected. The included 14500 is noticeably shorter than the replacement I got. What seems odd is that the replacement is very close to the size of regular AA batteries which this light is supposed to be compatible with. But I suppose the slight difference in size might be just enough to prevent correct contact.
Looking at the specs on the TAC AA, I don’t see anything about protection.
Bingo! its a length issue.
Depending which KeepPower model you bought, it might be longer than the USB batteries… for example this Protected Button Top is 52mm long
otoh, the USB 14500 cells, are probably 50mm long. Regular AA batteries are also 50mm long.
My guess is the Thrunite does not have enough spring travel to absorb the extra length, so the body tube is not able to screw in far enough to make contact with the driver in the head.
it seems the two Acebeam lights have longer body tubes and springs, and they are able to absorb the extra 2mm length of the KeepPower, and still allow the body tube to screw all the way into the head and make good contact.
I just happen to know that all 14500 USB rechargeable batteries have built in protection, and tend to be short, 50mm, like a normal AA.
It makes sense that your lights use protected batteries because they are dual fuel and can use AA batteries.
Dual fuel lights generally do not have built in protection, because the AA batteries can be drawn down to a lower voltage than would be safe for LiIon. So the LiIon batteries are protected, because the lights do not have built in protection.
My guess is the KeepPower you bought IS protected, so youre safe to use it in the two Acebeam lights that have enough spring travel, and Require Protected Batteries.
Welcome to the forum, I hope you enjoy your lights! 
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So I got a different one but it’s 52mm also.
Just for accuracy - the Thrunite actually works fine with the replacement as does one of the Acebeam Pokelite AA. It’s the Acebeam Tac AA that does not work.
What you said about the protection in the battery vs the flashlight itself makes sense.
Might send back the Tac AA lights (I got two). They were pretty reasonably priced but they have no special modes (no strobe) and they’re pretty large for a single AA type.
good to know… glad you have the option to return
fwiw, another battery variable you may want to learn to observe is the capacity…
your KeepPower is 800mAh, while the Acebeam USB cells have 920mAh capacity. That translates to slightly longer runtime. I tend to focus on that variable, and like to buy batteries with higher capacity, hence slightly longer runtime… for example the KeepPower I linked has 1000mAh capacity… but as you noted, at 52mm, the length can be an issue.
thanks for the detailed discussion and for sharing specific info about which lights you have, and which battery you bought.
its good to know that the Tac AA is size challenged… Size Does Matter… LOL
And congrats on your selection of USB batteries… I think they are very handy, and save you from needing to buy a separate charger, as long as you have a USB cable. 
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Yeah, actually I usually do the same (focus on capacity) but I’m sleep-deprived due to a new puppy in the house, and apparently was too brain dead to do the comparison. 
congrats on the new addition
hope you get to catch up on some sleep soon 
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It’s a pleasure to know you, ultradianguy!
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It seems the best way to facilitate ‘Low Voltage Detection’ in dual fuel lights is for the light to check the cell voltage at power-on, then set a flag (Li-ion/Primary) to enable / disable that LV detection function according to the measured voltage at that point. That’s how Fenix does it w/16340 / CR123 lights, but I doubt that all manufacturers go to that effort. They can then do LVD w/16340, but not when using CR123. That’s not too difficult, but of course it requires the will to do it.
agree
HDS is one of the few I know that can enable and disable LVP by auto voltage detect for a single CR123A/16340 light
however, HDS does not offer any ability for the operator to do a battery voltage check, without opening the light
If I open my HDS to check a LiIon battery voltage manually, I could potentially defeat the LVP if I was to reinsert a partly discharged LiIon that was at CR123a voltage…
I believe Sofirn SP10 can do voltage detect for AA/14500, but I hate its switch button, because it is so hard to find in the dark
Skilhunt H150 has a better switch, and also detects AA/14500
AA/14500 detection does not have the voltage overlap that could confuse a voltage detect in a single CR123A/16340 light
10-4. Fenix has been doing this for ages, and it works well in actual practice, as I’ve had the chance to exercise it a few times over the years.
The only thing I’ll add, just as an aside, is that there’s one other advantage to that implementation for me in practice. When those lights hit LVD threshold, they go thru a sequence of actions (and I don’t mean shutting the light down and leaving me in the dark;-). They are appropriate actions, but there are times when I am aware of the condition, but do not want those actions taken / continued. I don’t want it to step down a level, blink the output, or anything else. I want to run it uninterrupted and as needed until the cell LVP / over-discharge protection kicks in, if necessary; that’s not a big deal. A quick / simple ‘0-1’, off-on, of course disables those unwanted actions for the duration of use, as from that point, the ‘Li-ion flag / bit’ is toggled ‘off’. It’s an operator bypass / override. I appreciate that ‘side benefit’, if you will.
thanks for the details… sounds like a useful advance warning…
HDS does that too… it steps down and blinks, before the battery hits the LVP shutoff voltage.
glad you have options to stop the warnings while still keeping the LiIon LVP shutoff working…
I have one (protected) Keeppower 14500 (model: P1450C) and it is by far the longest 14500 that I have. I just measured it and it’s 52mm long. Due to its length it doesn’t fit/functions in some of my 14500 lights, like, for example, the Wurkkos TS10.
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You are not going into any detail about the light with wich the Keeppower battery has problems. So (longshot) do you have a spring at the front end leading to the driver? Or is it a shallow “cup”. This method is often used as a crude form of reverse polarity protection. If the positive pole, with the button, of the battery can’t touch the bottom of that “cup”, there will be no connection. The negative pole of the battery is flat, so there will be no connection with a wrong polarity. Flat top batteries will make no connection either. Short question of a long story is if the button on the 14500 Keeppower is too wide to make it to the bottom of the “cup”.
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