Battery extension tube question

wbexpress

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Oct 18, 2014
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I am new to the forum, and I just started using LED flashlights with 18650 batteries. I have a somewhat limited knowledge base regarding electronics, so I'm a NOOB. One of the lights I just purchased is what I guess would be called a knockoff, having the markings of UltraFire C8 and Cree XML-L2 on the body. Anyways, just to experiment a bit, I purchased a battery extension tube for it, thinking that similar to what can be done with AA's and AAA's, that I could just put the tube on, pop in the 2nd battery and it would extend the in between charge times. Well, instead of a bright, functioning beam, when I turn it on it sortof pops and then the beam is really low. When I take the 2nd battery out and return it to normal, everything is fine again. So, can someone please explain the flaw is in what I am trying to do? Am I correct that there are lights that can use 2 18650 batteries? If so, am I correct that doing so extends charge times? The batteries are Eagle Tac 2500's. TIA to all.
 

WalkIntoTheLight

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Jun 18, 2014
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I'm surprised your light still works at all. A cheap light likely drives the LED directly off the battery, because 1 lithium-ion cell is a pretty good match to the voltage that most LEDs use. By doubling the voltage, I'm surprised you didn't fry the LED. Perhaps there's some protection circuitry in your light that prevented that.

More expensive lights may take two lithium-ion cells (in series). These are designed with drivers that can handle the higher voltage.

In any case, don't do it. Or, run them in parallel if you really want to double run-time. (Thought it's easier just to replace the battery when it gets low.)
 

wbexpress

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Oct 18, 2014
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I'm surprised your light still works at all. A cheap light likely drives the LED directly off the battery, because 1 lithium-ion cell is a pretty good match to the voltage that most LEDs use. By doubling the voltage, I'm surprised you didn't fry the LED. Perhaps there's some protection circuitry in your light that prevented that.

More expensive lights may take two lithium-ion cells (in series). These are designed with drivers that can handle the higher voltage.

In any case, don't do it. Or, run them in parallel if you really want to double run-time. (Thought it's easier just to replace the battery when it gets low.)

Thanks for the input. Yeah, it was an experiment with a fairly inexpensive light just to see what happened.
 

oKtosiTe

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Jan 7, 2012
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Thanks for the input. Yeah, it was an experiment with a fairly inexpensive light just to see what happened.
"Experimenting" with Li-ion cells is not a good idea. If poorly matched, this can happen.
Also, I wouldn't call that a knock-off per-se, because TrustFire and UltraFire are known to be awful around here.
 
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