# LED flashlights

Discussion in 'LEDs and Optoelectronics' started by Edward Glenn, Mar 19, 2016.

1. ### Edward Glenn

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Mar 19, 2016
I have several led flashlights that use 3 AA or AAA batteries in an internal holder. I'd like to use rechargeable 3.7v batteries instead. Will they work? Does anyone sell plastic spacer sleeves to go over the outside, or will I have to make my own? Thanks for your help...Ed

2. ### Colin Mitchell

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Aug 31, 2014
Th problem is the circuit is designed for 4.5v and using 7.4v will be very inefficient as the difference between 4.5v and 7.4v will just be wasted. You will need to add an extra resistor.

3. ### cjdelphi

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Oct 26, 2011
7.4v?

3 x 4.2v = 12v, what are the?LEDs?

4. ### Edward Glenn

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Mar 19, 2016
What 7.4v??
I'm thinking of replacing the 3 pack of AAA with one big round 3.7v rechargeable battery. If the led will work at this lower voltage, all I need is a sleeve over the battery to keep it centered over the contacts.

5. ### Alec_t

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Jul 7, 2015
The flashlight could be relying on the internal resistance of the AAA cells to limit the current through the LEDs and drop the voltage down to ~3V, or whatever the forward voltage (Vf) of the LEDs is. The rechargeable battery would likely have a much lower internal resistance, so be prepared for fireworks if you don't add a suitable current-limiting resistor.

6. ### Edward Glenn

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Mar 19, 2016
Thanks. It's more complicated than I thought.
You troll your own forum? Shame on you.

7. ### cjdelphi

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Oct 26, 2011
What? How exactly do you come to that conclusion? (Troll?!)

8. ### Edward Glenn

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Mar 19, 2016
This was, to me, a nonsensical reply "3 x 4.2v = 12v, what are the?LEDs?". I figured you were just making fun of a newbie. For one thing, the math is faulty, =12.6v, and the led is the little round bulb that sucks up all the dark.
As I said, I have this little holder, it fits inside my flashlight, it holds three (3) batteries. Each battery is clearly labeled "AAA 1.5v Alkaline..umm, never mind....I'm just wasting my time and yours.

9. ### cjdelphi

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Oct 26, 2011
Faulty?

It reads 3 x 4.2v which is 12v (as lithiums settle to 3.8/4 quickly) i rounded it down!

From what you were asking, you implied you wanted to replace the AA's with lithiums i presumed 3 AA lithiums (14500 batteries)

3 x 4.2 gives 12v

I then asked what LEDs it has... 3mm? 5mm 1w LED? 3w? 10w 100w? A little round bulb sounds like an edison style bulb, pictures would be more useful than trying to get information out of you...

So, keep calling people trolls for no reason...

10. ### Edward Glenn

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Mar 19, 2016
OK, I owe you an apology about the troll comment, Also, I'm dumber than a box of rocks when it comes to most things electrical. I'm old and cranky, too, so sarcasm has crept into my normal good humor. My bad.
I'll just stick with the regular batteries.

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11. ### cjdelphi

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Oct 26, 2011
They are all standard 5mm / 20ma LEDs

To be honest you might get away with a single 18650, but you run the risk of them slowly burning out if the current is already maxed out...

Ideally the circuit needs modifying, but i'd probably use the lithium as it is (i've pushed 5mm up to 40ma and they lived, for a few days lol)