Batteries make flashilight hot.

I have one of thoe little 2.5" LED flashlights, that have 9 small LEDs and use 3 AAA batteries. It's like the ones that Harbor Freight sometimes gives away free, same size, but nicer.

The guy I bought the first one from said it would run for days on one set of batteries, but I forgot that it was still on and i put it in my pocket and an hour or two later, I noticed it was on because it was warm.

It's still really bright, but if it's noticeably warm, how long can the baterries really last? Certainly not two days. ?

I once, recently, forgot the screen was lit on my 5.5" smart phone and only noticed because it felt warm in my pocket.

It's got a proximity sensor but I don't remember what it's supposed to do.

The heat is from the LEDs, not the batteries, and no, the batteries will no t last indefinitely. I would guess (SWAG) that each LED will run about 50 m a. So you are pulling 450 ma, or thereabout per hour. If your AAA batteries are middle-of-the-road 1,000 MAH, you have a base of 3000 MAH, so, about 6

-7 hours +/- or so. Under normal use, that is 'days'.

Peter Wieck Melrose Park, PA

Did you notice that almost all such flashlights are made from aluminum, not plastic, which would be cheaper? That's because aluminum makes a good heat sink to get the heat away from the LED's. LED's loose output and efficiency when hot, so it pays to keep them cool. The COB style of LED mounting is quite common in larger flashlights, which offer a heat conductive path from the LED to the aluminum heatsink case.

No, it won't run forever. AAA alkaline batteries are good for

1000ma-hr each. At something less than 4.5V for 3 batteries, that's 4.5 watt-hrs. Commodity LED's generate about 50 lumens/watt, so if your Harbor Freight flashlight managed to belch 15 lumens, it will consume 0.3 watts. Runtime is therefore an optimistic: 4.5 watt-hrs / 0.3 watts = 15 hrs. It's probably less because I didn't bother throwing in losses in the current source, decrease in battery voltage as it runs down, and heating effects. But, it should give you a ballpark guess as to how long it will run. My guess(tm) is it will run about 8 hrs with a new set of batteries.

If your smartphone as a battery usage graph, like all Android phones, you'll find that it also has a list of which applications are sucking the most power. The backlighting for the OLED screen is invariably the highest. If you don't talk on your phone, and don't run any apps, the battery life of your phone will be totally dependent on the brightness setting and efficiency of the OLED display, which is about the same 50 lumens/watt as your white LED. (Yes, I know there are press releases for >100 lumens/watt, but those are under laboratory conditions).

Never heard of it. Maker and model number?

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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No brain function before my morning tea. That's wrong. There is no backlighting in an OLED display. That should be the LED "screen" power consumption, not backlighting power consumption.

--
Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
Santa Cruz CA 95060 http://802.11junk.com 
Skype: JeffLiebermann     AE6KS    831-336-2558

not last indefinitely. I would guess (SWAG) that each LED will run about

50 ma.

So you are pulling 450 ma, or thereabout per hour. If your AAA batteries are

middle-of-the-road 1,000 MAH, you have a base of 3000 MAH, so, about 6-7 hours

+/- or so. Under normal use, that is 'days'.

Might want to check that math.

to do

Isn't it just to switch off the screen and touch sensitivity when adjacent to your head so your ear doesn't swipe at random?

Mike.

The internal resistance of the batteries, and the internal resistance of the LEDs used, is very much a part of the design of those flashlights. Both the LEDs and the batteries do get warm in normal operation, and (as you suspect) it's likely that a fresh set of batteries will last well under a day of ON operation; Duracell AAA claims 3.5 hours at 1/4W output, so a three-cell light could deliver over half a watt for a useful amount of time; hours, not days.

Yeah, that's it. I guess I thought the screen would go off in my front pants pocket too, light weight pants, but it didn't.

I hadn't relied on that, just forgot, and I won't rely on it in the future. I guess my pants pocket won't swipe anything ilke my ear could.

OK. 50 x 9 = 450. 9 LEDs at 50 ma each.

3 x 1000 = 3000. 3 cells at 1000mah each. 3000 / 450 = 6.67. At 450 ma, that will go 6.67 hours.

The reality is probably different as all the shots/assumptions are center-mass. If the LEDs are 100ma outliers, if the cells are 1.4mah outliers, things do change, of course.

Peter Wieck Melrose Park, PA

3 cells at 1000 mah each do NOT give you 3000 mah. They give you 1000 mah but at a higher voltage (4.5 vs 1.5). If you use mwh (watts) instead of mah (amps), then you can add them.

(If the cells are wired in parallel instead of series, then you are correct, but that isn't very common.)

The three similar lights I have, they are in parallel. Which is why I probably wrongly assumed these were as well.

Funny thing, these types of lights are often give-away items at Solar Energy conferences - I have been to my fair share, and how they came into my hands. Otherwise I would not touch this cr*p on a bet.

Peter Wieck Melrose Park, PA

I just checked 3 of mine... Two give-aways and one UV of similar design. All three have the AAA batteries connected in series. That got me wondering how the LEDs are wired, but I was too lazy to take them apart. All have 9 LEDs so I will guess 3 sets of 3. Is 4.5v enough to power a 3 LED string? Just went to look it up and the answer to my own question is "no". UV and White LEDs typically have a Vf of 3.3 volts. So, all 9 are in parallel with their own current limiter? If so, why 9? Is there a cheap IC that supports 9 LEDs? If anyone knows, please educate us.

Pat

If they're in parallel, then they almost certainly aren't driving the LEDs directly... a blue LED (with or without a white phosphor) requires more than 3 volts in order to conduct.

So, if they're in parallel and are feeding only 1.5 volts into the LED head, there is probably a voltage-boost circuit (e.g. a "Joule Thief") which is stepping up the voltage before it reaches the LEDs.

And, the step-up comes at a cost. If the Joule Thief has to triple the voltage before it feeds the LEDs, and the LEDs are drawing a total of 100 mA at that voltage, then the Joule Thief will be pulling 300 mA or more out of the batteries.

There ain't no free lunch, alas.

A simple series arrangement is less expensive, but it'll end up wasting some power in a current-limiting resistor (unless the flashlight makers go _really_ cheap) and the flashlight will slowly dim as the battery runs down. A step-up circuit can feed a fairly constant current through the LEDs until the batteries are almost completely exhausted, and it allows for a "variable brightness" feature (pulse-width-modulate the LEDs, or step up to a lower voltage which results in a lower current).

I buy these at Walmart for $1 each. I use them a lot. I have left them on overnight and they still work, but that greatly shortens the battery life. If they are not left on for long periods of time, they last weeks and that is being used almost daily. For a buck each, I usually buy 5 at a time and that keep me with working flashlites for a few months. I keep one in the car, one in the truck, one in the house and a few other places.

Unless I can buy batteries real cheap at some dollar store, (like 8 batteries for a buck), it's not worth replacing the batteries. I just toss them and buy more flashlights which cost $1 WITH batteries. I have never had any of them get hot or cause any problems at all. I did have a few with switch problems, but very few. Since I have plenty ones with dead batteries, I just swap the batteries from one with a bad switch to one that still works.

I have saved the LEDs from a lot of them that I toss. Not sure why, but some day they might have a use... I guess these are just meant to be used and disposed..... But I have replaced the batteries a few times too.

Either way, they are 1000X better than the old flashlights I grew up with, that had very short battery life and almost always developed switch problems after a month or less.

I cut open one of the Harbor Freight nine LED flashlights that was damaged. All nine LEDs are in parallel, and all three cells are in series. They use the internal resistance of the cells to limit he current.

The forward voltage of a white LED is typically 3.7 to 4.2 volts, so parallel cells would require a boost converter which would double the cost of the electronics in a flashlight.

BTW, the Harbor Freight flashlight will slip into a cheap microphone boom for a handy bench light.

Mic-Arm-Stand-Microphone-Suspension-Boom-Scissor-Holder-For-Studio-Broadcast

--
Never piss off an Engineer! 

They don't get mad. 

They don't get even. 

They go for over unity! ;-)

It's certainly possible, but it's a bad design tradeoff, and a safety hazard, to put 3XAAA in parallel. I've never seen one that way.

Even if they were in parallel, you'd need about 3X the led current from the batteries, so your math is still questionable. watts from battery x efficiency = watts to led.

The cheapest lights use 3XAAA in series connected directly to the parallel combination of LED's. They ship with "heavy duty" batteries and the series resistance of the batteries limits the current. Alkaline batteries will overheat the LED's. Now you know why the LED's start to flicker after a while.

If you try to run one off a 18650 you'll burn out the leds, even though the 4.2V of the cell is less than the nominal

4.5V of three AAA cells. I had to put 1.5 ohms in series to make it work.

If are really bored, you can cut two FREE Harbor Freight lights in two and solder the parts back together (well, they're aluminum so use something like Welco 52) to make one longer light that accepts a 18650. Don't forget the series resistor. This made a lot more sense back before you could buy a single AA zoom flashlight for $2.

If you take out 8 of the 9 LED's and make the resistor larger, it makes a great emergency light that will run forever during a power outage. Just my luck...we haven't had a power outage since I built it. ;-)

Are you talking about the really cheap LED flashlights?

I've taken some apart, and I don't see anything but the LEDs. So they have to be in parallel, and rely on the bettery itself to supply some level of series resistance.

The cheap ones sometimes aren't even soldered together, just sort of pressed together, which may account for why some of the LEDs don't light, bad contact. Of course, these are just white LEDs in some package, not "chip" LEDs like in better LED flashlights and LED light bulbs.

Michael

Sometimes one or two of the bulbs go out, so I think that also means parallel.

I don't think I had had that either, which is why I posted. Of course maybe I'd never left it on so long before.

I don't have as many as you, I've had maybe 10 total and 2 or 3 have had the switch fail after very few uses. I expect the others to fail any minute....

That's caused me to think, Don't wear out the switch, leave it on, And that's what led to my leaving it on when I put it in my pocket!

Which might be the only reason it was warm.