Author Topic: Huge temporary voltage drop within 2 minutes on LED flashlight  (Read 1750 times)

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Offline John0922

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Huge temporary voltage drop within 2 minutes on LED flashlight
« on: January 17, 2017, 07:42:37 pm »
Hi- I just bought a Coast HP-7 flashlight. It takes 4 AAA batteries. On recommendation I put in Energizer Ultimate Lithiums, but I also tried Alkalines and Eneloop Pros and got a similar result. The flashlight draws 2.8 amps @ 6 volts. I noticed that within minutes the light output drops to less than half the original brightness. If I let the batteries rest, it bounces back to the same original brightness and then starts to dim all over again.

I read that 4 AAA's are not enough to power this flashlight. I became curious and took a look at the Ultimate Lithium data sheet:

http://data.energizer.com/PDFs/l92.pdf

It says that the maximum discharge per battery is 1.5 amps continuous. I'm assuming (maybe incorrectly) that this would be 6 amps using 4 batteries. But yet my result seems to suggest the number is actually much less.  What am I missing?

Thanks in advance.   -John
 

Offline Rerouter

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Re: Huge temporary voltage drop within 2 minutes on LED flashlight
« Reply #1 on: January 17, 2017, 07:51:19 pm »
As you said 6V, those cells are in series, which means the current rating is the same as a single battery,

In other words your trying to pull 2.8A off a battery rated at 1.5.

If they where in parrellel then the current rating could be added up, but you wouldnt have the voltage.
 
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Offline Tandy

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Re: Huge temporary voltage drop within 2 minutes on LED flashlight
« Reply #2 on: January 17, 2017, 08:07:30 pm »
In addition the cells have an internal resistance that prevents the maximum current being drawn from them. Trying to drive a 4W+ LED from AAA cells IMHO is just plain bad design.
For more info on Tandy try these links Tandy History EEVBlog Thread & Official Tandy Website
 
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Offline mmagin

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Re: Huge temporary voltage drop within 2 minutes on LED flashlight
« Reply #3 on: January 17, 2017, 08:30:37 pm »
Possibly some kind of thermal limiting in the LED regulator?

I have a fairly nice Fenix headlamp (the sort that uses a seperate 4x AA battery pack) and it has 4 power levels, but it will only stay on the highest level for about 5 minutes (by design.)
 
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Offline John0922

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Re: Huge temporary voltage drop within 2 minutes on LED flashlight
« Reply #4 on: January 17, 2017, 08:49:41 pm »
Thanks for the replies. I see I made a noob calculating error.

The flashlight has 3 power levels, but Coast doesn't tell you that you can't use the highest level for more than a minute. Oh well, caveat emptor.
 

Offline Brumby

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Re: Huge temporary voltage drop within 2 minutes on LED flashlight
« Reply #5 on: January 18, 2017, 12:28:52 am »
You're not a noob on this subject any more!

This is how a lot of knowledge is acquired.  Observe something that doesn't work as you expected and then work out why.  Confirm this knowledge in further exercises.


Now ... onto the next mystery ...
 
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Offline amyk

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Re: Huge temporary voltage drop within 2 minutes on LED flashlight
« Reply #6 on: January 18, 2017, 03:24:17 am »
That's why it's called a flashlight... ::)
 
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Online T3sl4co1l

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Re: Huge temporary voltage drop within 2 minutes on LED flashlight
« Reply #7 on: January 18, 2017, 07:01:57 am »
I have a 1W (hand made) flash light that works fine from a single cell... but that's a AA.  :-//  (It only fades while the cell is properly dying.)

Tim
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Bringing a project to life?  Send me a message!
 
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Offline eKretz

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Re: Huge temporary voltage drop within 2 minutes on LED flashlight
« Reply #8 on: January 19, 2017, 05:46:42 am »
Coast flashlights are unregulated. This means they don't stay at their initial brightness level for very long. A lot of the more popular lights are regulated so their light output is much more consistent.
 
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