Huge temporary voltage drop within 2 minutes on LED flashlight

[John0922]

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

[Rerouter]

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.

[Tandy]

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.

[mmagin]

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.)

[John0922]

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.

[Brumby]

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 ...

[amyk]

That's why it's called a flashlight...

[T3sl4co1l]

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

[eKretz]

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.