24 LED Circuit

[Magen]

Hello everybody,
I recently purchased a portable led light which has 24 Leds in it. After opening it up, I found that all the led's are wired in parallel and no current limiting resistors are used. They are driven by 4 x 1.5v AA batterys and are reasonably bright when lit. The voltage across the cells drops from 6.4V (no load) to 3.0V (full load). I do not have any specs for the Leds.

Question:1) Are the leds wired like this to give max brightness at the expense of led and battery life? The battery's are supposed to last 30 hours (according to the specs on the box) but I cannot see this being true.
2) Does the voltage drop to 3v because this is Vf of the Led's? If so, across what is the other 3.4 volts lost?
3) I am considering using 3 maxim chips (8596x) to drive the leds with 25ma and prolong battery life. Has anybody else used these chips with good results?

Sorry about the bad pic quality

 

[Hero999]

Have you measured the current?

Yes the drop in battery voltage is due to the LED forward voltage.

If each LED takse 50mA (some 5mm LEDs can do this) then the total current would be 1.2A which would drain the batteries in a couple of hours.

I don't see how the LEDs can not overheat, pehaps it relies on the internal resistance of alkaline batteries and it will blow if you used NiCads.

 

[audioguru]

Energizer rates the internal resistance of their AA alkaline cells at 150milliohms to 300milliohms when new. So four in series will have an average total resistance of about 900milliohms.
The LEDs are probably 3.5V white ones so the 900milliohms has a voltage of 6V - 3.5V= 2.5V and a current of 2.5V/0.9 ohms= 2.8A when the battery cells are new. That is 2.8A/24= 116mA for each LED which is crazy.

There must be some resistance in series in the circuit.

 

[Mikebits]

Maybe the LED's have internal resistors?

 

[Hero999]

The LEDs probably have a few tens of Ω internal resistance which could help.

 

[Magen]

The led's don't look like any of the standard types I've seen. I will measure the current and resistance and post some close ups of the led's.
Txs for the replies

 

[Willbe]

Do the LEDs get too hot to touch [>~53° C]?
Do they have the surface area of a 1/2 w resistor?

 

[matk95]

You can buy leds with build in resistors so this may be why there is non externaly

These miniature (5mm) LED's incorporate a in-built series resistor enabling the LED to be directly connected to a 5V or 12V (depending on purchase)