Heh Guys,
Please refer to the attached schematic for this question. Will the circuit work as drawn or do I need a series resistor for each LED? Please note that the LEDs are of the high intensity type (4 volts and 20 mAmps each) and all are on at the same time. Anyway using ohm's law I calculated a resistance value of 12.5 ohms (rounded up to 20). In addition the LEDs should shine for 2 hours (battery at 800 mAmp hours). Please reply and merry Christmas to all.

 

No, this will not work because not all LEDs have the same forward voltage. That being the case, the LED(s) with the lowest forward voltage will funnel a great deal of current compared to the others.

You would be best making pairs in series (8 volt drop) and 10 sets. Each set will draw 20 mA doe a 200 mA draw. You will need a 47 ohm resistor for each pair (10 resistors).

At 800 mAhour, this should last a minimum of 4 hours. It will last longer as voltage (and brightness) drops as the battery drains.

 

No, this will not work because not all LEDs have the same forward voltage. That being the case, the LED(s) with the lowest forward voltage will funnel a great deal of current compared to the others.

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Correct - but go into any bargain store and you'll find various flashlights/bicycle lights/headband lights etc which use 3x AA or AAA cells and no current limiting resistor. Number of LED can be anything from 7 to 24, just plumped in parallel.

Battery efficiency isn't good and it doesn't do the LEDs a lot of good, but it does the job for a lot of people who just want a cheap flashlight.