What is the current required for 4 led with a forward voltage4v @ 20mah each ?

Answer 1

I have to concur with 'One Step Down From God' on this.

Three 1.5V 'AA' batteries should be enough. A typical 'AA' battery is about 800mah so the four LED's should remain lit for 10hours.

1600mah batteries will give you twenty hours.

Of course, 3x1.5V=4.5V and you'll need to pop in a current limiting resistor to avoid damaging the LED's.

Using Ohm's Law "V = I x R" and rearranging the values thus:

R = V / I

R = 4.5 / 0.08 = 56.25 ohms

That's very precise and since resistors are offered with 'Preferred Values', I reckon a 56 ohm resistor will be fine.

To give yourself a little leeway, try a 68 ohm resistor.

There will be so little difference in the overall brightness, you won't notice it but your LED's will not be working at their limit.

Answer 2

further to Keith J's answer you should use 4 resistors, one for each LED. You need to drop about 0.5 v at 20ma, so you need 25 ohms. You would get extended battery life with 4 cells giving 6 volts, and would then need 100 ohms. The multiple resistors are important, as no two LEDs have exactly the same voltage, and with a single resistor the lowest voltage device will take most of the current.

Answer 3

the obvious answer is 80ma. Assuming no voltage variation, the current would remain the same.
Not sure why you've got mah, it's not a storage device.