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I am doing a school project. I am charging 4 aa batteries(Ni-Mh 800mAh) with a solar panel.I want to lit 3mm white LED with the battery. how many LED I can connect to this batteries and did I need to use a resistance?

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You can connect a lot of LEDs to the battery. The question is how long you want the LEDs to stay lit for.

Assuming you have the 4 batteries wired in series, they will provide 0.8 Amps (800mA) for 1 hour. The more LEDs you have, the quicker the battery will drain. Let's say you have one LED that draws 20mA, this LED will run for 40 hours (in theory). I figured this by dividing 800mAh / 20mA = 40 hours.

If you add more LEDs, the amount of time the LEDs will stay lit will decrease.

The LEDs should be wired in parallel, and you will need a resistor (preferably for each LED). You can figure out how much of a resistor you need here:
http://led.linear1.org/1led.wiz

If you don't feel like doing the math, 150 to 330 ohms is a fairly safe number to use for a 3mm LED. However, using too large of a resistor may decrease the brightness of your LED and efficiency of your circuit.