Time it will take a LED to discharge a battery

[joseche]

So, I have a family event soon and I need to build a lot of small paper lamps that will light a tree.

My idea was to use a AAA battery, a 75 ohms resistor and a 8mm led. So according to the wiki specification of AAA batteries, in the worst case I could have 120mAh.
120mAh of charge discharging at 20mA = 6 hours of life ?

That will be the worst case, given that the quality varies from 120mAh to 1200mAh, using 660mAh I will get 33 hours of light, right ?

Please let me know if you see an error or if you have another idea.

Thanks

 

[ronv]

Depending on the color of the led you will need more than 1 AAA battery to light it. Your math is okay but the voltage will only be about 1.25 volts each at 50%. A 500 mah 9 volt might be a better option if 25 hours is enough.

 

[4pyros]

One problem may be that at some point before the battery is used up the voltage will drop below the forward voltage of the LED and it will not light. You need to consider the loss of voltage as the battery discharges.

 

[alec_t]

Can't you power the LEDs from a wall-wart?

 

[cowboybob]

Sure. Just don't exceed the max allowable forward voltage level for the LED's used.

 

[joseche]

I can't use an wallwart because the lamps will be hanging on a tree

Thanks, I will do some testing with different leds and batteries to see what I get how long it lights

 

[audioguru]

A white or blue LED needs at least 3.5V plus a few volts for the current-limiting resistor.
If you use three AAA cells at 4.5V then their voltage quickly drops to 3.6V then the LED will barely light.
If you use four AAA cells at 6V then their voltage quickly drops to 4.8V and you will see the LED dim.

We don't know the recommended current for your LED. 120mA will instantly burn out most LEDs.

 

[RODALCO]

Look at building a simple joule thief, and go for an AA battery.

a lot more milli Amphours and not much bigger, costs usually the same.

It will run any colour led from a 1.5 Volt battery.

A small oscillator circuit needs to be built out of a toroid, BC547 and resistor. Google or look on this form for the simple schema.

Will run from a new quality battery at least 24 hours

 

[audioguru]

A Joule Thief produces a low output current so the LED will be as dim as a solar garden light.