homemade LED nightlight

[jfones]

I need some help with the math here. I made a night light with three 10mm superbright white 3.5 volt 20 ma LEDs in series. I put a 100 ohm resister in the line, a toggle switch for off and on, and two 6 volt lantern batteries in series to make it 12 volts. The lantern batteries are Raovac and supposed to last 4x longer than usual...and they were the heaviest ones on the shelf. If I let the LEDs run continuous on these batteries, how long would these batteries last or the LEDs keep lit up?

Thanks in advance for any feedback.

 

[JohnR66]

As the batteries run down, the current through the circuit drops and slows the discharge of the batteries. I'd expect a few days of high brightness with perhaps weeks of operation at lower brightness. With fresh batteries, you might be pushing more like 30ma into the LEDs because the voltage is higher than 12 starting out.