Random question about LEDs and photodiodes

Thread Starter


Joined Apr 4, 2009
This seems like a dumb question, but I'm no expert on optoelectronics, so I'm sure one of you more experienced in this area can help.

For New Year's I took a bunch of LEDs, current-limiting resistors, and small 3V batteries and made some festive lights for a party.

The batteries are 3V Sanyo MnO2-Li, and look about the size of 2 or three button cells stuck together. Anyway, after the party, I recycled the materials into other projects, but a couple of the individual light sets (1 LED, 1 resistor, one battery) remained on my counter. They've been there 3 1/2 months now, and while they emit no discernable light in daytime, at night they have a faint glow still.

The batteries don't have any indication of milliamp-hours, but I can't imagine that they pack more than several hundred. Initially the LEDs were running at about 20mA, so that would go pretty quickly.

I've concluded, perhaps erroneously, that sitting in the sunlight they must be operating as photodiodes during the day, inducing a current that charges the battery just enough to remain lit at night.

Is this possibly what is happening? Can standard LEDs act as photodiodes, and induce a charge when exposed to light?

Thanks for any help.
Last edited:


Joined Jul 17, 2007
As the battery voltage drops, the current through the LED becomes less and less, creating less of a drain on the battery. LEDs can actually continue to emit some light at very low current levels, as long as there is enough voltage across the LED to sustain some current.

While the LEDs may actually be generating some current when in sunlight, it would be pretty miniscule; not really enough to charge the battery.