I was reading this article and saw that it said that when the voltage of a 1.5V cell drops to 0.9V, it still has 10 percent of its capacity left. I have read in the past that the battery is considered used up when its voltage drops to 1 volt. I don’t know what the percent difference is between 0.9V and 1 volt, but I presume that the capacity is somewhat more than ten percent. I would not expect it to be 20 percent, though.
One point of this is that the amount of capacity left in the cell when it’s considered spent or dead is not very much. But the ten or whatever percent is there and it’s free, and can be used by the Joule Thief to make a LED light up,
One point I would like to remind readers about is that the typical conventional Joule Thief is only putting out half of the power that it takes in (and that’s when the input voltage is 1.5V; when it’s only 0.9V, it’s likely even less than half). Therefore the ten percent left in the cell actually looks like only five percent when it’s lighting the LED. One could get better efficiency if 5 of the spent cells are put in series and the resulting 4.5V is used to run the LED with a current limiting resistor. The efficiency with the current limiting resistor could be 70 percent.
Ok. My take on this is that as the battery dies the internal resistance goes up. This means for many uses e.g. in a camera, the battery battery may get get discarded much earlier than an open circuit voltage of 1v since the voltage drops under heavy load. A JT, especially with an input capacitor, drawing just 20mA will last a lot longer. Adding a CdS LDR ekes it out further as the light sensitivity increases when the voltage drops.
As for efficiency, I have yet to see the definitive work on this. If I had the test gear I would love to answer this question, I think there is room to put theoretical theories to the test varying / substituting individual aspects e.g. types of toroid, numbers of turns, turn ratios, different transistors, optimum frequencies. The is lots of room to experiment. One could optimise for light power and light output efficiency too. Paul
I’ve built so many that I have a sixth sense about JTs. 😉 I can pretty much guesstimate how it will work, and more often than not I’m like the side view mirror: “Closer than they appear.”