In late 2000 I bought a digital camera which was demanding of the AA cells, and it used quite a few. I decided to convert to NiMH rechargeable cells. That saved me a good bit of money, but at that time I hadn’t begun experimenting with Joule Thief circuits. This means that I really never had a device that used a lot of non-rechargeable AA cells. I have a few flashlights that use AA cells, but not in large quantities like my digital camera. A co-worker gave me a shopping bag full of about 5 pounds (2.3 kG) of AA cells that were ‘dead’, but of that, probably more than half were physically dead, with either no output or with electrolyte leaking out of them. Over a few months last year I put most of them on Joule Thiefs using germanium transistors, and I ran them down to almost zero, getting a few days’ light from each. They’re almost all gone now.
This means that I really never thought of a Joule Thief as a scavenger of used AA cells. My intent was to use the JT as a light source that runs off a single new, fresh AA cell and if it so happens to run down the AA cell more than a normal flashlight, that was just a side benefit.
My Joule Thief designs reflect this: they operate best when the supply voltage is 1.5V, or sometimes 1.25V for rechargeable cells. I read somewhere that when the voltage of AA cells start to drop, most of their capacity is used up, and only about 10 percent is left. The Joule Thief can use this remaining 10%, but how much can it use before the voltage drops to less than needed to keep the JT running? It has to be less than 10%. In other words, the JT gets the cell with 10% and then the cell voltage drops until the JT quits and there is still 4%, for example, left, and the JT quits. So the JT has used about 6%.
Let’s assume that we want to use the JT for a light. If I use a single fresh AA cell, it will last about 16 times as long as if I use a depleted cell, and I will have to keep 16 depleted cells with the JT and change them 16 times more often.
I need to design a circuit that uses the same LED to make an A-B comparison of the brightness a JT and a LED using just a current limiting resistor. This will eliminate having to measure the LED lighting current and calculate the power. No lux meter is needed if the eye can’t detect any difference in brightness. A CdS photocell or solar cell can be used in place of the eye to detect when the A and B LED brightnesses are the same.