Have you ever used battery operated hand mixer? Or any other battery operated high current drain device (typically one that has a motor inside)? If so, you know that typical batteries don’t last long in those devices. Alkaline might provide better service hour but still not very much.
So, what do you do with those batteries (or cells)? They can’t run those devices for any longer so throw them out perhaps? If you do that you are actually wasting quite a lot of energy. How so? Let’s start the explanation with an image from Wikipedia.
What this picture describes is an AA Alkaline cell that is being drained at 100mA constant current. In other words, you can imagine an Alkaline cell to run a device that consumes 100mA current constantly. If the device you have is designed in a way that cuts off at 1.3V (for sake of explaining) in other words it will not work if the cell voltage drops below 1.3V you are only using the Green Portion of the energy that was in the cell. If, however your device can use up to 1.1V it can use additional blue portion of the graph and as you can see the area has increased a lot. So, further you go down on the voltage more power you can extract from the cell. Although you can not really go below 0.8V because at that voltage or below this type of cells usually cannot provide enough current or majority of devices won’t run below this cell voltage.
Back to my original example, if you get some used cells out of your battery-operated coffee mixer and measure the voltage you would probably see something like 1.2V-1.3V which means they still hold a lot of energy in them. Energy in the blue portion of the graph is still unextracted. So, throwing away such cells is wasteful and not good for both environment and your wallet. You can easily use these cells on wall clocks, if you have good quality ones that can use Alkaline/Zinc Carbon cells down to 0.8V-1V. This way you can extract a lot more energy out of them.
A cell gotten out of such a mixer can power a wall clock for months or even year in case of Alkaline. Bottom line do make sure you have got the maximum capacity extracted from disposable cells before throwing them out. To ensure this use devices that can go down to such voltages, usually Calculators, Infrared Remote Controllers and Wall clocks can go down to that level. Do keep in mind all of them are not made equally so not all of them will be that good.
Disposable AA/AAA cells start their life at 1.6V and more you can go down more energy you can get out of them.