World's first real Free Energy Flashlight - no shaking - no batteries! No Solar

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If we assume the initial power consumption to be 3W the total amount of energy would be 5...6 Wh, which i would say would be a realistic value for 3 pieces of average quality AA-sized accus.

Three AA NiMH cells typically have the capacity of over 10Wh (3 x 1.3V x 2600 mAh). At some types of alkaline AA cells it could be over 15Wh.

The interesting thing now is: will it recharge ?  Normal accus don't recharge, at least not to their full initial capacity.

No accus recharge alone. It is the voltage, not the charge, that recovers. During the discharge, the chemical processes between the electrodes and the electrolyte saturate the electrodes, resulting in drop of potential (voltage). It is the dropping of the voltage, not the lack of energy that causes the LED to dim.

During the rest, the battery chemically and thermally recovers, and the voltage grows back (while the charge remains the same), and the flashlight will start again with the full (or almost full) voltage and hence luminosity. You may be able repeating it many times before you completely drain the battery. Although, the cycles will be shorter and shorter.

[skywatcher]

Three AA NiMH cells typically have the capacity of over 10Wh (3 x 1.3V x 2600 mAh). At some types of alkaline AA cells it could be over 15Wh.

Only theoretically. I have tested some 2500 and 2600 mAh cells and in practice they all had less than 2000 mAh or even less.

No accus recharge alone. It is the voltage, not the charge, that recovers. During the discharge, the chemical processes between the electrodes and the electrolyte saturate the electrodes, resulting in drop of potential (voltage). It is the dropping of the voltage, not the lack of energy that causes the LED to dim.

During the rest, the battery chemically and thermally recovers, and the voltage grows back (while the charge remains the same), and the flashlight will start again with the full (or almost full) voltage and hence luminosity. You may be able repeating it many times before you completely drain the battery. Although, the cycles will be shorter and shorter.

Voltage alone means nothing, if the charge remains the same the voltage will break down immediately if you apply any load.
For a normal accu i would expect a significant brightness only for some minutes if it had been discharged some time before.

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Voltage alone means nothing, if the charge remains the same the voltage will break down immediately if you apply any load.

Only if there is no sufficient charge, which is not the case when you power a LED. It is different at other appliances, not so sensitive about the voltage - they can drain the accu indeed faster and close to the limit of their capacity. Not so at a LED - as I explained, due to the chemical changes, the voltage will drop before the charge drops significantly. This potential (the voltage, not the charge) will then recover again, after some rest. The voltage at chemical batteries is not in direct correlation only with the charge as you assume. It depends on the chemical state and on the temperature too.

[skywatcher]

Only if there is no sufficient charge, which is not the case when you power a LED. It is different at other appliances, not so sensitive about the voltage - they can drain the accu indeed faster and close to the limit of their capacity. Not so at a LED - as I explained, due to the chemical changes, the voltage will drop before the charge drops significantly. This potential (the voltage, not the charge) will then recover again, after some rest. The voltage at chemical batteries is not in direct correlation only with the charge as you assume. It depends on the chemical state and on the temperature too.

Ok, so you run it down from 100% to 30%, LED goes off, you let it recover for some time, it remains at 30% charge but voltage increases so you can do another run which will run it down from 30% to 10%, etc... so the LED will work let's say 2 hrs for the first time, 30 min for the second time, etc...  until the voltage remains permanently below the minimum operation voltage for the LED.

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Ok, so you run it down from 100% to 30%, LED goes off, you let it recover for some time, it remains at 30% charge but voltage increases so you can do another run which will run it down from 30% to 10%, etc... so the LED will work let's say 2 hrs for the first time, 30 min for the second time, etc...  until the voltage remains permanently below the minimum operation voltage for the LED.

Since we did not measure the current, we cannot know how much of the charge you really drained. I would not exclude that you drained only 30% or 40% of the total charge, or even significantly less if high-capacity alkaline batteries were used. So it is hard to estimate how many cycles you will be able to do, but the times should indeed be shorter and shorter.

The effect of the batteries I am speaking about is well documented in scientific literature. It is called "relaxation effect". I just picked up the first paper from Google speaking about it - http://jes.ecsdl.org/content/141/4/982 - it refers to Li-ion batteries, so depending on the type of the battery in ELFE, it may differ a bit, but the principle is the same.