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Recharging non-rechargeable batteries

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andyf:
Chris-b’s voltage bargraph thread prompts me to describe a circuit I found in a magazine article 20 years back, which some may find interesting. The thrust of the article was that “non-rechargeable” zinc-carbon or alkaline cells can be recharged by “periodic current reversal”. The explanation given as to how periodic current reversal works whereas DC doesn’t (at least, not very well) is too long to repeat here.
 
The circuit applies AC current from the secondary of a mains transformer to the cell(s) to provide the periodic current reversal at 50 or 60Hz, depending on your local mains frequency. For each cell, a diode and two current limiting resistors are arranged so that (putting it simply) much less current is drawn during the discharge periods than is applied during the charge periods.

It is best not to let the cells get completely flat, but to recharge when the radio begins to sound distorted or the torch bulb begins to dim. I recharge alkaline cells (Duracell, for example) about 10 times before throwing them away. Their capacity is greater than NiCad or NiMh cells, and they give 1.5V rather than 1.2V. Thus, when a gadget calls for 6V supplied by four cells in series, it gets 6V rather than the 4.8V provided by four rechargeables.

My version was cheap to build, using a scrap mains to 4.5V transformer, a handful of resistors and diodes, battery holders and a box. I used 4 sets of resistors, diodes and battery holders to recharge up to four AA cells, and another 4 sets for AAAs. So far, none of the cells I have recharged has leaked, and if leaks do occur, this would be in the charger rather than in the appliance where the cells are to be used.





Being a cheapskate at heart, I feel very smug every time I use the thing  :ddb:

Andy

Darren:
I've heard of this trick before, but never from anyone that has actually done it and states that it works  :clap:

andyf:
It works fine, Darren. The alkaline batteries in the camera used for the photo have been recharged four times already - I put a scratch on the paint each time I do it, to keep track. Duracells seem to last rather longer between charges than the rechargeables which came with the camera. And alkalines which are running out of puff are effectively free :thumbup:

According to the 20-year old magazine article, the Japanese have been doing it for years;the batteries they export are marked "do not recharge", but the ones for their home market aren't.

Andy   

Bernd:
Won't have believed if you hadn't shown pics. I've got a few batteries laying around. Might just wip one of these little circuits up and give it a try.

How long do you leave them on the charger?

Bernd

andyf:

--- Quote ---Bernd: How long do you leave them on the charger?
--- End quote ---
Much longer than you would a regular rechargeable, Bernt. Up to 24 hours for AAs (which are the ones I'm most concerned with).  A typical AA cell has a capacity of just under 3Ah, which is about twice that of a NiMH rechargeable, and because you are using AC it's only being charged for half the time. Actually, rather less than half the time, because the sine waveform needs to rise to 2.2V:  0.7V before the diode will conduct, and a further 1.5V to start pushing electrons into the cell. For the other half of the time the cell is being discharged at about 20% of the charge rate.
The article uses a charge current of about 50mA.  :smart: Using 4.5V AC from the transformer, I = (4.5 - 0.7 - 1.5) / R2, where the 0.7 represents the voltage drop across the diode and the 1.5 is the voltage of the cell. So, where R2 is 47 ohms, I = 50mA (almost).

If you have a scrap transformer of different voltage V like 12V, find R2 using the same formula rearranged as:
R2 = (V - 0.7 - 1.5) / 0.05, where the 0.05 represents the 50mA charge current.
For R1, which comes into play in series with R1 during discharge,  use the formula R1 = [(V + 1.5) / 0.01] - R2, where the 0.01 represents the discharge current which is set at around 10mA, being 20% of the charge current. During discharge the diode doesn't conduct, so doesn't come into this calculation.
I suppose the resistor ratings need to be increased if a higher voltage is used.

Precautions:
Remember that mains voltage is on the transformer primary  :zap:
Despite my statement as to "no leaks so far", I don't use the thing on the polished dining table .....

Andy

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