Recharging non-rechargeable batteries

[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 

Andy

[Darren]

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

[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

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]

Bernd: How long do you leave them on the charger?

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.  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 
Despite my statement as to "no leaks so far", I don't use the thing on the polished dining table .....

Andy

[Bernd]

Thanks Andy. Looks like I'll be digging through my "junk" electronics box for a suitable transformer.

Bernd

[dsquire]

Bernd

I'm not going to    about this but am just curious if you ever got around to trying this battery charger. If nothing else it will restart the thread and get looked at by some new members. I am thinking of trying it as I have a few batteries that could use a bit of a jolt to bring them back to life.  

Cheers  

Don
565

[AdeV]

Glad you resurrected this one, I'd forgotten all about it, despite having seen similar circuits (and, indeed, commercial devices) before now...

One question (to Andy) - what happens if you accidentally leave your AA's on for, say, a week? Do they eventually explode, or do they just sit there & nothing much happens...?

[andyf]

One question (to Andy) - what happens if you accidentally leave your AA's on for, say, a week? Do they eventually explode, or do they just sit there & nothing much happens...?

Haven't a clue, Ade. I doubt they would blow up - I think the cells are vented * so they would just leak, but I don't really know.

I set the charger up in the kitchen after breakfast, using the socket normally occupied by my coffee machine. The charger has to be unplugged in the evening, because I can't stay awake until bed-time without a coffee after dinner. I suppose I could lash up a timer, but can't be bothered.

 * 50+ years ago, the zinc chloride carbon ones weren't vented. As a kid, I chucked a dead one on the living-room fire. Boom! Live coals all over the carpet, and no pocket money that week   .

Andy

[John Swift]

Hi Andy  ,

I've only recharged "non-rechagealbe" batterys using a  DC supply

this works well IF they are only partly used 

I think the charger you describe was in Practical Wireless or E.T.I when I first read it
but never got round to trying it

if I  remember correctly the asymmetric ac  current is supposed to improve the re-plating
of the cathode during the recharging
 
alkaline batterys and the likes of ever ready zinc carbon batterys worked  OK

but cheap makes of zinc carbon battery
 with the thin zinc case would'nt be , they leak the first time you use them


 John 

[andyf]

Hi John,

I have looked in the old file where I kept "useful" electronics articles, and it was in Everyday Electronics (now defunct?) for Sept '91. Your memory is good; the gist of the article was that the cathode needs to be replated, and a "normal" NiCad or NiMH charger supplying pure DC produces a spongy plating, whereas "periodic current reversal" does a better job.

I have only ever recharged alkaline, rather than zinc-carbon, cells, but what you say about the cheapos makes sense.

Incidentally, while I have the article in front of me, it says that cells should be revived before they are completely discharged (some, esp. zinc carbon, dry out as they discharge) and should not be left lying around in a partially discharged state for too long before being recharged. 

It also recommends limiting the recharge time - "Don't try to put more energy back into a cell than it originally contained" - but gives no guidance on timings. Timings might be hard to calculate, for the reasons set out in my post of 11 Jan 2010. Gosh, has a full year gone by already?

Andy

[AdeV]

Everyday Electronics (now defunct?) for Sept '91.

EE is still going, albeit it's merged with Practical Electronics (which was always the more complicated one), to become EPE. I'm not sure, but that may even have happened by '91? Certainly it had by '99, which was the last time I had a subscription.

IIRC, it mostly concentrates on computer interfacery these days; the old analogue circuits seem to have mostly bitten the dust. I recently built a "fuel gauge" for water tanks - the original design had no digital circuitry on it at all (unless you could an LED as digital). It was cleverly wired to show L(ow), I (in-between) and F(ull) on a 7-segment LED. I added some simple logic (recovered from a PET motherboard) to allow it to drive 3 LEDs. (If you're interested, the original project was called Caravan Dipstick, and was in the Sept 1984 issue of EE).

[John Swift]

 Hi Andy and AdeV ,

Everyday Electronics was a spin off fron Practical Electronics
now part of Everday Practical Elecronics  magazine ( P.E , E.E and  Electronics Today International  magazines combined)

at first I thought it may of been from Practical Wireless Take 20 series   (upto  20 components and 20 shillings limit ( £1-00))

found this link to some   http://vintageradio.me.uk/radconnav/radcon.htm


you will have a job to build a "joule thief " for a pound these days     

 another way to extract the maximum use out of a battery

( could be adapted to step up the voltage from one cell to charge 2 or 3 cell battery  in a phone , may be ?
since AAA cells work out more expensive than C cells for example )

http://www.emanator.demon.co.uk/bigclive/joule.htm

 John

[tylernt]

Duracells seem to last rather longer between charges than the rechargeables which came with the camera.

What kind of rechargables did you try? Cheap NiMHs do suck, but there are better alternatives on the market.

A typical AA cell has a capacity of just under 3Ah, which is about twice that of a NiMH rechargeable,

Actually, quality NiMH cells like Sanyo Eneloops have 2Ah and Eneloop XX have 2.5Ah.

One advantage for NiMH is for a camera flash. NiMH can supply far more current than alkalines, which translates into shorter flash (re)charge times. That's also why flashaholics generally reject alkalines in favor of Eneloops for their high-power flashlights.

[andyf]

What kind of rechargables did you try? Cheap NiMHs do suck, but there are better alternatives on the market.

Actually, quality NiMH cells like Sanyo Eneloops have 2Ah and Eneloop XX have 2.5Ah.

One advantage for NiMH is for a camera flash. NiMH can supply far more current than alkalines, which translates into shorter flash (re)charge times. That's also why flashaholics generally reject alkalines in favor of Eneloops for their high-power flashlights.

I was only comparing them with those which came with the Sony camera 12 years ago, and I can't remember the type. The camera is mainly where I use the recharged alkalines, and I am not interested in photography as such, so slow recharging of the flash is immaterial to me.

Welcome to Madmodders, by the way.

Andy

fast rechage

[dsquire]

Hi

A couple of years have gone by since this thread was started. How many people have built a charger like this and what were your results? Enquiring minds are curious. 

Cheers 

Don

2300

[mzt]

A couple of years have gone by since this thread was started

Glad You resurrected it again.
Marcello

[raynerd]

Has anyone got an build pictures for one of these. If anyone does build one, please take some step pictures if you don`t mind!?

Chris

[andyf]

I didn't take any build pics, Chris. But it's just simple soldering; the hardest bit is finding a suitable box and fastening the transformer and a 12 x 20 hole stripboard inside it, and the battery holders to its outside.

Here's a pic showing how things are laid out, and the track breaks.

Diagram deleted.  It contained so many errors that John Swift kindly took it in hand and his corrected version version appears a few posts further down. Read the following paragraphs in conjustion with his diagram. 

This only shows four battery holders; as you will see from the first post, mine has four for AAs and another four for AAAs. Just use a 12 x 40 hole bit of stripboard and put another four sets of resistors and diodes on it, with further track breaks.

Component values are in a table in the first post. The diodes needn't be IN4001; just about any rectifier diodes will do. If you have a scrap transformer giving more than 4.5V, the calculations for it are in the fifth post in the thread. You want unrectified AC out of the transformer, so it's no good substituting it with a phone charger giving DC output.

Andy

[modeng200023]

The layout in the picture won't work. The top transformer secondary wire is shown connected to an unused strip on the Veroboard. The wire should go to the first strip not the second.

John

[andyf]

Oops  ! Thanks, John.
I've amended the text (the pc is on P'bucket, which I hate using) to add an Erratum note. That, of course, is a more dignified way of saying "Glaring error I should have noticed before posting this".

Andy

[dsquire]

Oops  ! Thanks, John.
I've amended the text (the pc is on P'bucket, which I hate using) to add an Erratum note. That, of course, is a more dignified way of saying "Glaring error I should have noticed before posting this".

Andy

Andy

I think you better check that erratum text over. I think it should be as follows or am I loosing it.(wouldn't be the first time). Thanks for this idea in the first place.

Here's a pic showing how things are laid out, and the track breaks.
ERRATUM: The top connection from the transformer secondary should go to the second first track on the PCB, not the first second.

Cheers 

Don

[andyf]

No Don, you're not the one who's losing it. Duly edited (again!).

For me, once things start going wrong, they just get worse and worse

Andy

[philf]

Andy,

While you're in the swing of posting Erratum messages you might want to look at the diode symbol - you've got it the wrong way round - the white or silver band should be on the cathode side!

 

Phil.

[John Swift]

Hi Andy

I've re done your diagram

John