Project: Examining Capacity of Recharged Alkaline AA Batteries

As a young kid, my parents had told me about how great NiCad rechargeable batteries are for reducing waste and saving money. While we did have some alkaline cells around, we used rechargeable batteries whenever possible. Because these were quite expensive, I was always warned to treat those batteries with care and not to get them mixed up or thrown out with the disposable ones.

This naturally led my young and inquisitive mind to question why alkaline batteries couldn’t be recharged. I asked my parents and their response was along the lines of “there being irreversible chemical reactions”. If you read the label, it will say something like “Battery may explode if […] recharged.” Sounds dangerous.

Despite this, as a child, I felt that explosions weren’t particularly scary, so I made it a habit to try recharging alkaline cells. I was convinced that it worked to some degree and the battery tester agreed with me, but my Dad remained unconvinced. As I had recently rediscovered my Grandcell RAM battery charger and completed testing of regular disposable alkaline batteries, I thought it would be good to see whether alkaline batteries can really be recharged, how useful it might be and whether it was as “catastrophic” as the labels might suggest.

Disclaimer: The information presented is based on my own experiences. Your experiences will likely vary. The information is provided with no guarantees. Your use of this information is at your own risk and no liability will be accepted for damages howsoever incurred. You perform any experiments and ignore any labelled product advice at your own risk.

Grandcell Charger I-V Behaviour

After my alkaline battery testing, I was left with a bunch of cells which were mostly depleted at the high-rate discharge. These cells had (maybe) about 33% of their original charge remaining as it couldn’t be extracted at high rate and had been stored for a few months. I would use these cells to examine recharge behaviour.

I started by using the Panasonic Evolta cells. Initially, I just wanted to know what the Grandcell charger was putting out – on an oscilloscope it was a slightly-ripply DC as expected. As a result, I decided to connect it to my B&K Precision Model 8600 DC Electronic Load to graph the I-V curve of a single channel.

The graph of the I-V curve seems to show a pronounced kink, dropping off rapidly above 1.6V. Open circuit voltage is 1.65V, and at an unrealistically low cell voltage of 0.1V, it delivered about 560mA. It seems 400mA is delivered at 1.4V, so the labelling at the rear seems a little misleading.

Given this knowledge, there’s really no reason why a similar result could not be achieved using a lab benchtop supply.

Lab Benchtop Supply Battery Charging Characteristic

For this, I decided to employ my Keysight E36103A bench-top power supply which allowed me to monitor the charge results and my Keithley Model 2110 to monitor the cell temperature using a TME Electronics KA01 K-type thermocouple.

Instead of using the exact values from the Grandcell charger, I decided to push the voltage slightly to 1.7V and set the current limit to 400mA. This should result in a sort of “square” I-V curve, but should be sufficient to recharge the cell.

Cells are run on the 0.48W constant power discharge to 0.8V protocol as I used on the high-rate battery test in the past and then connected to the lab supply to recharge for at least 10 hours. This was one of the “typical” cycle results (from the charging for the third cycle):

The cell starts with a short constant current regime lasting about 20 minutes. From there, the current starts to taper as the voltage floats up to 1.7V. The current tapers down to about 200mA but then persists, while the cell temperature starts to rise somewhat more sharply between the 2 – 2.5 hour mark at about 600mAh into the cell. The temperature continues to increase but stabilizes at about 10 degrees C above ambient, suggesting that excess charge is generating heat. This may be due to causing side reactions, in which case, increases in cell pressure, rupture of seal and explosion are a possibility. But it can also be due to a “soft” short inside the cell which is resistively dissipating energy as heat. This rather “undefined” charging behaviour without a clear cut-off (maybe except temperature) suggests that recharging alkaline cells can be quite an inefficient process.

The actual current at which the cell “stabilizes” at can vary from cycle to cycle. At times it can be quite high, resulting in significant battery heating. I decided to abort the experiment if the cell temperature exceeded 50 degrees C for my own safety, but during the month of testing and nearly 80 cycles, this did not occur.

Recharging Results

In line with doing a methodical test, I wanted to explore several questions – is there any difference between the results recharging with pure DC from a lab supply and the Grandcell charger? How much capacity can be recovered with recharging? How many cycles can a non-rechargeable alkaline battery tolerate? Is there any unexpected side-effects?

Lab Power Supply versus Grandcell Charger at 0.48W CW to 0.8V

In this test, Panasonic Evolta cells were used owing to their high quality construction. Batteries were recharged for 10 hours minimum between tests and immediately moved from charger to load. This reduces dwell time and eliminates self-discharge from affecting results. Test load was 0.48W constant power to 0.8V cut-off as with the previous alkaline tests. A 5-second load voltage is also supplied as an indicator to the voltage after surface charge is bled off.

Using the lab bench supply, we can see that originally the cell delivered (out of the packet) about 1281mAh, which falls to 920mAh after the first charge cycle or about 72% of the original capacity. The capacity then slowly drops – by the 5th cycle we are down to 697mAh (54%) and by the 10th cycle, 533mAh (42%).  The capacity decline gets more gradual as the total of 30 cycles are accumulated, residual capacity of 267mAh (21%).

However, if we take into account the discharge voltage, we see that the mean discharge voltage actually falls as a function of the accumulated cycles. As a result, the actual capacity decline is slightly more steep (65% on the first, 50% by the 5th, 38% by the 10th cycle and 18% by the 30th cycle) and suggests an increase in cell internal resistance making it less suitable for high drain devices (the exact devices that they were initially targeting with their 1.5V chemistry claim).

A look at the discharge voltage curves shows that they are mostly continuously declining in capacity. The kink around 1.08V is a side effect of the B&K 8600 DC electronic load.

Repeating the same experiment but recharging the battery on the Grandcell charger resulted in an even lumpier graph but with very similar trends. Individual one-off poor readings can be explained due to the variability in the recharging process where a complete charge may not be obtained for a variety of reasons (e.g. contact dirt, internal charge leakage). The slightly worse result of this cell can be due to the lower charge cut-off voltage of the Grandcell, but also due to cell-to-cell variations as we’re only dealing with a sample size of one.

The individual profiles seem roughly similar in their declining nature with the exception of cycle #7 which seemed to have notably earlier voltage depression.

Based on these results, it seems that recharging with a pure DC supply can be just as effective if not better than with the Grandcell charger. The recharging efficiency is not particularly high, with a limited cycle life with diminishing cell average voltages suggesting increases in cell internal resistance. Recovered power capacity falls to 50% by the 5th cycle and 38% by the 10th cycle, making it of limited value. However, it does mean that it is possible to recharge alkaline cells and extend their lifetime slightly which might be useful if you’re in a pickle and want to take the risk.

Deep Discharge Recharge Ability at 25mA CC to 0.8V

Using a “fresh” Panasonic Evolta cell that was previously depleted using the 0.48W CW to 0.8V test protocol, I decided to run the test but with a much deeper discharge at 25mA constant current to 0.8V. This would fully deplete the cell and we can see how the capacity declines as a function of cycles. As the original 25mA CC capacity of a fresh cell was not measured, this test looks at decline as a function of the initial first cycle recovered capacity. A 5-second load voltage is also supplied as an indicator to the voltage after surface charge is bled off.

In this case, I only ran ten cycles owning to the significantly faster degradation of capacity. The first cycle recovered about 2229mAh capacity, which fell to 1131mAh by the next cycle. By the 5th cycle, we were obtaining just 310mAh and by the 10th, we have just 114mAh. Comparing by actual energy, at the 5th cycle, we have 60% of the first cycle. Considering that the first cycle may only be 70% of the original battery capacity, this alludes to the 5th cycle delivering around 42% of the original capacity at a guess.

The decline is most evident when looking at the voltage curves, but surprisingly, in the early cycles, the voltage actually improved sightly on the second cycle. This suggests there may be some cell geometry effects that affect the recharge-ability of the cells.

Brand Dependency of Recharge Ability at 25mA CC to 0.8V

Since all the tests were done on the Panasonic Evolta cells, I wanted to see whether the lower-cost cells were also as good. Some of these cells also have 25mA CC data from the addendum that we could use to gauge the first-cycle recharge-ability.

Comparing all of the cells, it seems that they all managed a certain level of recharge-ability which seems fairly competitive.

By the table, with the cells we have 25mA CC data for, it seems that the recovered capacity in total energy terms ranges around 50-60% on the first cycle on the deep discharge. As a result, while it is possible to recharge the batteries, expect only about half as much capacity if all goes well.

Side Effects

An issue with recharging disposable alkaline batteries is the possibility of side effects. One possibility is leakage due to cell pressure build-up causing an electrolyte leak. While no leak was experienced during the test protocol, in the past, I have encountered certain cell designs that have leaked. In fact, I remember the Grandcell batteries that were designed to be rechargeable having leaked at the end of life. As a result, it might be unwise to store recharged batteries or use them in precious devices out-of-sight as leaked electrolyte makes for a big corrosion hassle.

Another possibility is that of explosion. That didn’t happen either, but it could if the safety vents fail to open and the cell ruptures along its body. As I kept an eye on the temperature, I was expecting to abort the experiment if temperatures got too high for my own comfort. Regardless, if you choose to recharge batteries, don’t do it unattended and without monitoring.

Finally, low efficiency and self-discharge is another possible side effect. As I mentioned during charging that there didn’t seem to be a definitive end-point, the cell continues to consume current even after more-than-the-delivered amount of charge had been absorbed. As a result, it’s a slight waste of energy to just charge the cell not knowing if that charge is going to be recovered and excess energy could exacerbate the risk of explosion/venting. The current consumption also suggests the possibility that the cell has a soft-short developing that would cause charge to be dissipated within the cell by self-discharge. I didn’t test for this possibility, but it is one to be aware of.

From the Literature

Looking at patent documents proved to be quite instructive. While RAM battery technology had been under development even before the mid-60s, there were a lot of limitations with the idea. Changes to the electrode formulation and geometry were made continually to try and reduce the possibility of producing irreversible oxidation of the anode current collector and improve mobility of charges. A mountain of patents seems to be the results of research into improving RAM technology. As a result, I think that it can be concluded that there are formulation differences between RAM batteries designed for recharging compared to those which are not intended for recharging.

RAM battery testing is very few and far between, but Michael G Hains “The Doc” gave the Grandcell AA and AAA varieties a test. In both cases, the batteries performed rather unimpressively, with a low cycle life and rapidly declining capacity.

Looking at a subset of the patent documents which include cycle life testing, I came across these three graphs.



Adapted from US5340666 Rechargeable alkaline manganese cell having improved capacity and improved energy density


Adapted from US3530496 Rechargeable alkaline manganese cell

Based on these results, it seems that RAM technology performs similarly to what I observed from recharging recharging modern alkaline primary batteries. The first two graphs shows that there can be quite a bit of variance depending on the cell design, but an initial rapid decline seems characteristic. The last graph shows a clear dependence on the extracted capacity versus cycle life – making RAM suitable only for shallow-discharge applications if a high cycle life is demanded.

Because of the similarity in observed performance, this is probably why I was confused as to why RAM cells were even necessary – it may well be that better quality alkaline cells share innovations that RAM cells also used to perform better under heavy loads, but are not deliberately optimised or recommended for recharge applications.

Conclusion

Rechargeable Alkaline Manganese (RAM) battery technology has been obsolete for a while, but its existence was marred by its poor cycle life performance despite its claimed alkaline-like advantages and lower price. However, this bought up a bigger question – as RAM is based on the same chemistry as regular disposable alkaline batteries, what sets the two apart and are alkaline batteries rechargeable?

Through testing the Grandcell charger, I determined it was nothing special. It’s role is basically to apply current-limited/voltage-limited low-ripple DC to the battery. By substituting and comparing a clean DC lab power supply, very similar recharging results can be obtained.

As it turns out, it seems that alkaline batteries like RAM batteries can be recharged. You might be able to get out of a pickle this way, or save the environment a little. However, they also perform quite poorly, with rapidly reclining capacity (50-60% on the first cycle typically) and increasing internal resistance. The cells are also relatively “inefficient” with no clear recharge termination signal. Doing so also carries risks of battery leakage or explosion and damage to devices from leaked electrolyte. As a result, it’s probably not worth recharging such batteries when good low self-discharge Ni-MH cells are relatively affordable and are much more reliable.

It was still a good experiment to answer one of those “childhood questions”, even if it did take a whole month of testing …

About lui_gough

I'm a bit of a nut for electronics, computing, photography, radio, satellite and other technical hobbies. Click for more about me!
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6 Responses to Project: Examining Capacity of Recharged Alkaline AA Batteries

  1. Sleepwalker3 says:

    You might be interested in the discussions on recharging normal alkaline cells that occurred in the 1990’s (IIRC) in Electronics Australia (EA) mag. There was also discussion and experimentation in Elektor from memory. The EA discussions concluded that a biased AC charge (sometimes called a ‘Burp charge’) worked best and reduced temperatures and cell pressures. IIRC a number of ‘burp chargers’ were published, both as projects in the mag and also in the ‘circuit and design ideas’ sections. These type of charges also worked well for getting Nicads back to health according to some reports, but it depended a lot on how bad the cell was when this was attempted.

    • lui_gough says:

      You do bring up a very interesting point – I was only ever privy to the slow trickle-type chargers when I was young and I had always thought about whether burp charging was effective or not. From my research at the time, I found that the implementation details seemed to vary a lot – some of them claimed to “pause” charging every few seconds while others actually actively claimed to discharge the cells with a pulse to “cause gaseous bubbles formed during charging to collapse”. While I was almost bought into the hype, the prices kept me away and I never really had any performance issues to speak of using just old-fashioned trickle chargers with a timer. I thought that if it were truly revolutionary, everyone would have it by now – much like how dV/dT and dTemp/dT is common amongst fast Ni-MH chargers.

      When it comes to batteries, many similar things seem to sprout around them as an industry to “make money” as batteries can be pricey and will inevitably fail. I remember the dischargers for NiCads which were of limited utility and the mass hysteria around memory effect (which I didn’t truly believe in). There were the self-discharge cures for dendrite formation using high voltage/current from capacitors discharging into cells which was of limited lasting benefit (they would reform through punctures in separators sooner or later). There are RC-hobbyists who do the same with capacitors claiming it reduces the internal resistance of their cells. Likewise, moving over to Lead-Acid, there are a mountain of pulsed-high-voltage desulfators which claim to bring dead batteries back to life with a few positive anecdotes but rarely any lasting success. There are also additives which claim to bring starting batteries back to health, but again, are just a band-aid solution unfit for critical applications.

      I suppose, as a result, it’s important to take some claims with a grain of salt. I suspect burp charging might belong in this category – not entirely without benefit, but not significant enough to change the landscape. Unfortunately, as I don’t have such a charger and I’m not intending to build one, I can’t evaluate how it might work with Alkaline or modern Ni-MH, but I somehow suspect that if it “burps” through a negative current pulse (i.e. short discharge) method, it’s ultimately not going to help the battery purely because it’s going to be adding microcycling to the battery during recharge which might decrease its cycle lifetime. Maybe this can be offset by improving the resultant chemical distribution in the cell, but given the similar results from patent-filing data (which usually presents things “as good as it gets”), I doubt there is that much to be gained.

      Still, thanks for the comment – definitely bought back some recollections from flipping through EA in my local library when it was still around.

      – Gough

  2. Kerry Lourash says:

    My hazy recollection is that I’ve seen articles saying one can recharge non-rechargeable batteries, but you are the only one that actually has quantified it.

    Thanks!

  3. rasz_pl says:

    Guilty as charged (HA!)
    My mom had a friend working at a Photo store and would bring a ~1kg bag of AA cells fished out of recycle basket every 1-2 months. I sorted them out looking for >1V ones, which wasnt too hard as most came from early digital cameras notorious for very high voltage cutoff, and tried light recharging in modified NiMH charger. More than half came out great, rest either didnt take the charge or pissed base all over the place. Easy source of free AA batteries, perfectly usable in clocks, toys, flashlights, and other non critical use scenarios. I did that for almost 10 years, until Lion cells became popular/affordable.

    “burp charging” – I had 1980 vintage single channel CB radio (Warel Tukan 1) when I was a kid http://www.oldradio.pl/karta.php?1493
    it used 10 small NiMH coin cells, and charger was a primitive transformer + one diode design, I always assumed it was a cost saving measure 🙂 and not an actual clever charging technique.

  4. IAN HARPER says:

    Dear Gough

    Very interesting, thanks. With regard burp charging, as someone who owned a prius, I can tell you that with NIMH technology, micro discharging/charging cycles is of no problem (or, who knows, may even be a benefit). Depth of discharge makes a very big negative effect on battery life and I expect thats the same with LI and alkali.

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