Shadows of Forgotten Ancestors, or Batteries from the Past

Thanks to one of my readers, I got my hands on some very interesting batteries. I should say right away, though, that these days the people most interested in them are precious-metal refiners. I think many radio amateurs know who they are.

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As I mentioned, one of my readers suggested reviewing some very unusual batteries. No, they’re not rare, although you don’t come across them often. They were mass-produced and widely used, but even so, you were unlikely to see many of them around the home.
When we met, I was given this unusual set: a plastic bottle, a syringe with a tube, and a small bag with a handle :)


And inside the bag were... yes, three batteries. Not ordinary ones, but gold silver—or, more precisely, silver-zinc batteries. If you’d like to read more about them, you can do so here. I’ll keep it relatively brief.

The batteries are supplied dry, which helps them keep for a long time. I received them already filled—thanks again for that, because I’m no chemist at all. The bottle, of course, contains electrolyte, just in case.


The batteries were manufactured in October 1970, making them a few years older than me. They’re already 52 years old!


You can easily find lots of photos of these batteries on Google, with dates both earlier and later—for example, 1962 and 1988.


Naturally, I wanted to find out who made them, especially since all batteries of this type had the same logo.
My search first led me to an article mentioning the Jupiter plant in Pripyat. Here’s a quote from it


But I had my doubts, especially since I’d heard a little about this factory. So I took the easy route: cropped the logo from the photo, fed it to Google, and it immediately gave me the factory’s name. It turned out to be “Leninskaya Iskra.”


Before taking a closer look and running some tests, I should say a few words and share some facts about this type of battery.
I first heard about these batteries from a friend sometime in the late 1980s. Back then, people were talking about some kind of super-batteries that could charge to 80% in just 15 minutes. At the time, that sounded like pure science fiction, especially compared with the junk we had to use. By the way, I’m still looking for references to AA-like nickel-cadmium batteries that were about twice as long and used in police radios. For some reason, they seem to have disappeared completely.

Anyway, these batteries are in some ways similar to nickel-cadmium ones, at least because they also use an alkaline electrolyte. But since their plates are made of different materials, their electrical characteristics are different too.
In short, we have:
EMF: 1.85 V (operating voltage 1.55 V), somewhat higher than for nickel-cadmium and nickel-metal hydride batteries.

Theoretical specific energy: up to 425 Wh/kg
Practical specific energy: up to 150 Wh/kg
Here, they’re not far behind modern lithium batteries either, which are rated at:
Specific energy: 110…270 Wh/kg

But an even more interesting figure is:
Volumetric energy density: up to 650 Wh/dm³. In other words, how much energy you can pack into a given volume.
For lithium batteries, this figure is around 300 Wh/dm³.

So on this graph, silver-zinc batteries would be way off to the right and wouldn’t even fit on the horizontal axis.


Of course, it’s not all that rosy in reality, and that figure of 650 Wh comes with an “up to.” Still, it’s genuinely impressive, especially considering the difference is more than 50 years!
But even if we simply compare high-capacity lithium batteries with the SC-25 under review, the difference isn’t that great, especially since the photo shows the equivalent capacity in Wh. The SC-25 comes in at around 40 Wh, a pair of 21700 cells at around 38 Wh, and three 18650 cells at the same 38 Wh. But the SC-25 has a noticeably thicker case and takes up a considerably larger volume.


Silver-zinc batteries are also rated for very high discharge currents, up to 50C, provided the plates and current collectors are designed accordingly. But how many lithium batteries do you know that can deliver 50C? If the batteries under review were high-current models, they could deliver up to 1250 A.

At least four types were produced in this series:
SC-25
SCK-25
SCS-25
SCD-25

SC stands for silver-zinc; K means short cycle (high-current), S means medium cycle, and D means long cycle (high-capacity). Interestingly, all the model numbers include 25, which corresponds to the capacity only for the SC-25. The others have different capacities: 20, 30, and 40 Ah, respectively.


These batteries are also rated for a temperature range of -40 to +60.

Of course, many people will ask: if these batteries are so wonderful, why didn’t they catch on for consumer use?
Well, first of all, they did, in a way—just not as rechargeable batteries, but as primary cells, for example the SC watch-battery series. And secondly, silver-zinc batteries also have a number of drawbacks:
Cost: the SC-25 under review contains around 80 grams of silver in each battery!
Service life—or, more precisely, cycle count. The SC-25 is rated for just 50 cycles, while the SCK-25 is rated for only 10.
Long charging time: 18 hours for the SC-25.

Still, some people don’t worry much about the cost, and the cycle count isn’t always critical to them. Capacity and reliability matter more.
First, there’s aerospace use: silver-zinc batteries powered the first satellite (SCD-70 and SCD-18), as well as the lunar rover.


And, of course, the military—how could we leave them out? Batteries like these were used in torpedoes. There was even a series that used seawater as the electrolyte (so a torpedo like that wouldn’t work in fresh water), as well as in various radar stations and missiles. I wouldn’t be surprised if they’re used in some of the missiles that are hitting our cities now.


All right, enough digression. On to the review.
The batteries look a bit like ordinary nickel-cadmium ones. They also have a little plug with a valve, and the screw terminals seem to have been coated with something like solidol grease.


The rubber seal on the valve has degraded over time. Well, what do you expect after 50 years? It would be interesting to see what a smartphone battery looks like after the same amount of time :)


Thanks again to the person who sent them. He prepared the solution, filled them, and let them sit for a while. Chemistry isn't exactly my strong suit, either—this isn't like mixing lithium and cobalt in a basement :)
It's interesting to see the voltage appear as soon as they're filled. The battery on the left is still dry; the one on the right has been filled.


The recommended filling solution is described in the reference books. These batteries are a bit odd: there's plenty of information about them, but it's all very general. You don't find much in the way of modern datasheets for specific models, with charging and discharging currents and precise voltage and current values.


While the batteries were on their way to me, some of the electrolyte soaked in, which is why the sender included some solution and a syringe so I could top them up.


A view from the three main sides.


Dimensions. I decided to use an authentic ruler here—one made not only around the same time (1967), but in the same city, too. What are the odds that these two items would end up in my home more than 50 years later?
For those who don't know, the photo shows the kind of “calculator” your parents used at school—and quite possibly your grandparents, too. Just imagine: it needs neither a battery nor an eight-core processor, and “calculators” like this were even used to calculate space flights.


Measurements after receiving them: the impedance is about the same, but the voltage is odd. The first two are roughly alike, while the third is twice as high.


I'm topping up the electrolyte, and here's where one of the quirks of Soviet “datasheets” comes up: how high should I fill them? I found information online saying to fill them up to the red mark on the case. I inspected these batteries and looked through a ton of photos of them, but couldn't find any marks anywhere.
In the end, I went with my gut and filled them enough to make sure the plates were covered.


While I was at it, I noticed a crack in the case of one battery. I taped it up with electrical tape, just in case.


I let the batteries sit for a while, then repeated the measurements and weighed them as well.
The picture had changed, though. Only the second battery's results were more or less unchanged; the voltage and resistance of the others had both changed.


For the initial tests, I used an EBC-A10H battery tester, connected in four-wire mode, of course.


And right from the start, I ran into a lack of information.
Some sources said to charge them to 2.05 V, others to 2.10 V. The book says to charge at 1.5 A for 18 hours. I later noticed a mention of the electrolyte level, too: 5–10 mm above the plates. But I think that's allowing for evaporation; in my case, it's enough to make sure the plates are fully covered.
What annoys me most about the terminology, though, is the phrase “normal current.” I'm used to specifications like “charge at 0.5C to a voltage of XX, then stop when the current drops to 0.1 of the charging current”—which is much more precise.


I started charging at 1.5 A. The battery showed impressive voltage stability during charging; for the first three hours or so, the graph was practically a straight line, accurate to within a thousandth of a volt!
After about 8 hours, the voltage suddenly jumped.


Since I had a charging graph for similar batteries, I figured it had simply finished charging.


So I stopped charging. After that, the load simply monitored the voltage at the battery terminals.


After waiting for a while, I started discharging it at 2.5 A, or 0.1C in more familiar terms, figuring that was close enough to a “normal” current :)
The battery delivered almost 13 Ah, or about 20 Wh. The average voltage was 1.54 V, and the discharge was cut off when it dropped to 1 V. By the way, the discharge cutoff voltage is also specified differently, usually somewhere in the range of 0.6–1.2 V. I was thinking of setting it to 0.9 V, as with nickel-cadmium batteries, but a friend suggested setting it to exactly 1 V.


For the second cycle, I decided to run it longer. I was aiming for at least 18 hours, but technical issues meant I had to settle for 17 hours and 25 minutes.


Discharging at the same current, I got almost 25 Ah this time, which is much more satisfying. The average voltage was 1.56 V.

By the way, did you think it was hard to estimate the state of charge of lithium iron phosphate batteries because their discharge curve is so flat? Ha, think again. Now this is a flat curve—and what a flat curve. It's practically a straight line....


I ran the third charging cycle at 2.5 A, since I didn't see any point in waiting that long, and 0.1C seems perfectly reasonable to me.
But this time I did the charging a little differently: I decided to wait and see what would happen if I didn't stop it.

I ended up stopping it manually, but I still saw that the graph has two rises, roughly in the first quarter of the cycle and right at the end. Why was I interested in this? Yes, because they must be charged somehow, and not after being fully discharged, which means there has to be some way to determine when the process is finished.


Next came the tests. There were ten in total. Given how long charging took, it's hard to even work out how long the whole thing took in the end—and this was only the first of the three batteries.
I tested at 0.1C, 0.2C, 8.5 A (the recommended maximum), 10, 12.5, 15, 20, 25, and 30 A.


I monitored the temperature during the tests. Here are the readings at 20, 25, and 30 A. They seem a bit high to me, although the battery was operating at three times its rated load.


The charging graph for this battery was a little strange to begin with. Before the last test, I tried charging it at 4.5 A as an experiment. Oddly enough, that produced a nice graph and the charger shut off properly. But after all the tests, when I tried charging it again, I found that it wouldn't charge anymore.


I checked it with a thermal camera and spotted an unusually hot spot.


On closer inspection, I found some kind of abnormal growth there.


The battery itself also looked a bit strange.


The internal resistance had dropped, as had the voltage. On the right, for comparison, are the test results for the second unit after its first cycle.


Apparently, an internal short occurred, because when I picked it up about 10 hours later, it was noticeably warm.


During the tests, I also noticed that the plates inside had gone from white to almost black, so I decided to film the process.
The video is sped up about 30 times; the original shows 24 minutes of charging at 1.5 A.


And here's the result: the new battery is on the right, and the one on the left is shown about half an hour after charging began.


The second unit behaved much better during charging. I did, however, run its first cycle straight away for about 18 hours at 1.5 A.


The key point is that I set cutoff thresholds for voltage and current drop, and the battery then completed its charging cycle correctly. At the end, I tried charging it at a current well above the recommended value, and even then it finished charging properly.


For this battery, I decided to limit the discharge current to 25 A, since 30 A is clearly too much for them—the electrolyte starts to overheat.


Temperature at 20 and 25 A


The third unit was prepared in roughly the same way. For technical reasons, the process took a little longer than recommended; the tester has no option to limit charging time, only discharge time.


I also tried charging it at a current well above the recommended value. As with the previous unit, charging ended properly, though you can see that the process took longer here.


Results at the same currents: 0.1C, 0.2C, 8.5, 10, 12.5, 15, 20, and 25 A


And the temperature at 20 and 25 A


But the most interesting graph is the comparison of all three units in three modes: 0.2, 0.2, and 1C. You can see that the batteries differ not only in capacity delivered, but also in their discharge curves, especially at the beginning. You could say each unit has its own personality.


Final impedance, voltage, and weight measurements. The differences are clear here too: the first unit died completely, the second's resistance increased, and the third was the only one to come through the tests well.
That said, it's worth noting that:
1. The first delivered the most capacity, but died.
2. The second behaved best during charging and had average capacity.
3. The third had an average charging curve, but delivered the least capacity.


The electrolyte level dropped somewhat during the tests. The drop was noticeably greatest in the center, where the temperature was higher—in other words, it simply boiled away. Well, yes. If the case temperature was around 65 degrees, the temperature inside was clearly higher.
The photo shows the third unit, which they let me keep as a souvenir—many thanks for that :)


As a side note: while looking for various information, I came across a comment saying that alkaline electrolyte shouldn't be stored in plastic containers—or, more specifically, in ordinary PET bottles—because the plastic can degrade as a result of a chemical reaction with the electrolyte.
Well, I tested it myself, and sure enough, after about a month to a month and a half, the plastic had become very weak and the bottle started leaking. Some kind of white crystals also began to build up on the outside. So don't store this kind of electrolyte in PET bottles.
I had to pour some of it into a glass container and dispose of the rest :(


So, that was an unusual review. It's hard to draw any conclusions, since it would have been more accurate to test new batteries. On the other hand, I showed batteries that had been stored for more than 50 years, then filled with electrolyte, and still worked perfectly well. What's more, despite a rated discharge current of 8.5 A, they could deliver 25 A.
But what really surprised me was the almost perfectly flat discharge curve. I wonder how they used to determine the state of charge—or did they not need to, and simply calculate it from capacity, load current, and runtime?

That's all for now. Thanks to the sponsor for the batteries, and to the readers for the comments and for rating the review :)
By the way, does anyone happen to have an unwanted RTG lying around in their storage room? :)

P.S. Shadows of Forgotten Ancestors, a 1965 Ukrainian film whose title couldn't be more fitting for this review. Besides, the film is essentially from the same era as the batteries.