Ni-Cad Battery Answer

Unc Cece had it “half-right” on his Ni-Cad Battery answer recently published.

I used to manage a two-way radio shop. Now, as you can imagine, Ni-Cad batteries were very popular for the handheld radios, as you could simply recharge them instead of having to buy dry-cell or alkaline batteries. One of our biggest problems was “ni-cad memory.”

It would occur when a user would take his radio out of the charger, use it for 15-20 minutes or so, then put it back in the charger. Pretty soon the battery would have only 15-20 minutes of useful life left in it. I saw this happen more than once.

While I can not give you the “scientific” answer why this occurs, I can tell you it happens, and quite a bit. I had a Ni-Cad battery on a amatuer radio of mine, and with proper charge-discharge cycles, the battery lasted me 6 years! Not bad for a product that is designed to last 1 1/2 to 2 years!

There is one way of eliminating “memory effect” from Ni-Cad cells, and that is deliberate “overshock”, as we called it - the application of high voltage for a VERY short period of time, say 2 seconds or so. CAUTION!!! Do NOT try this unless you have a variable power supply, and TWO DVMs, one to monitor the power supply, the other the battery! This can result in explosion of the battery pack if the surge voltage is left on the pack too long!

Cece, you’re right in stating the voltage in the battery is still there underneath the “memory effect”, but aside from having a TRAINED TECHNICIAN perform the above on the battery pack, there is no way to access that stored votage underneath the memory effect.

There was a VERY informative article on Ni-Cad and Ni-MH (Nickel-Metal-Hydride) batteries in a recent issue of QST magazine, I invite you to visit the ARRL website at and do a search for Ni-Cad for the article. It explains the memory effect much better than I ever could.

Thanks and keep up the good work of informing the “unwashed masses”!


"It’s hard to be religious when certain people are not incinerated by bolts of lightning…

I have to agree with Rico that Cecil kinda missed the point of the question. In most applications, low voltage is as bad as none. Whether the condition is caused by battery memory, voltage depression, or gravity waves, the fact remains that nicads lose a great deal of their capacity if they routinely undergo a shallow charge/discharge cycle. I have three cordless phones whose effective battery life dwindled from 3 hours to less than 10 minutes over a few months’ time. Deep discharging/recharging every month helped, but the talk time inevitably withered away within weeks. Eventually I found some nickel-metal-hydride (NiMH) batteries that, with a little inspired soldering and handset case trimming, let me swap out the NiCd packs. The memory/voltage-drop condition vanished instantly, and permanently.
I’ve seen the same behavior in electric toothbrushes, cellphones, and cordless drills…but not in my little radio-controlled mouse that the cats routinely pursue to its last dying erg.
The former see intermittent use with idle time spent on the charger; the latter is routinely exhausted and fully recharged.
Empirical evidence, to be sure, but compelling nevertheless.
Technically, the NiCd memory effect may not be caused by the “memory effect” NASA discovered, but it’s far from mythical.

-=< RH >=-

Rico writes:

I think what you’re describing here is the phenomenon of “voltage depression,” which I distinguished from “memory effect.” I notice even in the technical literature there is a tendency to equate these terms, but you know me, I’m a purist.

Here’s an example of voltage depression in a nickel-metal hydride battery. You’ll notice in the chart that after repeated shallow discharges the cell drops to 1.15v prematurely. But there’s still quite a bit of charge left. NiCad batteries are more susceptible to voltage depression than NiMH, but the idea is that same. The cure is simple - subject the cell or battery to several full discharge/charge cycles. I agree with rhohm that over the long run NiMH will give you less trouble in this respect than NiCads.

Thank you Cecil for clarifying that. I had heard the term “voltage depression” before, but, as Duracell did on the link you provided, I have always equated it with the term “memory effect.”

You are correct in your statement that NiMH batteries will practically eliminate memor…oops, voltage deprerssion, but there is a trade off. NiMH batteries only have 25% of the overall lifespan of a Ni-Cad battery. While the voltage is constant for longer, and voltage depression is kept to a minimum, a NiMH battery will only be able to be recharged an average of 500 times, while a Ni-Cad is able to be recharged an average of 2000 times.

One other thing to take note of - anyone who thinks you can take a NiMH battery and rapid charge it using a Ni-Cad rapid charger, you are in for a fire! Ni-Cad “smart” rapid chargers “know” when it’s time to turn off the higher voltage charging by a “dip” in the voltage of the monitored pack. NiMH batteries also have the “dip”, however, it’s a much shallower dip, and 9 times out of 10, the Ni-Cad rapid charger will miss the dip and keep on charging the pack until a fire starts. If you wish to rapid charge NiMH batteries, you need a charger specifically designed for NiMH batteries to prevent this from happening. If you are using a regular rate charger (ie. “wall wart” or 16 hour charger) you have nothing to worry about, these chargers will not cause overcharge on packs (unless you leave them on for a week or so)

Once again I must point you to the ARRL web site for further discussion. About a year or so ago there was a very informative article in the magazine QST about NiMH versus Ni-Cad written by Gordon West. It is explained in detail in that article. Visit for more information.

Thanks again Cecil - and keep letting us know the truth about this stuff!


"It’s hard to be religious when certain people are not incinerated by bolts of lightning…

Just found another site with a major manufacturer warning against the use of NiMH batteries in devices designed for Ni-Cads - it’s You may want to check this out.


"It’s hard to be religious when certain people are not incinerated by bolts of lightning…

At the suggestion of Ed, I’ll post my
reply that I had originally e-mailed to you.
I had a conversation a year or so ago with
a physicist student friend of mine. Now, I hope I can get this right…
NiCad batteries contain a solution with
crystals floating around in them. When
these crystals dissolve, they give off
energy that is utilized by the circuit it
is closed within (and of course gives off
a bit of heat). When the crystals are
completely dissolved, the battery goes dead.
Recharging must obviously ‘regrow’ these
crystals from the pool of free NiCad, which you must keep in mind is of a definitive amount (this will be important in a bit).
Now, the problem lies in sticking a battery in the recharger when your crystals are only
partially dissolved - Your pool of free NiCad is much smaller now. As a result, the amount of small, healthy crystal you can grow is decreased (I say healthy because these little crystals have a small surface area - remember that the rate at which an object dissolves (or is digested, i.e. bits of pizza in your stomach) is directly proportional to the amount of exposed surface (which is why your mommy told you to chew up your food real good)). Meanwhile, you have the “old” crystals left over from before, which now grow bigger as a result - further decreasing the amount of NiCad going into healthy crystal. Because of the increased size of these mutant crystals, they will dissolve very inefficiently.
So what happens now? You stick the battery in your hardware, you use it… the decreased amount of smaller crystals is expended quicker, while the large crystal dissolves VERY slowly (to the point where you don’t get a detectable voltage anymore). So it goes dead prematurely. You recharge it, you add yet even more bulk to this ever-growing mass of NiCad Hell Crystal, further decreasing your pool of free NiCad for the healthy crystals…
and thus, over time your battery ceases to work. And there is actually the chance that this ever-growing crystal eventually bursts the inside of the battery, completely ruining it.
My own theory on ‘super-charging’ the dying battery is that these huge crystals are dissolved VERY quickly, reseting the whole soup of electrical goodness. But that is just my two cents.

I hope that I was at least relatively clear on my explanation. It is late, and my mind has begun to take on the consistency of curdled milk.

I will take leave of you now while that pleasant thought sinks in.

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As far as I know, Andrew was on the money about poor crystallization. I read an article in a ‘Videographer’ magazine a couple years ago (sorry I don’t have the exact reference) written by a physicist from an energy research lab who worked on rechargeable batteries. Apparently, the ‘memory’ rumor got started because poor crystallization is usually the result of cyclic small recharges. In other words, if you use your rechageable a little bit, then put it on the charger several times, your chances af having it’s capacity drop are much greater. You can see how a few personal experiences with this and no attempt to verify the phenomenon might make you come up with the ‘memory’ metaphor. Keep up the good fight, Cecil.
yer pal,

Thanks for all the info. Now I understand why my cordless phone kept losing its charge. Regarding a cordless phone, is it better to let it fully discharge before recharging, or just wait until it gets low and starts beeping at me? And why can’t they selectively set the charger for the cordless phones so you don’t ruin the battery, so you can charge it up only when you need it but can still hang up the darn thing!!

There are new types of batteries, the name eludes me right now. But my new portable phone has one. no ‘memory’ effect.
don’t ask how it works though. I’ll
do some digging and post something if I find it.

[this space intentionally left blank]

The three major types of rechargeable batteries on the market now are Nickel-Cadmium, which Uncle Cecil was talking about, Nickel Metal Hydride, which was discussed earlier in this thread, and the newest is Lithium Ion, which is starting to make an appearance on high-end two-way radio equipment.

Lithium Ion (LI) batteries have an advantage over NiMH batteries to the point that they can be recharged as many times as a Ni-Cd, but have the characteristics of a NiMH when it comes to memor…dang it, did it again - voltage depression. The major drawback is the high cost - you can expect to pay up to 5 times as much for a LI battery as you would for a comparable Ni-Cd, and 3 times as much for a LI as a NiMH.

And once again, the charger warning: Do NOT use a “smart” Rapid Rate charger designed for Ni-Cd batteries on NiMH or LI batteries - the packs will overcharge within hours resulting in a fire.


“It’s hard to be religious when certain people are not incinerated by bolts of lightning…”

The crystal theory would explain the cure somebody told me once: pick it up and drop it.

John Stracke

It seems to me that someone really needs to explain what is going on in a battery before any discussion of memory effect can proceed. The first point I should make is that these Cadmium “crystals” are not free floating in the solution, they are connected to the cathode. These electrodes are immersed in a solution of potassium and Nickel hydroxide (along with other complexing agents to make the reaction more efficient). The net reaction then is:

		Cd(s) + Ni(OH)2 -----&gt;   Ni(s) + Cd(OH)2

This reaction produces about 1.1 Volts. (I'm not sure exactly) When the battery is recharged, an opposing voltage is applied and the reaction reverses.
What the physicists have described is a phenomenon which does occur but is not entirely responsible for memory effect. When the Cadmium metal dissolves it may dissolve somewhat unevenly which will create high points and low points. The peaks and valleys will develop a higher current density when dissolving. This will generally even out the surface if the battery is completely discharged. If it is not, however, when the battery is recharged, the peaks will get a higher current density and the deposition will further increase the size of the peaks. When the batter is then used the higher current density at the peaks results in a higher electrochemical potential.
A naive individual might expect that this would cause the peaks to dissolve faster and once again level out the cathode. The problem is that now the potential is high enough that another reaction can occur.

		H2O  ------&gt;   2e-  +  2H+   +  1/2 O2(g)

This reaction occurs at about 1.2 V. Not only is this electrochemical power permanently lost (The reverse reaction is kinetically unfeasible), but the oxygen forms gas bubbles at the surface of the electrode. Since these gas bubbles are non conductive, the effective surface area of the electrode is significantly reduced.
Dropping the battery may help free some of the bubbles from the electrode which could at least temporarily fix the problem. Quickly applying a high voltage to the cell will do several things. It will produces lots of gas which may actually wash the smaller bubbles of the surface. It may also level some of the high current density areas.
I apologize if any of the information I have given is not entirely correct since I am writing this off the top of my head. The general principle is there, however. It is the gas bubbles which reduce the surface area of the electrode causing the memory effect. I hope I have helped clear things up.
Incidentally, I am not familiar with any kind of voltage dip, since all of the standard potentials remain the same. This coiuld be the result of a more complicated issue such as diffusion of the species or complexing agents. I just took a test on this subject, and you don’t want to hear the explanation.

I saw this problem with hand-held calculators, specifically the TI-58 and TI-59. Someone in the calculator club came up with this suggestion, and it works. It seems that after many cycles, tiny growths arise on the surface of the battery material. (I guess stalactites would be a good analogy.) The cure is to charge a 50-volt, very high capacity capacitor with a variable power supply, then discharge it across the battery terminals. (Unfortunately, I don’t remember if the connection is + to + or - to +. At any rate, many of us tried it, and it worked.

It is dangerous to be right when your government is wrong

Bottom line question: Is it OK to leave my rechargable toothbrush, telephone, flashlight, etc. plugged in all the time? Will their circuitry automatically prevent overcharging? Will the full power remain available?

OK - every NI-CD battery occasionally must be discharged to maintain optimum life. As far as leaving it ON the charger, it depends. If you have a rapid charger, where the voltage of the charger is significantly higher than the voltage of the battery, yo should not leave it on for more than 24 hours MAXIMUM - it should charge fully in only 3 hours or so. If you have a SMART rapid charger, where the charging rate automatically switches from rapid charging to trickle charging, you can leave it on safely for quite a while, a week or so at the most. With a regular rate (trickle) charger, where the voltage of the charger is only slightly higher than the votage of the battery, the battery will charge in 16-24 hours, and should not be left on charge for more than three days or so.

As I said earlier, discharging a Ni-CD battery is essential to maintaining optimum battery life. Do NOT discharge the battery COMPLETELY, just low enough so that the low battery warning comes on, or the device starts to “run down”. Then recharge from there.

If you have any questions regarding batteries or battery life, please feel free to e-mail me. I’m not a scientific expert, but I can give you “real world” solutions to maintaining your rechargeable batteries.


“It’s hard to be religious when certain people are not incinerated by bolts of lightning…”

Nicad musings are an example of what I call a “Black Box Urban Legends”.

Whenever there is a bit of technology that is everywhere, but sealed up, misunderstood, and prone to random glitches, this is a “Black box”. Batteries are a fruitful source of these legends. You’ve probably heard: (1) Keep your camera batteries in your pants pocket for a while and they’ll recharge". or (2) Don’t put a car battery on the garage floor, it’ll discharge". or (3) “Nicads have a memory” or (4) “Drop a nicad to break up the bubbles”.

IMHO the reason these legends are so popular is that they make us feel better. You’ll note that (1) and (4) are “cures”, and (2) and (3) are excuses.

The legends persist because they’re hard to disprove. None of us keep detailed records of how we charge and discharge or nicads. When a nicad pack fails on us it’s easy to point to one of the common scenarios where we are not to blame.

You’ll notice almost nobody has mentioned IMHO the main reason nicads fail-- over-discharge, either due to overuse, or underuse. If you leave the device turned on and forget about it, many devices will completely drain the battery. Alternatively, if you put the device away for a few months, it slowly loses its charge, maybe taking 3-4 months to get down to zero charge.

This is a dangerous charge level. In any nicad pack made up of several cells, the cells may vary 20% or more in their ability to store charge. If you let the nicad pack get below 20% charge or so, the weakest cell may be driven into a reverse-voltage condition. This quickly grows metallic crystals that then short out the cell. Recharging it usually doesnt undo the short, so the battery pack now puts out about 1.2 volts less. If the powered device has a voltage sensor, it’s going to turn on it’s “battery low” warning that much sooner.
Even if it doesnt have a sensor, the device is going to run low on voltage sooner than it should.

Search for dueling bagpipes and you’ll see that they attract the ladies :slight_smile:

Moderator Note

drewder, don’t bump zombies to post non sequiturs. This is closed.