Author: Ken A. Nishimura (KO6AF)
E-Mail: kennish@kabuki.EECS.Berkeley.EDU I have decided to write this diatribe due to the continuing Ni-Cd battery saga. Yes, batteries are LOW tech -- they can't compare to the bells and whistles of our latest HTs, but... your new HT is but a paperweight (albeit an expensive one) without power from your batteries. This is not a response to any particular prior post, and is unsolicited, so in short, I'm not flaming anyone.... But, I thought it may be useful, so, without further ado, let us take a more careful look into NiCd battery management.
Note:
First, a bit of nomenclature. A cell is a single electrochemical device with a single anode and a single cathode. A battery is a collection of cells, usually connected in series to obtain a higher terminal voltage.
Batteries, whether they are primary (use once) or secondary (rechargeable) are devices which convert chemical energy into electrical energy. In the case of the latter, they can take electrical energy and store it as chemical energy for later use.
The key to electrochemistry are the processes of oxidation and reduction. Remember the phrase" "LEO (the lion) goes GER (grr??)
LEO = Lose Electrons Oxidation
GER = Gain Electrons Reduction.
When one oxidizes a material, it gives up electrons it becomes more positively charged, or enters a higher oxidation state. Likewise, when one reduces a material, one is adding electrons to it and either making it negatively charged or reducing its oxidation state.
Now, one can make a cell using two materials, say A and B and immersing them in a solution which can conduct ions, called an electrolyte. (An ion is a charged atom or radical of a molecule capable of transferring electrical charge). Now, let us say that material A is easily oxidized -- it likes to lose electrons, while B is a material that likes to be reduced. When these two materials are immersed in an electrolyte, and a circuit is completed from A to B, A is oxidized and electrons are released to flow to the circuit. After performing electrical work, the electrons flow into B, where B is reduced. The circuit from B to A is completed by the flow of ions in the electrolyte. A secondary cell can be reversed by forcing electrons into A, and reducing the oxidized A to regain unoxidized A for use again.
This, of course, is an oversimplified view, as only certain combinations of materials and electrolytes provides useful and practical batteries.
Oh, one more bit of nomenclature: The cathode is where reduction takes place, and the anode is where oxidation takes place. So, in a battery which is producing current, the positive terminal is the cathode, and the negative terminal is the anode. Yes, this is counterintuitive from our understanding of diodes, where the cathode is negative with respect to the anode...
Now, the NiCd system itself:
When the cell is fully charged:
The cathode is composed of Nickelic Hydroxide.
Now, nickel is one of those elements that has multiple oxidation states -- it can lose a different number of electrons per atom, depending on how hard it is coerced. Nickel is usually found with oxidation states of 0 (free metal), +2, +3 and +4. The +2 state is referred with a -ous suffix, while the +3 and +4 states are referred with a -ic suffix. So, nickelic hydroxide is really NiOOH (the nickel has a charge of +3) or
Ni(OH) (the nickel has a charge of +4)
4
The anode is composed of free cadmium metal (zero oxidation).
The electrolyte is usually a solution of potassium hydroxide (KOH).
When one connects a load to the cell, as explained earlier, the anode is oxidized and the cathode is reduced. Electrons leave the anode where the cadmium is oxidized and forms:
Cd(OH) , plus 2 free electrons.
2
These two electrons go to the cathode where they reduce the nickelIC
hydroxide to form nickelOUS hydroxide or:
Ni(OH) (where the nickel has
2
a charge of +2)
This reaction can take place until the materials are exhausted. In theory, cells are manufactured so that both anode and cathode are spent at roughly equal rates.
In the NiCd system, the cadmium hydroxide is being re-converted into cadmium, and the nickelous hydroxide is being re-converted to nickelic hydroxide.
Note that the electrolyte in both charge and discharge is a means to move the hydroxyl (OH-) ions around. Unlike the lead-acid system, the electrolyte really doesn't change in composition too much between the charged and discharged state.
This of course is bad. Oxygen + hydrogen = BOOM. Cell manufacturers, or at least their lawyers, frown on this from happening. So, they cheat. During manufacture, they deliberately oversize the negative plate, and they partially discharge it. That is, they put a fully charged positive plate, but put a slightly discharged, but bigger plate of cadmium in. The amount of free cadmium in the oversized plate is matched to discharge in step with the amount of nickelic hydroxide provided in the positive plate.
Now consider what happens as full charge is achieved. Oxidation of water starts at the anode, but since the cathode is oversized, and has excess hydroxide, the current continues to produce cadmium metal instead of hydrogen. At the same time, the separator (the material used to prevent the plates from shorting) is designed to allow oxygen gas to diffuse through, from the positive to the negative plate. The free oxygen then oxidizes the cadmium metal to form more cadmium hydroxide to prevent hydrogen from being formed. Voila -- a safe battery.
Another problem is that the process of generating oxygen, and recombining it at the cathode generates heat. With a moderate amount of current, the cell temperature can rise considerably, to 50 or 60 degrees C. If after charging, the batteries are hot, then you have overcharged them -- slap yourself on your wrist...
So even though the cells may not vent, the heat by-product is wearing down the cells. Specifically, hydrolysis or degradation of the separator material, usually polyamide, is greatly accelerated at high temperatures. This leads to premature cell failure (see below).
The price one pays for this is reduced capacity. Everything takes space in the cell, and space for carbon means less space for active material. Also, there have been some indications that carbon can cause the cadmium metal to corrode, possibly leading to a shorter life.
So far, the highest capacity sintered plate (best for low resistance) cell I have seen is the Sanyo KR-800 cell, rated at 800 mAh.
The Panasonic 900 mAh cell is of the foam type, and may work for a specific application, but expect higher resistance. I also suspect (but am not sure) that the Millenium cells are also foam type. For most consumer applications, the internal resistance isn't an issue -- for high power transmitting (e.g. more than 1A of current), it can be a concern.
Just as everyone is running around and saying that the memory effect is a myth, here I am, saying that it is true. OK, so, why is this? First of all, the term memory effect is quite unscientific. People tend to attribute any failure of a NiCd to memory.
Let us define memory as the phenomenon where the discharge voltage for a given load is lower than it should be. This can give the appearance of a lowered capacity, while in reality, it is more accurate to term it voltage depression.
Memory is also hard to reproduce, which makes it hard to study. Originally, memory effect was seen in spacecraft batteries subjected to a repeated discharge/charge cycle that was a fixed percentage of total capacity (due to the earth's shadow). After many cycles, when called upon to provide the full capacity, the battery failed to do so. Since we aren't in space, the above is not really relevant...
Let us look at various causes of "memory" or voltage depression.
Memory can be attributed to changes in the negative or cadmium plate. Recall that charging involves converting
Cd(0H) to Cd metal.
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Ordinarily, and under moderate charging currents, the cadmium that
is deposited is microcrystalline (i.e. very small crystals). Now,
metallurgical thermodynamics states that grain boundaries (boundaries
between the crystals) are high energy regions, and given time, the
tendency of metals is for the grains to coalesce and form larger crystals.
This is bad for the battery since it makes the cadmium harder to dissolve
during high current discharge, and leads to high internal resistance and
voltage depression.
The trick to avoiding memory is avoiding forming large crystal cadmium. Very slow charging is bad, as slow growth aids large crystal growth (recall growing rock candy). High temperatures are bad, since the nucleation and growth of crystals is exponentially driven by temperature. The problem is that given time, one will get growth of cadmium crystals, and thus, one needs to reform the material. Partial cycling of the cells means that the material deep with the plate never gets reformed. This leads to a growth of the crystals. By a proper execution of a discharge/charge cycle, one destroys the large crystal cadmium and replace it with a microcrystalline form best for discharge.
This does NOT mean that one needs to cycle one's battery each time it is used. This does more harm than good, and unless it is done on a per cell basis, one risks reversing the cells and that really kills them. Perhaps once in a while, use the pack until it is 90% discharged, or to a cell voltage of 1.0V under light load. Here, about 95% of the cells capacity is used, and for all intensive purposes, is discharged. At this point, recharge it properly, and that's it.
The more common "memory effect" isn't memory at all, but voltage depression caused by overcharging. Positive plate electrochemistry is very complicated, but overcharging changes the crystal structure of the nickelic hydroxide from beta-Nickelic Hydroxide to gamma-Nickelic hydroxide. The electrochemical potential of the gamma form is about 40 to 50 mV less than the beta form. This results in a lower discharge voltage. In a six cell (7.2v) pack, this means a loss of 300 mV. Trick? Don't overcharge. Leaving cells on a trickle charger encourages formation of gamma nickelic hydroxide. Expect the cells to discharge at a lower voltage.
The best method is the so called delta-V method. If one plots the terminal voltage of the cell during a charge with a constant voltage, it will continue to rise slowly as charging progresses. At the point of full charge, the cell voltage will drop in a fairly short time. The amount of drop is small, about 10 mV/cell, but is distinctive. There are circuits out there built specifically to look for this. The Maxim MAX712 and 713 ICs are ones that come to mind now. This method is expensive and tedious, but gives good reproducible results. There is a danger in this though. In a battery with a bad cell this delta - V method may not work, and one may end up destroying all the cells, so one needs to be careful. If one ends up putting in more than double the charge capacity of the cell, then something is wrong.
Another cheap way is to measure the cell temperature. The cell temperature will rise steeply as full charge is reached. When the cell temperature rises to 10 degrees C or so above ambient, stop charging, or go into trickle mode.
Whatever method one chooses, a failsafe timer is a requirement with high charge currents. Don't let more than double the cell capacity of charge current flow, just in case. (i.e. for a 800 mAh cell, no more than 1600 mAh of charge).
If one wants to get a bit more aggressive, a C/3 charge will recharge the cells in about 4 hours, and at this rate, most cells will handle a bit of overcharge without too much trouble. That is, if one catches the cells within an hour of full charge, things should be OK. No overcharge is best of course.
Only with automatic means of full charge detection should one use charge currents above C/2. At this current level and above, many cells can be easily damaged by overcharging. Those that have oxygen absorbers may not vent, but will still get quite hot.
With a good charge control circuit, charge currents in excess of C have been used -- the problem here becomes reduced charge efficiency and internal heating from ohmic losses. Unless one is in a great hurry, avoid rates greater than C.
This is the big danger of battery cycling to prevent memory. Invariably, unless one is very careful, one ends up reversing a cell. It does much more harm than the cycling does good. Also, keep in mind that cells to have a finite life. Each cycle is a bit of life.
However, in reality, there will be bumps and valleys. When there are bumps on both the positive and negative plates are adjacent, separated only by the separator, the resistance between those two points is slightly less than in other regions of the cell. So, the current density there rises. This means that more material is deposited there, contributing to even more "bumpiness". In reality, needles called dendrites form, and given time, they can force themselves through the separator to short the cell.
A cell that appears to self-discharge in a couple of days has dendrite problems, and will soon completely short out. Plan to replace the cell.
Degradation of the active plate material is just a normal aging process of cycling. Both of these mechanisms are very good reasons to avoid cycling the cells after each use. Cells should live to about 1000 cycles if treated properly. Anything over that is gravy.
This works, and can revive an otherwise shorted cell. However, it is a stopgap measure at best. First, the fact that one dendrite has formed means that another is not too far behind. Second, the material that was vaporized has now permeated the separator material, forming a resistor that shorts the plates. The cell may no longer be shorted, but will still have a poor charge retention.
Besides, unless done properly, this can be dangerous as large currents are necessary.
The downside: They are expensive (all new technology is). They have a horrible self-discharge rate (I have measured between 3 to 10 percent per day -- useless after 1 month). They are trickier to charge. Delta V works, but the voltage drop is very small (2.5 mV/cell). Better to charge them to a point where the voltage stops rising. And, yes, the same thing goes with hydrides as with cadmium. They can suffer from memory, though it is much harder to see than in NiCds. Expect to see a bit less touting of the "memory free" operation of NiMH cells in the future.
To the well informed, however, 'memory' is a term applied to a specific phenomenon encountered very infrequently [emphasis mine - RLM] in field applications. Specifically, the term 'memory' came from an aerospace nickel-cadmium application in which the cells were repeatedly discharged to 25% of available capacity (plus or minus 1%) by exacting computer control, then recharged to 100% capacity WITHOUT OVERCHARGE [emphasis in the original]. This long term, repetitive cycle regime, with no provisions for overcharge, resulted in a loss of capacity beyond the 25% discharge point. Hence the birth of a "memory" phenomenon, whereby nickel-cadmium batteries purportedly lose capacity if repeatedly discharged to a specific level of capacity.
The 'memory' phenomenon observed in this original aerospace application was eliminated by simply reprogramming the computer to allow for overcharging. [Note that no mention is made of adding an intentional *discharge* to clear the problem - RLM] In fact, 'memory' is always a completely reversible condition; even in those rare cases where 'memory' cannot be avoided, it can easily be erased. Unfortunately, the idea of memory-related loss of capacity has been with us since. Realistically, however, 'memory' CANNOT exist if ANY ONE of the following conditions holds:
This note goes on to list the following as the most common causes of application problems wrongly attributed to 'memory':
This has been an interesting thread, but one which took some slightly wrong turns. I hate to sound like a self proclaimed expert, but I will anyhow. NASA has paid me for the last 5 years to study and model Nickel-Cadmium batteries. Check out my battery modeling home page if you care to, at:
http://mashtun.jpl.nasa.gov/section342.html
Getting back to the origninal question, is reconditioning useful. The answer is definitely a yes, but marginally. If you were to completely discharge your cells every time you used them, you would dramtically shorten thier lives. If however, you need max capacity and have the $$ for new cells, go for it. I would recommend a recondition once every three months for most applications. Too much hassle? then don't do it and buy new packs sooner.
Recondition should be done to 1.0 volts per cell or there abouts, if no individual cell let down resistors are present. The strong cells tend to drive the weak into reversal, generating hydrogen and causing some permanent dammage to the cell, although the hydrogen will eventually recombine. Lots of hydrogen is a dangerous thing. If you feel must go lower, you should switch to a low rate reistor for the last bit. With individual cell monitoring you could go all the way to 0.0 v theoretically, but that never happens.
But I prefer to run my packs to various depths, and get a mild "stealth" reconditioning that way. This reduces the total number of cycles on the pack and should help prolong the useful life.
I was stated that with modern Ni-Cd's rigorous battery charge control was nearly superfluous. I couldn't disagree more. The trend for rechargable applicaions is to push them harder and harder, with expectations of greater cycle life, faster recharge, better voltage regulation, and more temperature tollerance. The basic commerical design has not changed very much, although some manufacturers are doing a very good job in tuning thier production to give consistent cells. The desire for 15 minute chargers is very real, and you need special equipment to do it.
Pulse charing is a hot topic that alot of people are trying stake out patents. There seems to be some advantage at the highest rates 15 -30 minute charging to this technique for nicds. The Cristie charger was designed for Lead-Acid systems, where liquid phase stratification was a big problem. It is quite helpful in "stirring" the acid in the big cells they typically market. There is also some evidence that the pulse charging changes the morphology of the cadmium electode, in an advantageous way. High rates and current reversals tend to give higher areas and better utilization. This is good, expecially for cheap plastic roll bonded cadmium electrodes found in your average commerical cell.
The comments about storage shorted or trickle charged is right on. What wasn't said is the benefits of keeping them in the fridge or frezzer. Keep them above -20f and you'll be sure not to freeze the electroltye.
Jonathon was asking about bubbles and charging and heat. It sounds like you got some sales literature thrown at you. The bubbles are oxygen, and you don't shake them loose, you electro- chemcially recombine them. Bill, they are generated at the Nickel electrode. The real solution is to not make so many of them. You do that by controlling the cell potential, and thus the driving potential on the positive electrode. Pulsing may do that by relaxing the proton diffusion gradients in the postive electrode, or it may not! Like I said, at high rates, good idea. The effect on dead cells can be to burn out the cadmium shorts and give you a few more cycles. Jon, I could tell you what to look for but then I'd have to kill ya! :-)
The circuit that Gerald posted seems to be working off 60 hz AC. I am not sure, but I think that is a little fast for optimal results. (So move to europe 50 hz :-) The problem is that at two high a frequency you just access the doulbe layer capacitance in the cell and you don't exercise the main reactions, or the overcharge ones. I seem to remember 15 hz as being indicated by the work of McBreen on the Zinc electrode.
I could go on, but have probably said to much already. I hope somebody finds it useful. If somebody has a specific application or problem, I can handle it by email at.