Another problem is that in long term, NiCd cells can have longer life spans than NiMH cells, but why is it that in so many applications people find that the NiMH cells outlast the NiCd cells that they used to use?
We'll answer that question along the way.
The economics and convenience of rechargeable cells:
Even if they don't last very long (in terms of years) rechargeable cells are almost always much less expensive to own and operate than their non-rechargeable Alkaline cousins, but there is a convenience factor involved:
- You can probably get more run time (from many - but not all - devices) from a set of alkalines than you can from a single set of rechargeables.
- When you put in a fresh set of alkalines, you have a pretty good idea how long that device will run. Unless you pull rechargeables off the charger - and you know that they are good, you don't know before-hand how long the device will run.
- A set of alkalines can sit around in the package, unopened, for several years and still be good. Again, with rechargables you don't know their charge state for certain - particularly if you haven't used them for a while!
Alkaline Cells: A comparison to NiCd and NiMH:
Other than the fact that they are rechargeable, what are the main differences between alkalines and NiCd/NiMH cells? As it turns out, voltage isn't really an issue since modern devices will happily run at 1.2 volts per cell - the same as NiCd/NiMH and a half-discharged alkaline.
What about capacity?
A good-quality alkaline AA cell has a capacity of about 2.5-2.8 amp-hours. Comparing an AA-size NiCd, its capacity will be in the area of 0.6-1.1 amp-hours and a NiMH will have a capacity of between 1.8 and 2.8 amp-hours, depending on the brand and specific type. In general, the rechargeable cell will have less capacity than the equivalent sized alkaline, but why is it often the case that it runs an electronic device longer?
Internal resistance is the answer. When fresh, the internal resistance of a good-quality AA alkaline cell is on the order of 0.15 ohms per cell, increasing to 0.3 ohms per cell when the it is 50% discharged and over an ohm when 80% discharged! If your camera uses a battery of 4 cells in series that means that the total resistance of new cells (excluding resistance of battery contacts and wiring) is about 0.6 ohms, rising to 1.2 ohms when the battery is just 50% discharged - and it only gets worse (much worse!) as the it is further-depleted!
If the digital camera consumes, say, 800 milliamps (a reasonable amount when a flash is charging, a backlit display is operating, etc.) then cell resistance alone will dictate a voltage drop of 0.48 volts for a battery with new cells, and 0.96 volts or so for cells that are 50% discharged.
Again, this does not take into account other resistive losses - such as contacts and internal wiring - some of which can be significant!
For new cells in a 4 cell battery, this voltage will (optimistically - assuming a nominal 1.5 volt unloaded output) amount to about 5.5 volts under these conditions, dropping to about 4 volts when the cells are 50% discharged - a voltage that may be inadequate for operation of the camera.
There is yet another problem. Often, cameras contain switching-type voltage converters. While these are efficient in their energy conversion, they attempt, by their nature, to maintain a constant power output over a varying input voltage. What this means is that, as the battery voltage drops, the current consumption will increase as the voltage converter attempts to maintain the constant voltage output - exacerbating the problem of already-low voltage and resistance. This problem can get worse when the camera's load changes because of a charging flash, a backlit display being illuminated, or the camera's CPU pulling more current when processing the image and saving it to memory.
In other words, the cells may be, say, only 50% discharged, but the equipment (the digital camera, in our example) may simply be unable to use the energy that is still available. If this is the case you'll probably get plenty of life out of those same batteries if you put them in a small flashlight or portable FM radio, or TV remote control.
In other words - don't throw them away just yet!
NiMH, NiCd cells and internal resistance:
NiCd and NiMh cells, on the other hand, typically have a much lower internal resistance over their charge life and under typical conditions, this resistance is typically lower than that of an alkaline cell - even when the NiCd or NiMH cell is significantly discharged.
According to info from several well-known manufacturers, a relatively new AA NiMH cell typically has about 0.17 ohms per cell when fully charged (as opposed to 0.15 ohms for a "fresh" Alkaline AA cell of good quality) but this rises to just 0.18 ohms at the "100% discharge" point.
As we demonstrated above, a typical AA Alkaline cell can be expected have over an ohm of internal resistance at 80% discharge - and this value skyrockets as the battery is discharged further! From what information that I have been able to find, a typical NiCd seems to have about half the internal resistance of the same-sized NiMH cell and is one of the factors that explains its suitability in very high current situations.
What this means is that while an alkaline cell may be able to run the digital camera (our example from above) only until the cell is at its 50%-70% charge level, a NiCd or NiMH battery can probably output the required current and voltage until it is at or below its 15% charge level. The lower intrinsic resistance also means that they are more likely to be able to tolerate impulse loads (i.e. additional current drawn by the flash charging, for example) without causing the camera to shut down due to low voltage.
At the current level of technology, NiCd cells are often preferred over NiMH cells for certain applications, most notably those requiring very high current consumption such as in battery-powered tools, etc. In these applications, the high current drawn by the tool would over stress a typical NiMH cell and likely result in shorter operational and useful life than a NiCd cell.
There are NiMH cells that are specially designed for "high drain" applications, but these are special purpose cells that often trade this high current capability for capacity, putting their amp-hour ratings below those of other types of NiMH cells.
When cells go wrong: "Memory"
One of the best-known properties of NiCd cells is this thing that people refer to as "Memory" - that annoying property of cells seeming to go dead much sooner than expected.
It is unfortunate that this effect. while called "memory" by many people (and some manufacturers of electronic devices) is almost never that phenomenon that is really the "Memory Effect." Instead, this phenomenon is usually due to cell damage caused by reversal - more on this later.
One of the first places that the so-called "memory effect" was first noticed and quantified was when NiCd cells were first used in communications satellites. These satellites rely on solar panels for their power, but the Sun is eclipsed by the Earth at times and it is during these periods that the satellite must operate from battery power alone. For many satellites these eclipses were typically of very similar duration which means that during the "eclipse season" the battery was run down by about the same amount, time after time.
The "memory" was noticed when, after several eclipses, the battery voltage would relatively quickly drop to the voltage approximately that attained during the latter part of the eclipse - and typically stay there. It was also noted that this "memory" effect could be reversed simply by charging the battery and then discharging it to a different point for several cycles and was done by clever management using multiple battery strings onboard the satellite and preventing a battery string from being discharged to the same point repeatedly.
It is important to know that little (or no) permanent damage was actually done to the cells by this "memory" effect - the result was (more or less) a temporary reduction in the cells' capacity until they were conditioned appropriately.
In typical use by consumers who use NiCd-operated devices, it is unusual to discharge the battery to precisely the same point time-after-time. Typically, the amount of discharge is somewhat random - and just one or two variations from a precise cycle will largely "erase" a weak memory effect. Among the very few documented cases of "terrestrial memory effect" were been in pager service where, regular as clockwork, the batteries would be run down during the day and recharged overnight. This was a long time ago - back in the days when pagers were those half-brick sized things that only VIPs and doctors wore - and batteries only lasted a day or two anyway!
It has been reported that NiMH cells can also exhibit this same "memory" effect - but remember that it is atypical to expose a cell to very precisely repeated discharges of equal depth time after time: Most people just don't use their battery-operated devices that way!
"What is this thing (mis)called 'Memory' then?"
Abused NiCd cells will typically exhibit a loss of capacity and/or the inability to take or retain a charge, and it this property that is too-often misidentified as "memory." But, this is not "memory."
What is going on, then?
Good quality battery packs are made from individual cells that have been matched in terms of resistance and capacity. This is important in terms of maximizing battery life.
A battery typically consists of cells wired in series for higher voltage. Ideally, all cells will run down at exactly the same time. This is not usually the case, however, especially as the cells age and some get weaker faster than others.
Temperature also has a large impact on cell longevity. A cell that is operating at a higher temperature will generally have a shorter overall lifetime than one that is cooler. An effect of this can be noted in a large battery pack (such as that on a cordless drill) in which a large number of cells are grouped together. Often, the cell(s) in the "middle" of the pack die first as these are surrounded by other cells. Not only can these "inner" cells not get rid of their own heat as easily as those cells on the "outside" layer of the pack, but they are also exposed to heat from the cells that surround them!
How important is heat to the life of a cell? One oft-quoted statistic (that I've not verified personally with NiCd cells) is that for every ten degrees F of temperature rise above 80 degrees F, the usable lifetime of the cell will be halved! Even if these numbers aren't exactly correct, cells that are warmer will die sooner!
Inevitably, one or more cells will run down sooner than the rest and its voltage will drop. Because the other cells still have some charge, current is still flowing through it and the now-dead cell's voltage will not only drop to zero, but it can go below zero and effectively start to "charge" backwards because at least some of the remaining cells are still outputting voltage.
The effect of this is a very quick death to a NiCd cell!
It comes down to chemistry. When a NiCd cell is reverse-charged, a strange thing happens: Conductive metallic "hairs" (often called dendrites) begin to form - and they "grow" from one electrode to another. Eventually, this dendrite forms a short across the cell - one that can have a range of resistance from high to low, depending on the severity of the damage and the size of this dendrite.
Once this dendrite has formed in the NiCd cell it is permanent and cannot be "dissolved" by charging the cell correctly or by doing any sort of "conditioning." Furthermore, this dendrite can form a leakage path that can cause the cell to run down by itself - the rate at which can vary depending on the resistance and relative size of the dendrite. The effect can range from a cell that just doesn't "hold a charge as long as it used to" to or, in extreme cases, the dendrite may be big enough that the cell won't even seem to take a charge at all (except, maybe, on a "quick charger.")
Perhaps the worst thing about the dendrites is that they represent an amount of electrolyte that can no longer be used to contribute to the charge capacity of the cell. What this means is that not only is the cell likely to run itself down more quickly because of charge leakage due to the dendrite, but even if it is fully charged to begin with it will be the first in the battery pack to run down next time it is used and go into reversal - again - and will be prone to forming even more, bigger, and better dendrites! (In other words: A vicious little circle...)
Note: NiMH cells do not seem to exhibit this "dendrite growth" problem, but cell reversal tends to cause gasses to be generated. If these gasses are produced as too high a rate, they cannot be reabsorbed internally and pressure will build within the cell, causing outgassing when the safety vent releases and resulting in a permanent loss of cell capacity.
You may have heard about a technique for "restoring" NiCds often referred to as "Zapping." As the name implies, one dumps a brief surge of energy into the cell and, almost as if by magic, the cell is "restored" to operating condition.
Well, not quite!
The surge of energy should be limited - often, a "zapper" consists of a very large capacitor (50,000 to 200,000 microfarads) charged to 50-100 volts, the source voltage disconnected, and the energy of this capacitor is dumped into a cell via a very heavy switch or a beefy SCR. This "one shot" burst of capacitor-stored power prevents too much energy from being dissipated by the cell and blowing it (and the person doing the "zapping") up.
Another method uses a lower voltage - but much higher current - say, from a large power supply: The obvious disadvantage of this latter method is that it is not "self limiting" as is the one-shot nature of the capacitor discharge and one can easily "pop" a cell either by burning open internal conductors or cause the cell to rupture due to a sudden buildup of heat and gasses. Needless to say, neither situation (especially the latter) is particularly desirable!
What is supposed to happen in this process is that enough energy applied to "fuse" (or blow away) the dendrite that is shorting (or "almost" shorting) the cell. Once this low-resistance path is removed, the cell can be charged again. This doesn't completely remove the dendrite, but "disconnects" it (hopefully) but it still represents a degradation of the cell.
It should be kept in mind that such a "repaired" cell, although it may be more able to take a charge than before, will still have reduced capacity and, when used in a battery, is still very prone to discharging early and going into reversal - again.
Remember: The material that formed the dendrite no longer contributes to the charge capacity of the cell - even after you "zap" it. Furthermore, the cell contains a separator material that will often be damaged by dendrite growth and "zapping" - something that further contributes to self-discharge.
If you do this technique, make sure that you have completely disconnected the cell/battery from the appliance being operated to prevent the voltage surge from the "zapping" process from damaging it.
Finally, while you may get some additional use out of a battery as a result of "zapping" I consider that "zapping" a cell may simply be buying me enough time to get a replacement ordered and on its way!
It should go without saying that this "zapping" procedure can be hazardous: Not only are potentially dangerous voltages and currents involved, but there is a chance that the cell may explode and/or leak hazardous material.
Finally, this procedure should be done only on an individual cell and not the entire pack at once - That is, you must be able to access and test each cell you plan to "zap", individually.
Getting the most out of your NiCd/NiMH cells:
For reasons unknown to me, some manufacturers of battery-operated equipment recommend that you "condition" NiCd battery packs by running them completely down, and then charging them again. I guess that the claim is to prevent a "memory" condition from occurring - but it is already known that to cause this "memory" the cell would have to be precisely depleted to exactly the same charge state repeatedly: This just doesn't happen with most people's usage of equipment.
Why do they make this recommendation, then? The cynical side of me says that they are just trying to sell more batteries or devices: By recommending you go through some steps that are guaranteed to shorten battery life, they can increase sales! The other side of me would guess that the person writing these instructions is just poorly informed or just doesn't know any better.
Here are a few things you can do to prevent premature failure of NiCd battery packs:
- NEVER, EVER run a NiCd battery pack completely down. Inevitably, one or more cells will go into reversal before the others, immediately causing permanent damage to the cell(s). The only safe way to run a NiCd battery pack completely down is to guarantee that no cell can possibly go into reversal. This can only be done by monitoring each individual cell and preventing reversal by bypassing it - but almost no manufacturer of consumer goods does this due to cost and complexity. NiMH cells aren't totally forgiving either: While they may not be immediately damaged by reversal, such operation can result in loss of capacity due to outgassing. (Note: Rechargeable lithium-ion packs use exactly this sort of protection because Li-Ion cells are completely unforgiving of a complete discharge/reversal.)
- DO NOT try to drill that "last hole." Have you ever been using a cordless drill when, just before the battery goes completely dead, it suddenly slows down and loses most (but not all) of its power? At that moment, one or more cells have collapsed and are going into cell reversal. Your battery pack will last much longer if you stop using it the instant that the motor slows due to the voltage drop. Unlike alkaline cells, NiCd and NiMH cells will put out (more or less) the same voltage until they are almost totally dead - at which point their voltage will suddenly drop. Again, if NiCd battery packs had the same sort of circuitry in them that Lithium-Ion battery packs did (e.g. a circuit that "disconnects" the pack when any one or more cells' voltage drops too low) they would, on average, last much longer.
- Do not overcharge the cells. Nowadays, "smart chargers" are pretty good about preventing cell overcharge, but if a battery pack gets unusually hot, something may be wrong. So-called "trickle" chargers won't destroy a battery pack too quickly if they are left connected after the battery is fully charged, but it isn't a good idea to leave it connected forever. If the battery is noticeably warm when connected to a trickle charger, it is already overcharged. Overcharging NiCd or NiMH cells can cause gasses to form in the cell's electrolyte and if this pressure builds up, a safety vent in the cell can open (which is better than having the cell explode...) and the gas will be vented. This venting represents a loss of material - which also means a loss of cell capacity. Another phenomenon that can shorten life of a trickle-charged cell is the breakdown of the plastic separator due to its continuous exposure to oxygen at elevated temperatures.
- Don't leave the pack on the charger and walk away! Related to the above, it is a terrible idea to leave a battery on a charger all of the time. If you can detect any warmth from the battery when it is left on the trickle charger for a day or so, it is being trickle-charged too strongly! Unfortunately, it's very easy to "charge and forget" many power tools and slowly kill the pack. Of course, there's the desire to have the portable device always at the ready, so the temptation to leave it on the pack is almost irresistable!
At first glance, it would seem that NiMH cells are just "better" versions of NiCd cells as they have the following advantages:
- Their energy density is better: A NiMH cell has more charge capacity than the same-sized NiCd.
- They do not contain Cadmium - a toxic heavy metal - and thus do not pose as much of a disposal problem.
- They are (apparently) not prone to forming dendrites when they go into reversal - something that can kill a NiCd by shorting it out internally and/or raising self-discharge current - not to mention loss of capacity.
- Their lifetime with respect to the number of charge/discharge cycles is lower (250-500 for NiMH versus 500-1000 for NiCd - but this is improving)
- They have a relatively high self-discharge rate: Just sitting around they tend to run themselves down more quickly - especially as they age.
- They have a slightly higher internal resistance and a lower current-carrying capacity than an equal-sized NiCd: This generally makes them inappropriate for use in high-current drain devices such as cordless power tools where the load may be several "C" (i.e. 2-3 amps of load per amp/hour of cell.) Newer types of NiMH cells are beginning to appear that do not have quite this limitation.
- It is more difficult to tell when NiMH cells are fully charged
NiCds. When NiCds are nearly fully charged, the voltage suddenly
rises - and then starts to go back down again when an overcharge
condition is approaching. NiMH cells will do this, but the
magnitude of this voltage rise is only a fraction of that of NiCd cells
and, under many charge conditions, may go completely unnoticed, hence
the need to monitor the temperature of NiMH cells as well as to limit
the amount of time over which a charge is applied.
Again, a lot of NiCd cells "die" due to cell reversal (see above) and the resultant effects while NiMH cells do not readily form dendrite shorts when they go into reversal. Damage to a NiMH cell may still occur, though: Cell reversal of a NiMH cell causes gases to form and it is possible that pressure will build up faster than its chemistry can reabsorb these gasses and the cell will vent. The resultant loss of gas means a loss of electrolyte material and a subsequent loss of capacity.
Even without loads, all cells slowly lose their charge over time as the cell's chemistry slowly changes. In all cases, the rate of self-discharge increases dramatically as temperature also increases.
The table below shows the approximate amount of time that it takes to lose 10% of the cell's capacity at different temperatures.
A - Storage or use of this type of cell at 60C violates the manufacturers recommendation for consumer-type cells and one may expect poor lifetime. It is not recommended that any cell be exposed to such high temperatures for an extended period of time.
B - The self-discharge rate of LiIon cells varies widely according to its chemistry and manufacturer. Information on self-discharge rate at temperatures other than 20C was not available at the time of writing but, as in the case of other types of cells, it increases dramatically with increasing temperature and the age (and past use) of the cell.
The chart above compares various cell chemistries and their approximate rates of self-discharge showing how quickly one can expect to lose 10% of the cell's capacity. Please note that these rates are typical published specifications by various manufacturers and, in the case of rechargeable cells, represent the sort of performance that may be expected from new cells. In general, independent testing has shown that the manufacturers' specifications concerning self-discharge are more-or-less in line with what is actually observed. Also, reduction of self-discharge is one of those parameters on which the manufacturers are continually improving.
As can be seen the clear winners are the non-rechargeables and the two cheapest type are the alkaline and Zinc Chloride (the so-called "heavy duty" batteries) which do a respectable job of retaining their capacity over time. Ahead of the pack are the non-rechargeable Lithium types (The Lithium-Iron Disulfite and the Lithium Manganese Oxide) and these two chemistries also perform better than the others even when they are very cold.
The worst of the bunch is clearly the NiMH cell which could easily be found dead after having been left in a vehicle for a month during the summer (if you have hot summers, that is...) It is certainly worth repeating that NiMH cells are NOT the proper choice for your car flashlight, for example, or even for any item that is left idle for months at a time and is then expected to work (such as an emergency radio.)
What about putting cells in the freezer to "keep" them? For the non-rechargeable types, it can be seen that freezing them will certainly slow the self-discharge rates, but if you plan to use them within a year or two you'll probably not see any real difference in longevity of those stored in your freezer and those simply kept at room temperature.
What is clear from this chart is that you should not be storing them in your attic or garage - or anywhere else that may tend to get warm: It is preferred that they be stored simply in a cool location (such as a basement) as compared to a warmer room or a vehicle.
Replacing NiCds with NiMH cells:
Can you simply drop NiMH cells in place of NiCds?
For optimal cell lifetime and performance under ideal conditions, the answer is probably no.
For "good" performance (that is, where overall lifetime and charge capacity will probably exceed that of NiCd cells) the answer is likely yes - as long as a few rules are observed:
- You (probably) can't/shouldn't use NiMH cells in very high-current devices such as power tools. These sorts of demands on the cells will result in a very short lifetime and could be hazardous due to cell overheating and venting. Again, there are certain types of NiMH cells designed especially for this type of service but you will have to do research on where to find them and if they will, in fact, be suitable for your intended application!
- A NiCd-only "smart charger" or "quick charger" may not be able to detect when a NiMH cell is fully charged. This could result in undercharging (the cell isn't charged completely) or (more likely) overcharging and "cooking" the cell if the charger cannot detect a full-charge condition. Make sure your quick charger is specifically designed for charging NiMH cells before you use it.
- Charging a NiMH cell from the original NiCd "slow" charger should work, but it will probably take more than twice as long as it did with the NiCd charger. Typically, "slow" chargers will charge a NiCd pack in 12-16 hours, but this means that the same charger will probably take 30-36 hours to charge the NiMH pack. This extra charge time is required because the storage capacity of the NiMH cells are likely to have at least twice the capacity of same-sized NiCd cells that they replace.
- Dispose of the dead NiCd cells properly - do not just throw them in the trash. Do a bit of research and find out where to dispose of the dead cells. (Your local recycling or trash-collection agency can probably tell you where to go... so to speak...)
- Take note of the guidelines in the above section when you charge NiMH cells: They may not charge properly in a "smart" or "quick" charger and a slow charger will take much longer to charge NiMH cells.
- They have a much higher self-discharge current (see the chart above.) If you charge the battery pack and forget about it, do not expect it to still be charged months later!
It should be remembered that one of the main reasons why NiMH cells seem to last longer than their NiCd counterparts is just that they can better-tolerate the abuse typically inflicted upon them. At the risk of repeating myself, here are some examples:
- In a power tool: Continuing to use it after one or more of the cells in the battery have gone dead and it slows down. This is guaranteed to very quickly kill ANY NiCd cell!
- Putting it on a charger and walking away: For a number of reasons, NiMH cells seem to be better-able to tolerate this sort of abuse than NiCd's, but it is still a bad idea! If any rechargable battery pack is noticeably warm after being fully charged and is left in the charger, it is already overcharged and is likely being (slowly) damaged!
What could manufacturers (and you!) do to prolong NiCd/NiMH cell life?
Again, it somewhat irks me that the appliance manufacturer's recommendation (i.e. to completely discharge a NiCd pack) is precisely the thing that can kill NiCd cells prematurely due to the inevitable reversal that will occur in a series-wired battery pack. What is so terrible about this is the cost of replacement and inconvenience that results: Often, the user will simply throw away the entire appliance and effectively wasting money!
There are several things that could be done to greatly lengthen cell life of both NiCds and NiMHs:
- Don't recommend that the batteries be completely discharged. Ever! There is absolutely no need for this in most cases. Most often, one cannot completely discharge a pack without causing permanent damage from cell reversal! Again, NiMH cells are more resistant to damage due to reversal.
- Build into the packs (or the appliance) a device that will cause current consumption to cease if any cell drops below, say, somewhere between 1.0 and 0.6 volts. This will prevent cell reversal from ever happening in the first place. An example of this sort of protection is found in all Lithium-Ion rechargeable battery packs because allowing them to be run completely down and then recharged constitutes a very real safety hazard!
For a longer version of this article with links to related pages, go to the "About NiCd and NiMH rechargeable batteries" - link web page.
This page stolen from ka7oei.blogspot.com