|MadSci Network: Engineering|
Greetings, John: First of all, let me state that I cannot claim to be an expert on batteries, and so I may not be the best person to be answering your Question. However, I can make a stab at it, and after that.... Next, from your description I cannot know how many times your nickel-cadmium cells have been cycled (charged and discharged). They do have a limit, regardless of the memory effect. Past that limit, your only option is to REcycle them (PLEASE don't just dump them into a trash can!). If the the cycle life of the cells hasn't been reached, then you may be able to try a few things. My first notion was to see what I could find on the Web (just as you did). Here is a site that offers a good description of how a nickel-cadmium cell works: http://www.powerstream.com/batteryfaq.html#nicd I am quoting two sentences from there, about the memory effect: "In the former case, a few cycles of discharging and charging the cell will help correct the problem, but may shorten the lifetime of the battery." "An important thing to know about "conditioning" a NiCd battery is that the deep discharge spoken of is not a discharge to zero volts, but to about 1 volt per cell." This implies that your cells have been TOO fully discharged. Then there is this site: http://www.repairfaq.org/ELE/F_NiCd_Memory.html which proclaims that the memory effect is largely a myth arising from misinterpretation of what actually happens. While well-detailed, it does conflict a bit with what I quoted above, and I cannot say which is correct. Neither offers much advice regarding your problem, because you need to charge your cells rather more than the minimal amount that you described. Here is a third site: http://user.aol.com/ralph234/cb-page/f_nicd_b.htm with far more information than the other two put together. This one agrees with the first, about discharging only down to one volt or so. "Zapping" the cell is described, which MAY be an option for you, after all. -------------------------- Finally, since this is a place for Mad Science, I don't mind dreaming up a few oddball notions. After all, IF, after browsing the above sites (and any others you happen to find), you decide that the thousand dollars invested in your nickel-cadmium cells is indeed about to become waste, why not Experiment? BE CAREFUL, OF COURSE! CHARGED CELLS ARE DANGEROUS! You might take one of those dead cells and try hooking up the charger to it backwards. (I think the instructions probably warn against overcharging, or tossing into a fire, but not against negative charging.) If it doesn't overheat or smoke or some other scary thing, leave it hooked up for an hour or two. Then TEST it to make sure it is still dead. I doubt that it will acquire a charge, but best be certain. THEN hook it up the normal way, and see if it can acquire a decent charge. Sure, I know that I just quoted that the cells shouldn't be fully discharged, and now I am suggesting you go for MORE than full discharge? But like I said, if the thing is dead anyway, why not? For another and somewhat more risky mad idea, you could take a dead cell (perhaps the same cell as in the first experiment, now deader than ever), and open the case. Try to open it in a fashion that will let you re-seal it. The risk in this case comes from the contents of the cell; cadmium is poisonous, and potassium and sodium hydroxide can cause chemical burns. You should wear suitable gloves. If there is no way to open the cell so that it can be re-sealed, then consider this a sacrifice to learn the details of how those particular cells were constructed. Then take ANOTHER dead cell, and drill a small hole into a place where you will not damage the internal electrodes. (You MUST avoid causing an internal electrical short!) Add some water. This could be a slow process, because droplets of water will not go through that hole easily, unless the innards of the cell is "thirsty". Possibly a syringe will be necessary, and a whole other set of risks will enter the picture. I cannot say how much water should be added; that's part of the Experiment! So start small, and then seal the hole with putty. You can always add more water later. Why add water? Because, if you noticed the particular electrochemistry of the nickel-cadmium cell, you will have seen that water plays a key role in the reactions. Just like ordinary lead-acid automobile batteries, some water can be "used up" whenever a cell is overcharged. That is, the water is electrolyzed to hydrogen and oxygen, which are gases that can probably (almost certainly in the case of the hydrogen!) leak out of the cell. And, just as the performance of a lead-acid battery degrades as water is used up, so I expect the performance of a nickel-cadmium cell to degrade. I would expect poor performance from a never-used NiCd cell, if it had been constructed loosely enough for the water to evaporate and escape. WAIT a time, after adding the water, before attempting to charge the cell. Give the water time to permeate throughout the innards of the cell, first. Well, for someone who doesn't claim to be an expert on the subject, I've probably said too much, and will now stop. Good luck!
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