Care and Feeding of Nickel Cadmium (NiCd) Batteries

12-13-06

Bob Kenyon – K8LJ

There is a lot of folklore about batteries in general and Nickel Cadmium batteries in particular. Although being supplanted by newer battery technologies such as Nickel Metal Hydride and Lithium-ion cells, NiCds are still widely used in our emergency communications systems and supporting devices. In a future training tip we will compare and contrast the three technologies, but for the moment, let’s focus on NiCds.

We have all heard much about the NiCd “memory effect”. So much so that most of us probably believe it actually exists. Actually it apparently does, but its effect is so slight that most of us could not duplicate it if we tried. The whole concept dates back to the early use of NiCds in the space program where they were being repetitively charged and discharged to EXACTLY the same level each time. By exactly, this means to within a thousandth of a volt or so and in subsequent attempts to utilize the batteries below that charge level, they acted as if they had lost some amount of capacity and hence the term “memory effect” was coined. In the normal use of an HT or other NiCd powered device this will simply never occur. In short, it’s a non-issue, but current battery advertisements for newer battery technologies still push the no “memory effect”.

The so called “memory effect” spawned the concept that to counteract it, one should “deep cycle” the batteries occasionally. While that’s basically a good idea, the cure can be worse than the disease if not done properly. Most battery packs have multiple cells in series. Over time one or more cells will likely become weaker than the others and if the battery pack is deeply discharged by a load connected to it, one or more cells might be reversed in polarity, which will likely be very damaging. A better method is to discharge each cell individually (admittedly difficult to do in a series sealed battery pack). Contrary to what many may believe, discharging each cell to zero and then recharging them is beneficial because it re-balances the cells (if they’re still in good condition). The bottom line is, memory effect was not the problem to begin with. Also, contrary to some opinions, allowing a NiCd pack to self-discharge to zero will not damage anything. Each cell will eventually self-discharge to zero and there will be no cell polarity reversal because there is no current draw.

So what is the problem? There is a big problem and it’s not memory, but better characterized by the term “voltage depression”. Voltage depression is caused by repeated over-charging of a battery, which causes the formation of small crystals of electrolyte on the plates. These can clog the plates, increasing resistance and lowering the voltage of some individual cells in the battery. This results in a seemingly rapid discharge as those individual cells discharge quickly and the voltage of the battery as a whole suddenly falls. This effect is very common because most of us over-charge our batteries because we leave them on charge too long and the result is often falsely attributed to the “memory effect”. Heat is the enemy of almost any electronic device, and in time will do serious damage and that’s what happens when batteries are subjected to charging for too long a period. Smart chargers and smart radios that sense the voltage level and know when to quit obviate this problem, but the typical wall charger and HT don’t know when to quit. As examples, we have several cordless phones that have semi-dead batteries that aren’t very old (2-3 yrs), but they’re always in the cradle, getting the batteries fried. On the other hand, I have a couple of HTs and a cordless drill that have been properly cared for and in the case of the latter is well over 20 years old and the original batteries are still going strong. I just used it this past week. To properly charge NiCds, they should be charged at 10% of their rated capacity for about 15-16 hours and no more. I use a simple timer to do that.

So what can we do about batteries that have suffered voltage depression? If it isn’t too severe, discharging each cell individually and then recharging will sometimes yield positive results. More serious cases might require zapping each defective cell with a high voltage (and resulting high current) that sometimes clears the cell and allows it to function again. Don’t attempt this unless you are sure of what you are doing. Wear protective clothing and eye shields. This method sometimes works but often doesn’t. When in doubt, be safe, buy new batteries and care for them properly.