Memory

Memory effect

What is memory?

Does the memory effect exist?

(See Below)

What is Memory ?

The term ‘memory’ has become a catch-all ‘buzzword’ that is used to describe a raft of application problems, being most often confused with simple voltage depression. To the well informed, however, ‘memory’ is a term applied to a specific phenomenon encountered VERY INFREQUENTLY 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. 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:

A. Batteries achieve full overcharge.

B. Discharge is not exactly the same each cycle - plus or minus 2-3%

C. Discharge is to less than 1.0 volt per cell.

Remember, the existence of any ONE of these conditions eliminates the possibility of ‘memory’.

The most common causes of application problems wrongly attributed to ‘memory’:

1. Cutoff voltage too high - basically, since NiCds have such a flat voltage vs. discharge characteristic, using voltage sensing to determine when the battery is nearly empty can be tricky; an improper setting coupled with a slight voltage depression can cause many products to call a battery “dead” even when nearly the full capacity remains usable (albeit at a slightly reduced voltage).

2. High temperature conditions - NiCds suffer under high-temp conditions; such environments reduce both the charge that will be accepted by the cells when charging, and the voltage across the battery when charged (and the latter, of course, ties back into the above problem).

3. Voltage depression due to long-term overcharge.

NiCds can drop 0.1-0.15 V/cell if exposed to a long-term (i.e., a period of months) overcharge. Such an overcharge is not unheard-of in consumer gear, esp. if the user gets in the habit of leaving the unit in a charger of simplistic design (but which was intended to provide enough current for a relatively rapid charge). As a precaution, I do NOT leave any of my NiCd gear on a charger longer than the recommended time UNLESS the charger is specifically designed for long-term “trickle charging”, and explicitly identified as such by the manufacturer.

4. There are a number of other possible causes listed in a “miscellaneous” category; these include - -

Operation below 0 deg. C

High discharge rates (above 5C) in a battery not specifically designed for such use

Inadequate charging time or a defective charge

One or more defective or worn-out cells (NiCds DO have a finite life; they won’t keep charging and discharging FOREVER no matter how well we baby them.)

Conclusion: To recap, we can say that true ‘memory’ is exceedingly rare. When we see poor battery performance attributed to ‘memory’, it is almost always certain to be a correctable application problem. Of the...problems noted above, Voltage Depression is the one most often mistaken for ‘memory’... This information should dispel many of the myths that exaggerate the idea of a ‘memory’ phenomenon.”

Q: Does the memory effect exist?

A: YES. 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. 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. 2 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.

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