The Dreaded Memory Effect

Original article “The NiCd FAQ,” posted by D. Bushong, OPEN/image Recognition Products, written by K. A. Nishimura (KO6AF), originally entitled “Some Ramblings About NiCd Batteries.” Updated and used by permission of P. Parry, WPP Ltd., Hemel Hempstead, UK. Added by Research International.

Does the Memory Effect Exist? Not in Research International’s Li-ion Cell. Yes and no. The ‘real’ memory effect as reported in certain space borne applications and caused by persistent under charging of a partially discharged cell is almost certainly never seen in mobile phone batteries or any others in terrestrial service except in certain very unusual circumstances.

Much more commonly, there is AN effect, properly called voltage depression, which people now tend to refer to as ‘memory effect.’ This is a bit unfortunate as the cause (and cure) is different from the true memory effect. For details, read on:

First, 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.

Voltage depression is more severe in NiCd batteries than in NIMH. The explanation below concentrates on NiCd, the same principles apply to both.

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(OH) to Cd metal. 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 is 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 replaces 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 (impossible in a phone), one risks reversing the cells and that really kills them.

Perhaps occasionally, use the pack until it is 90% discharged, or to a cell voltage of 1.OV under light load. Here, about 95% of the capacity of the cell is used, and for all intensive purposes, is discharged. At this point, recharge it properly, and that’s it.

In the case of mobile phones the phones are designed to switch off at the 90% discharge point before the battery voltage gets sufficiently low to damage the individual cells so the phone itself make the ideal discharger —just leave the phone on until it switches itself off after bleating about your battery being low! Many so-called ‘conditioners’ actually take the battery down to virtually zero volts and far from conditioning the battery cause cumulative damage to the cells.

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 (7.2 V) 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.